de leeuw 2011 thesis

Paleomagnetic and geochronologic constraints on the Miocene evolution of semi-isolated basins in southeastern Europe

Johannes Hendricus Wilhelmus Maria de Leeuw

Members of the dissertation committee:

Prof. Dr. Sorin FilipescuDepartment of Geology, Babeş-Bolyai UniversityCluj-Napoca, Romania

Prof. Dr. P.L de BoerFaculty of Geosciences, Utrecht UniversityUtrecht, The Netherlands

Prof. Dr. G. Bertotti Faculty of Civil Engineering and GeosciencesTechnical University DelftDelft, The Netherlands

Univ. Doz. Dr. Mathias HarzhauserGeological-Paleontological Department, Natural History Museum Vienna, Austria

Dr. J. WijbransFaculty of Earth and Life Sciences, VU University,Amsterdam, The Netherlands

The research for this thesis was carried out at:

Paleomagnetic Laboratory ‘Fort Hoofddijk, Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 17, 3584 CD Utrecht, The Netherlands.

and at:

The Department of Isotope Geochemistry, Faculty of Earth and Life Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

Copyright © JHWM de Leeuw, Faculteit Geowetenschappen, Universiteit Utrecht

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, with the prior written permission of the author.

ISBN: 978-90-8570-769-1

Printed and bound by Wöhrmann Print Service, Zutphen, The Netherlands


Een Paleomagnetisch en geochronologisch kader voor de Miocene evolutie van semi-geïsoleerde bekkens in zuidoost Europa

(met een samenvatting in het Nederlands)

Proefschrift

ter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de rector magnificus, prof.dr. G.J. van der Zwaan, ingevolge het besluit van het college voor promoties

in het openbaar te verdedigen op dinsdag 14 juni 2011 des ochtends te 10.30 uur

door

Johannes Hendricus Wilhelmus Maria de Leeuw

geboren op 10 september 1982te Heythuysen

This thesis was accomplished with financial support from the Netherlands Research Centre for Integrated Solid Earth Sciences (ISES) and by the Earth and Life Sciences Division (ALW) of the Netherlands Organization for Scientific Research (NWO).

Promotor: Prof.dr. C.G. Langereis

Co-promotoren: Dr. W. Krijgsman Dr. K.F. Kuiper

Contents

Preface and acknowledgements 13

Introduction 16

Problem statement and objectives 1640Ar/39Ar dating 16

Magnetostratigraphy 17

Cyclostratigraphy 18

Chronology and integrated stratigraphy of the Miocene Sinj Basin (Dinaride Lake System, Croatia) 21

1.1 Introduction 22

1.2 Geological Setting, paleogeography and basin stratigraphy 22

1.3 The Lučane section: lithostratigraphy, biostratigraphy and earlier works 24

1.4 Methods 261.4.1 Isotopic Dating 261.4.2 Palaeomagnetism 27

1.5 Results 281.5.1 Isotopic Dating 281.5.2 Palaeomagnetism 29

1.6 Discussion 301.6.1 Age model 301.6.2 Lithostratigraphy, fossil flora and climate change 301.6.3 Age of the large mammal sites Lučane and Ruduša 321.6.4 Patterns in the mollusk fauna 33

1.7 Conclusions 35

Acknowledgements 36

A chronostratigraphy for the Dinaride Lake System deposits of the Livno-Tomislavgrad Basin: the rise and fall of a long-lived lacustrine environment 39

2.1 Introduction 40

2.2 Geological setting 40

2.3 Sections, lithology and depositional history 412.3.1 The Tušnica Section 412.3.2 The Ostrožac section 422.3.4 The Mandek section 432.3.5 The Drage West and Mokronoge sites 442.3.6 Depositional history 44

2.4 Radio isotopic dating 462.4.1 Sampling and methods 462.4.2 40Ar/39Ar ages 46

2.5 Magnetostratigraphy 472.5.1 Sampling and methods 472.5.2 Demagnetization results 47

2.6 Chronostratigraphic framework 49

2.7 Discussion 502.7.1 Comparison with the other lakes of the Dinaride Lake System 502.7.2 The rise and fall of Lake Livno: interplay of climate and tectonics 522.7.3 Towards a time-indicative endemic mollusk biostratigraphy 52

2.8 Conclusions 53

Acknowledgments 54

Palaeoenvironmental evolution of Lake Gacko (Southern Bosnia and Herzegovina): impact of the Middle Miocene Climatic Optimum on the Dinaride Lake System 57

3.1 Introduction 58


3.3 Material and methods 593.3.1 Mollusc palaeontology 603.3.2 Geophysical logging and spectral analysis 603.3.3 Magnetostratigraphy 603.3.4 Geochronology 62

3.4 Depositional facies analysis and lake-level change 623.4.1 Lithology 623.4.2 Carbonate depositional facies 663.4.3 Mollusc palaeontology 683.4.4 Geophysical logging 703.4.5 Transgression-regression cycles 72

3.5 Chronology 743.5.1 Magnetostratigraphy 743.5.2 Isotopic Dating 763.5.3 Correlation to the GPTS 76

3.6 Astronomical tuning 763.6.1 Spectral analysis 763.6.2 Periodic changes in eccentricity as forcing factor of the observed lake-level variations 773.6.3 Correlation to the astronomical curves 78

3.7 Discussion 783.7.1 Dinaride Lake System mollusc phylostratigraphy revised 783.7.2 Dinaride Lake System palaeo(bio)geography revised 803.7.3 Impact of the MCO on the Dinaride Lake System 80

3.8 Conclusions 81

Acknowledgements 82

Magnetostratigraphy and small mammals of the Late Oligocene Banovići Basin in NE Bosnia and Herzegovina 85

4.1 Introduction 86

4.2 Geological setting 864.2.1 Regional stratigraphic and paleogeographic development 864.2.2 Description of the basin infill 864.2.3 The Turija Section 884.2.4 The Grivice Section 88

4.3 Magnetostratigraphy of the Grivice Section 894.3.1 Sampling and laboratory methods 894.3.3 Demagnetization and magnetostratigraphy 92

4.4 Small mammal taxonomy and biostratigraphy 924.4.1 Material and methods 924.4.2 Taxonomy 924.4.3 Biostratigraphy 96

4.5 Chronology for the Banovići basin 98

4.6 Discussion 1004.6.1 The Turija Hemicyoninae (bear-dog) fossils 1004.6.2 Implications for mammal chronology 1004.6.3 Regional implications 101

4.7 Conclusions 102

Acknowledgements 102

Paleogeographic evolution of the Southern Pannonian Basin: 40Ar/39Ar age constraints on the Miocene continental series of northern Croatia 105

5.1 Introduction 106

5.2 Geological Setting 1065.2.1 Mt. Kalnik area 1065.2.2 Karlovac and Glina sub-depressions 108

5.3 40Ar/39Ar geochronology 1085.3.1 Sections and materials 1085.3.2 Methods 108

5.4 Discussion 1125.4.1 Initiation of extensional tectonics in the South Pannonian Basin 1125.4.2 Paleogeographic changes and faunal bioprovinces 1135.4.3 The Badenian transgression in the Southern Pannonian Basin 115

5.5 Conclusions 116

Acknowledgments 116

Paleomagnetic and geochronologic constraints on the geodynamic evolution of the Central Dinarides 119


6.2 The intra-montane basins of the Central Dinarides 120

6.3 A chronostratigraphic framework for the Dinaride basins 1226.3.1 Pag Island (Croatia) 1226.3.2 Sinj Basin (Croatia) 1226.3.3 Livno-Tomislavgrad Basin 1226.3.4 Gacko Basin 1236.3.5 Bugojno and Sarajevo Basins 1266.3.6 Banovići basin 1266.3.7 South Pannonian Basin 1266.3.8 Ugljevik basin 127

6.4 Late Oligocene to middle Miocene paleomagnetic rotation data 127

6.5 A compilation of Mesozoic and Cenozoic paleomagnetic data: the differential rotation of crustal fragments in the Dinarides 128

6.6 Neogene to recent deformation: AMS as an indicator of post-depositional strain 136

6.7 Consequences for the post-orogenic evolution of the Dinarides 138

6.8 Conclusions 139

Acknowledgments 139

Age of the Badenian Salinity Crisis; impact of Miocene climate variability on the Circum-Mediterranean region 141


7.2 The Badenian Salinity Crisis (BSC) of the Paratethys 142

7.3 Radio-isotope dating 142

7.4 The onset of the BSC 144

7.5 Progression and termination of the BSC 144

7.6 Comparison to the Messinian Salinity Crisis of the Mediterranean 146

7.7 Conclusions 146

Acknowledgments 147

Paleomagnetic & chronostratigraphic constraints on the evolution of the middle Miocene Transylvanian Basin: implications for Central Paratethys Stratigraphy & emplacement of the Tisza-Dacia plate 149



8.3 Stratigraphic framework and sampling approach 152

8.4 40Ar/39Ar dating of key stratigraphic horizons 1548.4.1 Methods 1548.4.2 Results 154

8.5 Resulting time frame and sedimentation rates 156

8.6 Paleomagnetism 1588.6.1 Methods 1588.6.2 Results 1588.6.3 Correlation to the timescale 160

8.7 Biostratigraphy 1638.7.1 Methods 1638.7.2 Results 163

8.8 A new chronology for the Middle to Upper Miocene of the Transylvanian Basin 1668.8.1 Late Karpatian and early Badenian 1668.8.2 Late Badenian and Sarmatian Biostratigraphy 1668.8.3 The Sarmatian-Pannonian Boundary 1678.8.4 The Pannonian 167

8.9 The rotation of the Transylvanian Basin during the Miocene 168

8.10 Comparison with structural geological observations 169

8.11 Conclusions 170

Acknowledgments 172

Epilogue 174

References 177

Samenvatting 196

Curriculum Vitae 203

Publication list 204

Preface and acknowledgementsThis thesis is the culmination of five years of intensive paleomagnetic and chronostratigraphic research in southeastern Europe. I could, by no means, have achieved this work alone. It will become clear in the forthcoming chapters that the integrated approach taken required the efforts of scientists from various disciplines to coalesce. I am grateful for their contributions and for their continuing stimulus and support. I am likewise greatful for all the support I got from my girlfriend, my friends and my relatives. And certainly not to be forgotten: all of you that have ever so often lured me away from my computer. Without a proper dose of distraction I would certainly not have made it.

Beste Wout, ik weet eigenlijk niet hoe ik je kan bedanken voor je vertrouwen in me, voor je steun en je toewijding. Ik kon altijd bij je aankloppen, en je had altijd tijd voor me. Manuscripten kwamen bij me terug voor ik erg in had. Toen ik er voor ging, gaven we samen gas. Fijn dat je zo recht door zee bent. Je gaf me altijd onverbloemd je mening, of ik dat nou leuk vond of niet. Ik waardeer dit echt enorm. Wat ik ook erg bewonder is je pragmatisme en wat je zou kunnen noemen ‘de kunst van de simpele papers’. Bedankt dat je me de mogelijkheid hebt gegeven onder jouw begeleiding te promoveren. Klaudia, ook jij stond altijd voor me klaar. Ook al was dit niet gepland, toch bracht ik een groot gedeelte van mijn tijd onder jouw hoede door. Ik voelde me altijd welkom in het argon lab en leerde van jou nauwgezet de kneepjes van het vak. Bedankt voor al je hulp en geduld, en bovenal voor het warme hart wat ik net voorbij Amsterdam Zuid vond. Cor, jouw kantoor en huisdeur stonden ook altijd voor me open. Samen spitten we geregeld door mijn data, net zolang tot alles keurig op een rijtje stond en er als vanzelfsprekend een mooie conclusie uit kwam rollen. Hard werken combineert bij jou met een stukje humor en sfeer. Als stuurman van het fort, zorg jij ervoor dat wij ons vooral met wetenschap bezig kunnen houden. Duidelijk, doordacht en doelgericht. Hier kan menigeen een puntje aan zuigen. Oleg, what a great time we had. In the field, in the office, at your home in Vienna and at my home in Utrecht. Rough work, rough driving, rough music and rough jokes. If I’ll ever be low on irony, I now know whom to call. Thank you for everything. It could not have turned out better. Marius, thank you for the late evenings at your house, where I always found a warm welcome. Thank you for the late nights in whatever bar in Romania. And most importantly, thank you for all the fielwork support. If I am ever going to explore jungle sections again, you’ll be the first to be invited. Sorin, thank you for all the kind advice. And thank you for the open door in Cluj. I really enjoyed the field tour to Garbova and the jungle adventures in northern Transylvania. Krzysztof, thank you very much for your help. It was very kind of you to provide me to give me your samples. I wish everybody would have such a positive attitude towards cooperation. Grzegorz, thank you for inviting me to the core storage in Poland. I really enjoyed the days we spent there. Poppe, ook jou wil ik graag specifiek bedanken. Vanaf het moment dat ik mijn master bij je begon ben je me blijven inspireren. Begrip en inzicht komen bij jou op de eerste plaats. Op de een of andere manier weet je altijd juist die vraag te stellen die me nog eens goed laat overpijnzen wat ik nou eigenlijk denk te weten. Bedankt ook dat je me hebt gevraagd mee te gaan naar Tremp. Ik vond het in alle opzichten verrijkend en heb er enorm van genoten. Jan, bedankt voor de gezelligheid aan de VU, en bovenal bedankt dat ik zo welkom was in het argon lab. Andrei, thank you for the very good time we had in the field. I spent more time with you in Transylvania than I spent with anybody else in the field. It was always fun. Thank you for all the support and for being such a good companion. Giza, thank you too for the good times in the field. Alan, thank you for coming to Utrecht and helping me on the mineral separation. And thank you for the good time in Napoli. I will come and visit you in Zagreb or Dubrovnik, one day. Tvrtko, hvala ljepo! Bolje dva velika od jednog malog. Pasha, I really enjoyed working together. Thank you for your friendship and for the good time we had. Arnoud en Jaap, die 10 dagen Transylvanie met de tent, of beter gezegd in de open lucht, zullen me lang heugen. Bedankt voor het harde werk en de lange uurtjes in het veld en in het lab. Ik vond het top. Beste Jurgen, Karen, Tineke, Uros, Marlies, John, Marten en Sandra, bedankt voor de gezelligheid op de VU. I would also like to thank the clay quarries in Transylvania and the coal mines in Bosnia and Herzegovina for their benevolence. Withouth entrence to their sites this thesis would not have been possible. Roel,

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zonder jouw opgewektheid en gezonde dosis humor was het in de kelder van de VU erg kleurloos geweest. Ik kwam altijd met plezier in je lab. Bedankt voor de goede tijd. En reken maar, als ik ooit nog ga scheiden, klop ik bij je aan. Tom, bedankt voor de ondersteuning en bedankt voor het technische circus wat in het fort opgesteld staat. Bedankt ook voor de goede discussies en de fijne tijd op het fort. Mark, bedankt voor de hulp, voor de babbels tussendoor en voor je enorme feitenkennis. Leuk dat ik je vanaf het moment dat ik aan het University College ging studeren keer op keer weer tegenkwam. En natuurlijk bedankt voor de aanbevelingsbrief die me de master binnenloodste. Liviu, thank you for the support, for the answers to my sometimes silly questions and for helping me out whenever I knocked on your door. Iuliana, Maud, Martijn, Silja, Ahmed, Christine, Dzihong, Marcella, Nuri, Pinar, Roderick, Lennart, Maxim, Chris, Côme, Mark Sier, Andy, Piet-Jan, Bora, Anja, en Tarek, bedankt voor de toptijd op het fort! Viktor, thanks for being as shocked as I was the day I electrocuted myself in the pmag lab. Guillaume, Douwe, Hemmo & Frits you are thanked for very many, and sometimes useful geological and non-geological discussions. Verder wil ik ook nog graag Paul Meijer, Reinhoud Vissers, Sjors Postma en Quintijn Clevis bedanken omdat jullie mij in het verloop der jaren heel wat geologie hebben geprobeerd bij te brengen. Hans de Bresser wil ik in het bijzonder bedanken voor de GPS. João, Maurits, Dario, Mathieu, Frank, Wolfram, en Gert-Jan voor de grappen en grollen in Tremp en het altijd open hebben staan van de deur van Noord drie nogwat. Isabel, Menno, Mike, Fauzie, Dida, Suyoko, Christine and all the other members of the expedition, thank you for the adventures in Indonesia. Mathias Harzhauser, Davor Pavelić, Kamil Ustaszewski, Stefan Schmid, Senecio Schefer, Patrick Grunert, and Miloš Bartol, thank you for the discussions! Wieske, bedankt voor te leuke dagen in Transylvanie en in Wenen. En die foto’s van de floradita gaan voorgoed in de doofpot! I would also like to thank the reading committee, who took the effort to read this bookwork. People who have conttributed to a specific part of this thesis will, by the way, be acknowledged at the end of that particular chapter. And then, last but not least, my gratitude to all the people of Transylvania, Bosnia and Herzegovina and Croatia who have shown me the meaning of hospitality.

Dan wil ik natuurlijk ook al mijn vrienden bedanken voor de gezonde dosis afleiding die ze me verzorgden. Mijn vrienden Mark, Joop en Joost, die zich geregeld afvroegen waar ik nou in hemelsnaam mee bezig was en of ik nog een keertje echt ging werken. Het dissidente vertikale uitschot wat mij geregeld uit de sleur trok. Arnoud, Maarten, Hans, Niek, Bas, Thomas, Tjerk, Christiaan en Wodi. Gelukkig houden jullie net zoveel van buitensport als ik. Sil, voor al de tijd die ik met jou klussend, skieend, rennend, fietsend en klimmend doorbracht. Christiaan, zonder jou had ik mooi van het ziekenhuis naar huis kunnen hinken. Bedankt voor de toptijd. Toch wel frappant, dat ik met jou meer tijd op de Karpaten door heb gebracht dan met eenieder van mijn veldwerkpartners. Aart, bedankt voor al het sarcasme en de wiskey als het nodig was. Veertje en Nik, voor de gezelligheid. Maxi, Daniel, Wade, Joseph, Birka, Zoki, Tamara, Sara, Nienke, Aida, Marianne, Maarten, Otto, Nerminka, Timo and Erin for the good times. Borja, bedankt voor alles. Schneesportschule Wildkogel, danke für die gut Zeit. Henk en Willemien, bedankt dat ik me bij jullie thuis zo welkom voel. Beide oma’s voor het stukje levenslust wat jullie uitstralen. Rens, je bent en blijft mijn beste kameraad. Bedankt dat je er bent, je betekend meer voor me dan ik ooit op zou kunnen schrijven. Pap en mam, bedankt voor het doorzettingsvermogen. Zonder jullie was ik er nooit gekomen! En Janne, bedankt voor de enorme dosis energie die jij me iedere dag weer geeft. Jou lach maakt mijn dag goed. Fijn dat ik in het heetst van de strijd de stretcher in het fort mocht verruilen voor een net ietwat confortabeler onderkomen.

En verder wil ik natuurlijk iedereen die ik vergeten ben bedanken dat ze het me niet te erg kwalijk nemen.

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Preface and acknowledgements

16


IntroductionDuring the Cenozoic, convergence between Europe and Africa led to the collision of multiple micro-continents along the southern margin of Europe, and thereby triggered the uplift of a variety of orogens including the Alps, Carpathians, Dinarides, Hellenides and Pontides (e.g. Csontos 1992; Schmid et al. 2008; Ustaszewski et al. 2008). The complex geodynamic evolution of the area induced a similarly intricate paleogeographic history (e.g. Rögl 1998, 1999; Popov, 2004; Harzhauser et al. 2002, 2007a; Magyar, 1999). Over time, the former Tethys Ocean fragmented, and the Mediterranean and Paratethys Seas came into existence (e.g. Rögl, 1999). Persistent orogenesis induced the progressive isolation and further fragmentation of these basins. Their evolution is intimately linked with the history of the adjacent orogens.

This thesis addresses the evolution of the Central Paratethys Sea and the neighboring Dinaride Mountains. The Central Paratethys was a vast epicontinental sea that covered the larger part of central and southeastern Europe during the Oligo-Miocene. At the advent of the Middle Miocene the Central Paratethys was connected with the Mediterranean as well as the Eastern Paratethys. Its preserved Middle and Upper Miocene deposits tell a tale of recurrent isolation from the neighboring seas. These had a severe impact on the water chemistry (e.g. Matyas et al., 1996; Latal 2004, 2006; Harzhauser et al. 2007b; Vasiliev et al., 2010; Peryt, 2006) and fauna (e.g. Steininger and Wessely, 2000; Magyar et al., 1999; Harzhauser et al. 2002, 2003; Kováč et al. 2007) of the Central Paratethys and caused a number of pervasive regional extinction events (Harzhauser and Piller, 2007).

The Dinarides are situated along the southwestern border of the former Central Paratethys and separated it from the Mediterranean. An extensive and long-lived lake system thrived in the intra-montane basins of the orogen during the Late Oligocene and Miocene (Mandic and Harzhauser, 2007; Harzhauser and Mandic, 2008; Mandic et al. 2007). Its deposits provide an excellent record of the concurrent geodynamic and paleogeographic history of the mountain range, and offer an extraordinary opportunity to study evolution in an ecosystem subject to long term-isolation. The suite of large and small land-mammal fossils, preserved within the lacustrine deposits, testifies of the contemporaneous developments in the terrestrial realm.

Problem statement and objectives

The Dinaride Lakes and the Central Paratethys share a key problem. The intrinsic isolation of the Dinaride Lakes and the progressive isolation of the Central Paratethys led to severe faunal endemism, which hampers straightforward biostratigraphic correlation of their deposits to radio-isotopically or cyclostratigraphically dated sedimentary records elsewhere. High resolution age constraints on the sedimentary infill of the corresponding basins are thus generally lacking. This obstructs global or even regional correlation of events apparent in the geological record and thus limits our insight in the geodynamic as well as paleogeographic history of southeastern Europe. The lack of reliable age constraints is, in our opinion, one of the most fundamental geological problems to be solved in this area.

The main objective of this thesis is therefore to provide new age constraints for the Dinaride Lake System and the Central Paratethys trough 40Ar/39Ar dating, cyclostratigraphy, and magnetostratigraphic correlation of key sedimentary records. The acquired results will moreover be place in a comprehensive geological framework and their implications for the paleogeographic and geodynamic evolution of South-Eastern Europe will be explored. 40Ar/39Ar -, cyclostratigraphic-, and magnetostratigraphic dating techniques are independent of the faunal record and thus particularly valuable tools if the fossil record is scarce or endemism prevents straightforward biostratigraphic correlation.

40Ar/39Ar dating

When volcanoes erupt, they often eject large amounts of volcanic material. This material returns to the surface some distance away from the volcano and might accumulate as layers of tuff or tuffite at

17

Introduction

the bottom of a marine or lacustrine basin. The minerals that form during eruption often contain significant amounts of potassium (K), which is a main constituent mineral of the Earth’s crust. Potassium has three naturally occurring isotopes, of which 40K is radioactive and decays to 40Ar over time. The 40Ar gas that forms within the mineral grains of a tuff since these crystallized

is generally trapped within the mineral structure and preserved. The rate at which 40K isotopes decay to 40Ar, i.e. the decay constant, is known. The age of a rock sample can therefore be calculated when the quantity of parent (40K) and daughter (40Ar) isotope it contains are measured. In conventional K-Ar dating, the amounts of solid 40K and gaseous 40Ar in a sample are determined with different measurement techniques. In order to overcome many of the inherent analytical problems, the 40Ar/39Ar dating technique was developed. In order to enable 40Ar/39Ar dating, a small mineral separate from a tuff sample is first irradiated in a nuclear reactor. Here 39K is transformed into 39Ar by neutron capture. The amount of 39Ar produced is directly proportional to the amount of 40K in the sample since the 40K/39K in terrestrial materials is assumed to be constant. Since both 40Ar and 39Ar are gaseous, their ratio can be measured on a mass spectrometer during a single experiment. 40Ar/39Ar ages for tuff levels intercalated in marine and lacustrine deposits provide a direct estimate of the age of the respective stratigraphic horizon and represent invaluable tie-points for cyclostratigraphic and magnetostratigraphic correlations. More information on the basic principles of 40Ar/39Ar research can be found in Kuiper (2003).

Magnetostratigraphy

The Earth’s field has frequently reversed its polarity over the course of geologic history. A polarity reversal typically takes several thousands of years to occur, and can be considered globally synchronous on geological time scales. When fine-grained sedimentary rocks accumulate and lithify, they record the contemporary local magnetic field direction. This primary remanent magnetization is later often partially overprinted. Paleomagnetic measurements can, nevertheless, reveal the primary remanent magnetic field direction preserved. In order to retrieve the characteristic remanent magnetization, rock samples are demagnetized by stepwise heating in a field-free oven, or by subjection to alternating magnetic fields of stepwise increasing strength. The primary component reveals itself as a sequence of vector endpoints that approach the origin in a straight line. Intensive study of the sedimentary record as well as marine magnetic anomalies has pointed out that reversals occur at irregular moments in time. Each time-period in the geological history therefore has its own characteristic pattern of reversals. If a characteristic pattern of reversals is recognized in a sedimentary succession, the corresponding strata may be correlated with other records over large distances or across biogeographical barriers. This makes magnetostratigraphic correlation a powerful tool for chronostratigraphic research. The basic principles of magnetostratigraphic research are explained in detail in Langereis et al (2010).

An impression of the paleogeography of the Africa-Eurasia collision zone in the Miocene (after Rögl, 1999)

Paratethys

Africa

Eurasia

Mediterranean

Dinarides

Central Eastern

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Cyclostratigraphy

Cyclic variations in the Earth’s orbital parameters affect the distribution and amount of incident solar energy and cause periodic changes in global climate. These are often recorded as cyclic lithological or faunal changes in a sedimentary succession. If recognized, these cycles can be matched with known variations in insolation as computed from astronomical solutions for the behaviour of the solar system (e.g. Laskar et al., 2004). Cyclostratigraphy can, in this way, provide a very high resolution time-frame. The general principles of cyclostratigraphy are explained in detail in Strasser et al (2007).

SynopsisIn the first part of this thesis we turn to the Dinaride Lake System. Our objective is to acquire better age constraints on the lacustrine sediments of the intra-montane basins of the Dinarides and to explore their geodynamic and paleogeographic significance. In chapter one, we construct a detailed time-frame for the lacustrine sediments of the Sinj Basin in Croatia. Our paleomagnetic results for the Lučane section provide a magnetostratigraphy. 40Ar/39Ar ages for three volcanic ashes present in the Lučane section provide tie-points for correlation of the acquired magnetostratigraphic pattern to the geological timescale. According to this correlation, the lacustrine sediments of the Sinj Basin accumulated between 18.0 and 15.0 Ma. In chapter two, we investigate the lacustrine deposits of the neighboring Livno-Tomislavgrad Basin. Combined magnetostratigraphic and 40Ar/39Ar results indicate that Lake Livno thrived between 17.0 Ma and approximately 13 Ma. The life-time of Lake Livno and Lake Sinj thus largely coincided. The disappearance of Lake Livno is most likely attributable to a change in tectonic regime. Calcarenites and breccias, derived from the basin margins, first enter the lake around 14.8 Ma and subsequently coarsen and thicken upwards. The basin margins were apparently gradually uplifted before subsidence stalled. In chapter three, we explore the evolution of Lake Gacko in southwestern Bosnia and Herzegovina and identify cyclic lake-level changes in its stratigraphic record. These are tentatively tuned to the astronomical target curves guided by our integrated magnetostratigraphic and 40Ar/39Ar results. According to the astronomical tuning, the investigated sediments of Lake Gacko accumulated between 15.8 and 15.2 Ma. The cyclic nature of its sedimentary record reveals that the lake was particularly sensitive to climatic changes induced by the 100-kyr and 400-kyr eccentricity cycle. The newly derived ages for Lake Sinj, Lake Livno and Lake Gacko demonstrate that these lakes existed during the Middle Miocene Climatic Optimum. It is consequently likely that the optimum climatic conditions stimulated the formation of these long-lived lakes in the interior of the Dinarides. Our new chronostratigraphy for these Dinaride basins pinpoints the age of a number of mollusk occurrences important for the regional biostratigraphic scheme. These endemic mollusks can now be used as age indicators for fossil sediments that are inappropriate for either magnetostratigraphic or 40Ar/39Ar dating. In chapter four, we combine magnetostratigraphic and small mammal results to establish a late Oligocene age for the Banovići basin in NE Bosnia and Herzegovina. The determined 24–23 Ma lifetime of Lake Banovići coincided with optimum climatic conditions and an elevated global temperature, in analogy with the investigated Miocene lakes. Our small mammal results indicate that a limited exchange of fauna from western Asia to central Europe existed in Oligo-Miocene times. In chapter five we explore the evolution of the northern margin of the Dinarides on the boundary between the realm of the Dinaride Lakes and the Central Paratethys. Back-arc extension triggered deposition of up to 500 m of continental fluvio-lacustrine deposits with a high biogeographic affinity to the Dinaride Lakes in the Southern Pannonian Basin. Our 40Ar/39Ar results pinpoint the onset of extension at 18 Ma concurrent with the initiation of lacustrine deposition in the intra-montane basins. 40Ar/39Ar ages for two ashes in the upper part of the continental series demonstrate that fluvio-lacustrine sedimentation continued until at least 16.0 Ma. The fluvio-lacustrine series is overlain by marine deposits of the Central Paratethys that flooded the Southern Pannonian Basin at 14.8 Ma (Ćorić et al. 2009). Our 16.0 Ma age for the uppermost part of the continental series implies a significant hiatus or interruption of sedimentation. An alternative explanation might also be that marine flooding

19

Synopsis

was not simultaneous in the lacustrine basins along the northern margin of the Dinarides. In chapter six we finally synthesize all acquired chronostratigraphic results from the Dinarides. We supplement these with AMS and paleomagnetic rotation results and present a review of paleomagnetic data for the Dinarides and Adria. We use the acquired paleomagnetic and geochronologic results in order to constrain the geodynamic evolution of the Central Dinarides since the Mesozoic but with particular emphasis on the last 20 Ma. We conclude that a first phase of intra-montane basin formation occurred in the late Oligocene when strike-slip faulting related to the extrusion of the Alcapa block penetrated into the orogen. A second phase of basin formation took place between 18 and 13 Ma, concurrent with profound extension in the neighboring Pannonian Basin. The acquired chronostratigraphic results suggest that a particularly beneficial combination of tectonic and climatic circumstances permitted the accumulation of thick piles of lacustrine sediments in the Dinarides in the Late Oligocene and Middle Miocene. Our paleomagnetic results moreover indicate that the Dinarides did not experience any significant tectonic rotation since the late Oligocene. This implies that the Dinarides were decoupled from the adjacent Adria- and Tisza-Dacia Mega-Units that both underwent major rotation during the Miocene. The Dinaride orogen must consequently have accommodated significant shortening, which is corroborated by our AMS results. A review of paleomagnetic data from the Adria plate constrains its rotation since the Early Cretaceous to 48 ± 10° counterclockwise (CCW) and indicates 20° of this CCW rotation took place since the Miocene. The amount of rotation within the Adria-Dinarides collision zone decreases with age and proximity of the investigated rocks to the center of the orogen. These results significantly improve our insight in the post-orogenic evolution of the Dinarides and resolve the existing apparent controversy between structural geological and paleomagnetic rotation estimates for the Dinarides as well as Adria.

The second part of this thesis addresses the Middle Miocene evolution of the Central Paratethys that is characterized by several isolation events for which we aim to provide better age control. The first of these Middle Miocene restriction events triggered the Badenian Salinity Crisis (BSC) which resulted in evaporite deposition in large parts of the Central Paratethys and induced the extinction of a great number of species. In chapter seven we present a new chronology for this catastrophic event based on 40Ar/39Ar dating of volcanic tuffs below and within the Badenian salts in southern Poland. The onset of evaporite deposition is dated at 13.81 ± 0.08 Ma. The entire event is estimated to have lasted 200 to 600 kyr. Correlation to oxygen isotope records shows that the Badenian Salinity Crisis evaporites are just preceded by glacial event Mi-3b, and consequently suggest a causal relationship. The sea level fall that occurred in conjunction with the Mi-3b event most likely restricted the open marine connection between the Paratethys and Mediterranean, thereby trapping the salt in the deep Paratethys basins. In chapter eight we construct a chronostratigraphy for the ca. 3 km thick Middle Miocene infill of the Transylvanian Basin. 40Ar/39Ar ages for several tuffs that occur in the basin infill provide solid anchors to the geological timescale. Due to the dense network of seismic lines, all investigated sections can be traced into a synthetic seismic section in the basin center. Based on the acquired absolute ages sedimentation rates of around 1.5 m/kyr are calculated for the late Badenian to Sarmatian part of the basin infill. In the Pannonian, rates of deposition decrease to 0.36 m/kyr in good correspondence with structural geological observations. A second step in the progressive isolation of the Central Paratethys can be dated with higher precision than before on the basis of the established chronostratigraphy. The transition from the regional Badenian to the Sarmatian stage is characterized by a large change in the faunal assemblages of the Paratethys, indicative of a decreased water exchange with the surrounding seas. Our results from the Transylvanian Basin indicate that this transition occurred 12.80±0.05 Ma ago. The acquired chronostratigraphically constrained paleomagnetic results moreover provide first hand insight into the tectonic rotation of the Transylvanian Basin during and since the Middle Miocene and indicate that the Transylvanian Basin rotated approximately 20° CW (clockwise) from the late Sarmatian to early Pannonian and another 6° CW since then. These rotations, which are driven by the eastward roll-back of the European slab during its subduction below the Eastern Carpathians, are corroborated by a number of structural geological observations. Their magnitude and timing provide new constraints on the emplacement of the Tisza-Dacia plate into the Carpathian embayment.

A contemporary lake in the Dinarides

Chapter 1Chronology and integrated stratigraphy of the Miocene Sinj Basin (Dinaride Lake System, Croatia)

Arjan de Leeuw, Oleg Mandic, Alan Vranjković, Davor Pavelić, Mathias Harzhauser, Wout Krijgsman, and Klaudia Kuiper

In the Miocene, the intra-montane basins of the Dinaric Mountain Chain harbored a series of long-lived lakes constituting the so-called Dinaride Lake System. The thick lacustrine sedimentary records of these lakes provide an excellent opportunity to study evolution and radiation of mollusks in an isolated environment. The 500 meter thick infill that accumulated in the Sinj basin is one of the key records because of its excellent mollusk preservation. Recent studies on the depositional history, pollen assemblages and large mammals have enhanced the understanding not only of Lake Sinj, but also of the regional climatic developments and faunal migratory patterns.

A reliable chronology of the development of Lake Sinj, which is crucial for global correlation of its endemic realm, was still lacking. In this paper we present a detailed time-frame for the Miocene Sinj basin based on palaeomagnetic and 40Ar/39Ar data. We conclude that deposition took place between 18.0 to 15.0 Ma, a time span that correlates with the upper Burdigalian and lower Langhian Mediterranean stages and Ottnangian, Karpatian and lowermost Badenian Paratethys stages. Furthermore, we determined the timing of several events in mollusk evolution, important for correlation across the Dinarides. An age of 15.0 Ma is attributed to the large mammals Conohyus and Gomphotherium, preserved in the upper part of the basin stratigraphy.

This chapter is based on: de Leeuw A., Mandic O., Vranjković A., Pavelić D., Harzhauser M., Krijgsman W., and Kuiper K.F. (2010) Chronology and integrated stratigraphy of the Miocene Sinj Basin (Dinaride Lake System, Croatia). Palaeogeography, Palaeoclimatology, Palaeoecology, 292: 155-167.

22


1.1 Introduction

During the Neogene, a series of lakes occupied many of the tectonic depressions in the Dinarides (Rasser et al. 2008). The strictly endemic nature of the aquatic fauna preserved in the lacustrine deposits of the Dinaride Lake System (Krstić et al. 2001, 2003; Harzhauser et al. 2008) has up to now prevented a reliable correlation with the global time scale (Pavelić, 2002; Jiménez-Moreno et al. 2008). This hampered regional palaeogeographic reconstructions and comparison to the Mediterranean as well as Paratethys realms. This is unfortunate since the Dinaric-Anatolian land occupied a crucial position between the Mediterranean and Paratethys and potentially acted as a land bridge for mammal migration between Africa and Europe.

The lacustrine sediments of the Sinj basin are particularly interesting because of their palaeontology. The thick lacustrine series contains a remarkably well preserved mollusk fauna (e.g. Brusina, 1874, 1897, 1902), providing an excellent opportunity to study evolution in an isolated system. Moreover, lignite deposits intercalated with the lacustrine series have provided an interesting vertebrate fauna with remains of the ancient pig Conohyus as well as the elephant ancestor Gomphotherium at the top of the Lučane section (Olujić, 1999, Bernoir et al. 2004) and the rhinocerotid Brachypotherium at presumably slightly higher stratigraphic position in the coal layers of Ruduša NW of Sinj (Takšić, 1968). Although the thickness of the sedimentary succession suggests Lake Sinj was long-lived, i.e. persisted more than 100 kyr following classification by Gorthner (1994), the strictly endemic character of its fauna inhibits straightforward biostratigraphic estimation of the age and longevity and therefore calls for an independent age estimate.

In this paper we present the results of an integrated 40Ar/39Ar and magnetostratigraphic study of the calcareous lacustrine sediments of the Sinj Basin (Fig. 1.1). For this purpose, we selected the Lučane section in the western part of the basin (Fig. 1.2, 1.3). The Sinj Basin thereby becomes the first lacustrine basin in the Dinarides of which the age is reliably constrained. The chronostratigraphic results were constructed in a joint research effort and will be directly integrated with the simultaneously acquired palaeontological results.

1.2 Geological Setting, paleogeography and basin stratigraphy

The geological setting of the Sinj Basin and the upper part of the Lučane section in particular were described in detail by Mandic et al. (2009) and Jiménez-Moreno et al. (2008). Here, a summary is given, with special emphasis on facts important for the chronostratigraphy.

The Sinj basin is situated in southeastern Croatia, in the western marginal part of the Dinaric Thrust Belt. The main phases of thrusting in this area occurred in the Middle and Late Eocene to possibly earliest Oligocene (Tari, 1994) and involved mainly Mesozoic to early Cenozoic limestones from the Adriatic-Dinaric Carbonate Platform. These thrust sheets eventually formed the basement for the Sinj Basin, which started to develop in the Early Miocene. The Sinj basin is situated along the major Split-Karlovac Fault that crosscuts the Dinarides (Schmid et al. 2008; Tari 2002). Basin evolution is controlled by strike-slip movement on the fault and the main decollement level is formed by Permo-Triassic pelitic and evaporite rocks.

The basin was mapped in detail by Kerner (1905b, 1916a, b), Marinčić et al. (1969, 1977), Šušnjara & Šćavničar (1974), Raić et al. (1984), Papeš et al. (1982) and Šušnjara and Sakač (1988) and has a rhomboidal shape. The infill can be subdivided into 3 units: a lower unit with freshwater limestone and considerable terrigenous input, authigenic carbonate in the middle unit and carbonate with coal intercalations in the upper unit (Šušnjara and Sakač 1988) (Fig. 1.2). The main, 4 m thick, coal seam was formerly mined and it was from this layer that the large mammal fauna was collected (Olujić, 1999). Carbonates developed throughout the whole basin. Breccia intercalations in the upper part of the succession, located near the basin margins, point out the original relief (Mandic et al., 2009).

From the results of Mandic et al. (2009) and Jiménez-Moreno et al. (2008) it is clear that Lake Sinj was a carbonate hard water lake in a karst environment. Authigenic carbonate production prevailed and

23

Chapter 1: Chronology and integrated stratigraphy of the Miocene Sinj Basin (Dinaride Lake System, Croatia)

due to a fl at morphology most of the lake remained in the photic zone. Periodically, perennial increases in water level occurred and at times the lake suffered acidifi cation. Pollen records show that humid and colder climatic phases coincided with the deposition of limestone while arid and warmer phases provided coals.

Palaeobiogeographically, Lake Sinj belonged to the Dinaride Lake System (DLS). This long lasting and complex system generated a locally very thick infi ll in the intra-montane basins of the Dinarides (Krstić et al. 2001, 2003; Mandic et al. 2009; Pavelić 2002; Rasser et al. 2008). In the Sinj Basin, sediment thickness comprises ~500 meters (Fig. 1.4), while in the adjacent Livno basin the thickness reaches up to 2 km. Although its thick sedimentary succession and great species richness hint at longevity, accurate age constraints are lacking.

A rough age indication of the Sinj succession can be derived from its large mammal fauna represented by Gomphoterium angustidens, Conochyus olujici and Brachypotherium brachypus. These indicate a latest Early Miocene to earliest Late Miocene age (Mandic et al. 2009, Palfy et al. 2007). Conohyus olujici has been described as a new species on the basis of eight mandibular specimens, by Bernor et

AlpsDinarid

Carpathians

es

Hr BiH

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Dinaride Lake System, Lower MioceneExtension after Krstic 2003Extent of Dinaride Lake SystemA�ected by the marinetransgression from the Paratethys

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political boundary

Croatia

Bosnia andHerzegovina

Adriatic SeaSARAJEVO BASIN

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Fig. 1.2

SINJ BASIN

Figure 1.1. Overview map showing the main Neogene basins within the Dinarids. The location of the Sinj basin is highlighted within the grey frame. Modifi ed after Pavelić (2002).

24


al. (2004). Their cladistic analysis suggests it is an early member of the clade. Such evolutionary stage inference for C. olujici led Bernor et al. (2004) to estimate the age of the topmost coal layer between 17 Ma and 16.1 Ma. The lower part of the Lake Sinj deposits contain abundant Doderleinia sinjana (Kerner 1905a) (cf. Jiménez-Moreno et al., 2008). While this plant is rare elsewhere, it is abundant in an Early Miocene horizon in NE Austria and SW Czech Republic, which corresponds to the Late Ottnangian (app. 17.5 Ma after Rögl et al. 2004).

1.3 The Lučane section: lithostratigraphy, biostratigraphy and earlier works

The studied Lučane section is located in the westernmost part of the Sinj Basin. Although Mandic et al. (2009) and Jiménez-Moreno et al. (2008) described only the uppermost coal bearing unit, all three (Fig. 1.2) lithological units defined by Šušnjara and Šćavničar (1974) are reflected in this section and were sampled for magnetostratigraphy (Fig. 1.4). Several volcanic tuffs intercalate with the lacustrine limestone. The most prominent tuff is located at the section base i.e. at the base of the lowermost of the three units. The most fossiliferous part of the section is found in its upper unit.

Mollusk preservation as well as species richness provides the Sinj Basin with a high potential for the study of isolated evolution (Harzhauser and Mandic, 2008, 2010). The amount of species described is one of the highest in the Dinaride Lakes and might be partially influenced by the exceptional faunal preservation in some of its classic localities (Brusina, 1884). The taxonomic content of the Lučane mollusk fauna was referred to by Brusina (1884, 1897), Kerner (1905b), Kittl (1895), Olujić (1936, 1999), and Mandic et al. (2009). Compared to other mollusk localities in Sinj, the Gulf of Lučane shows rather moderate fossil preservation. Yet, as pointed out by Kerner (1905b), due to good outcrop conditions, showing the most complete and well exposed lacustrine succession in the Sinj basin, it is certainly the best place for a relational study on the biotic and palaeoenvironmental evolution in this palaeolake. Several distinct evolutionary lines from the upper limestone and coal bearing unit were already described by Olujić (1936, 1999). This stratigraphic interval also records the late stage optimum of Mytilopsis evolution in the DLS. As postulated by Kochansky-Devidé & Slišković (1978) this implies the biostratigraphic correlation of the topmost Lake Sinj infill with the youngest sediments of the DLS elsewhere.

While the fossil content and lithology of the uppermost unit in the Lučane section was described in detail by Mandic et al. (2009), the lower- and middle unit deserve a short description here. The Lučane section is a composite, built up from three partially overlapping segments (Fig. 1.4), not to be confused with the three units defined in figure 1.2. While the upper- and middle unit (Fig. 1.2) are very well exposed along the Lučane river valley, the lower unit crops out in a meadow and several parts remain unexposed. This could be related to a considerable amount of terrigenous input in this unit, the marly facies being more prone to erosion. At the base of this unit, a prominent, several meters thick, layer of volcanic origin crops out. Above it, between 8 and 100 meters, dominantly coarser grained limestones surface. The almost complete lack of fossils hints at post-depositional leaching, which is supported by the presence of operculi of Bythinia around 28 m and Mytilopsis hercegovinensis coquina around 65 m. Several thin, mostly benthonized tuff layers occur in this interval. Furthermore, intercalations of breccia-conglomerate could indicate sporadic fluvial and/or debris flow influence. The lime mudstone dominating the interval consists of very fine-grained micrites. The deposition of the fine-grained components of lacustrine carbonates is dominantly induced by photosynthetic activity of macrophytes and microphytes and is characteristic for deeper and protected lake parts.

From approximately 100 m upward, exposure and fossil preservation are better. The sediments consist mainly of limestone and Mytilopsis jadrovi, M. hercegovinensis, Melanopsis bicoronatus, Fossarulus tricarinatus, Orygoceras dentaliforme and Doderleinia fruits are common in its lower part and Mytilopsis drvarensis in its upper part. These limestones are usually light-colored, thin to medium bedded, and commonly soft and porous. They primarily consist of calcite and its CaCO3 content reaches up to 99%. The dominating facies types are: detrital limestone generated from redeposited clastic material of older lacustrine and/or palustrine deposits exposed on mud flats, charophycean mudstone or

25


wackstone with rooted submerged macrophytes indicating vegetated littoral environments, stromatolite bindstone consisting of alternating thin micritic laminae, or alternatively micrite to microsparite laminae both containing micrite-calcified microbial filaments, and finally mollusk wackstone with coquinas. The latter ones suggest a calm, very shallow fresh-water environment prone to occasional storm events. The interval correlates with the units 2 to 7 of Kerner (1905b).

alluvial / soil

Figure 1.2. Geological map and generalized stratigraphic column of the Sinj Basin (modified after Jiménez-Moreno et al. 2008). Subdivision and lithostratigraphic units according to Šušnjara and Sakač (1988).

Figure 1.3. Detailed geological map of the Lučane area, located in the western part of the Sinj basin. (modified after Kerner, 1905b) The traces of the individual segments of the studied section are indicated.

26


Jiménez-Moreno et al. (2008) made a detailed analysis of the pollen spectra along the topmost 100 m of the Lučane section. They distinguished four plant ecosystems in the pollen data. Those are a swamp and riparian environment, a broad leaved evergreen forest (sea level – 700 m in altitude), an evergreen and deciduous mixed forest (>700 m) and finally a mid-altitude deciduous and coniferous mixed forest (>1000 m). Surprising is the high abundance of Engelhardia and the low presence of Taxodium-type and Quercus-type in comparison with other studies from Croatia, Central and Western Europe. These results also show that periods characterized by thermophilous and xeric plants alternate with periods characterized by a high abundance of conifers. These changes in the section’s pollen spectra generally coincide with changes in the sedimentary regime. Increases in thermophilous and xeric pollen, likely indicating a warming-induced upslope displacement of broad-leaved evergreen forests, are generally associated with the frequent deposition of coal in the basin. This denotes periods of low lake levels and peat forming paludal swampy conditions. Peaks in pollen originating in the higher altitude conifer forest and simultaneous decreases in thermophilous pollen generally coincide with the deposition of deep littoral and organic-poor limestones. These periods denote a higher lake level and downslope displacement of the conifer vegetation belt in response to a colder climate.

1.4 Methods

1.4.1 Isotopic Dating

Eleven volcanic ashes are intercalated with the limestone- and coal beds of the Lučane section. All of them were sampled for 40Ar/39Ar dating. The bulk samples were crushed, disintegrated in a dilute calgon solution, washed and sieved over a set of sieves between 63 and 500 µm. Only 3 of the 11 tephra samples contained potentially suitable minerals. Of each of the three suitable samples the largest appropriate mineral fraction was subjected to standard heavy liquid and magnetic separation techniques for sanidine as well as biotite. Samples Lučane 1, 2 and 3 held sanidine crystals in fractions 355-500 µm, 120-200 µm and 250-500 µm respectively. In addition, samples Lučane 1 and 3 contained biotite crystals in fractions larger than 355 and 400 µm respectively. All samples were subsequently handpicked. Sanidine separates were leached with a 1:5 HF solution in an ultrasonic bath during 5 min. Biotite separates were cleaned in an ultrasonic bath using pure demineralized water.

The mineral separates from Lučane 1 were loaded in a 6mm ID quartz vial together with Fish Canyon Tuff (FC-2) and Drachenfels (Dra-1, f250-500) sanidine. Separates from Lučane 2 and 3 were put in a 9mm ID quartz vial together with the same standards. Additional Drachenfels (Dra-2, f>500) sanidine was added to the latter vial as a third standard. Both vials were irradiated in the Oregon State University TRIGA reactor in the cadmium shielded CLICIT facility for 10 hours.

Upon return to the VU Argon Laboratory, both standards and Lučane samples were pre-heated under vacuum using a heating stage and heat lamp to remove undesirable atmospheric argon. Thereafter, samples were placed in an Ultra High Vacuum sample chamber, degassed overnight and were either fused or incrementally heated using a Synrad CO2 laser in combination with a Raylase scanhead as a beam delivery and beam diffuser system. After purification the resulting gas was analyzed with a Mass Analyzer Products LTD 215-50 noble gas mass spectrometer. Beam intensities were measured in a peak-jumping mode in 0.5 mass intervals over the mass range 40–35.5 on a Balzers 217 secondary electron multiplier. System blanks were measured every three to four steps. Mass discrimination was monitored by frequent analysis of aliquots of air. The irradiation parameter J for each unknown was determined by interpolation using a second-order polynomial fitting between the individually measured standards. All 40Ar/39Ar ages have been calculated with the ArArCalc software (Koppers 2002) and applying the decay constants of Steiger and Jäger (1977). The age for the Fish Canyon Tuff sanidine flux monitor used in age calculations is 28.201 ± 0.03 My (Kuiper et al. 2008). The age for the Drachenfels sanidine flux monitor is 25.42 ± 0.03 My. Correction factors for neutron interference reactions are 2.64±0.017×10−4 for (36Ar/37Ar)Ca, 6.73±0.037×10−4 for (39Ar/37Ar)Ca, 1.211 ± 0.003 × 10− 2 for (38Ar/39Ar)K and 8.6 ± 0.7×10−4 for (40Ar/39Ar)K. Errors are quoted at 1σ level and include the analytical error and error in J.

27


1.4.2 Palaeomagnetism

Over 300 palaeomagnetic cores were taken along the 500 meter thick Lučane Section in 3 successive sampling campaigns. The gross resolution thus achieved was approximately one sample every 2 meters. Only in the lower part of the section resolution remained lower due to bad exposure and non-suitable lithologies. Samples were collected with a hand-held electric as well as gasoline-powered drill. The orientation of all samples was measured by means of a magnetic compass. Bedding planes were similarly determined at regular intervals.

In the laboratory, the obtained cores were cut into several specimens that were subsequently stepwise demagnetized. Samples from the earlier sampling campaign were all demagnetized thermally. Cores from

segment 1

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limestone coarse grained limestone coal seam

clayey coal

tephra

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Volcanic Ash with Ar/Ar age (My)

Mammal site with Conohyus remains

A

B

C

D

EF

I

J

G

H

mammal site

Figure 1.4. Lithological column, fossil distribution and magnetic results for the Lučane section. The polarity pattern derived is correlated to the ATNTS using the 40Ar/39Ar ages as primary tie points. ATNTS according to Lourens et al., (2004) with adjustments by Hüsing (2008).

28


the later campaigns were subjected to robotized alternating field (AF) demagnetization. Results were in good correspondence with those of thermal demagnetization. The natural remanent magnetization (NRM) of the samples was measured after each demagnetization step on a 2G Enterprises DC Squid cryogenic magnetometer (noise level 3·10-12Am2). Heating occurred in a laboratory-built, magnetically shielded furnace employing 10-30 °C temperature increments up to 285 °C. AF demagnetization was accomplished by a laboratory-built automated measuring device applying 5-20mT increments up to 100mT by means of a degausser interfaced with the magnetometer. The characteristic remanent magnetisation (ChRM) was identified through assessment of decay-curves and vector end-point diagrams (Zijderveld, 1967). ChRM directions were calculated by principal component analysis (Kirschvink, 1980).

1.5 Results


The results of the 40Ar/39Ar total fusion experiments of the Sinj Lučane samples 1, 2 and 3 are given in figure 1.5 and table 1.1. For Lučane 1 and 2, 10 multiple sanidine grain fusion experiments have been analyzed. We do not observe a homogeneous age population. However, based on the corresponding probability density distribution, we assume that those experiments comprising the youngest cluster contain no or only a negligible detrital component and represent a maximum crystallization age for the ash. We selected these experiments for calculation of the weighted mean age and error. For sample Lučane 1 the percentage radiogenic 40Ar* is on average 93%, which is relatively low for sanidine. The average radiogenic 40Ar* for sample Lučane 2 is 98%. The isochron ages are concordant with the weighted mean age and the trapped 40Ar/36Ar component is atmospheric. This provides and age of 15.43±0.05 Ma for Lučane 1 and 16.23±0.16 Ma for Lučane 2. The peak probability density age calculated based on all 10 experiments coincides exactly with the calculated weighted mean ages for both samples. Errors only slightly increase to respectively ±0.054 Ma and ±0.164 Ma when uncertainties for respectively the age of astronomically calibrated standard and decay constants, as reported in Kuiper et al. (2008) and Steiger and Jäger (1977) respectively, are included. This is a conservative error estimate including all sources of uncertainty. The largest part of the total uncertainty for these samples arises from the analytical uncertainty in J-values where we use a conservative approach. All relevant analytical data and an overview of the applied J-value calculations as well as error determination can be found in the online supplementary material. The age distributions as depicted in figure 1.5 are slightly heterogeneous due to the presence of a small fraction of reworked crystals in the samples.

For Lučane 3, initial analyses failed as indicated by the variable ages spanning a range between 17.5 and 40 Ma. Subsequent more thorough grain selection resulted in 5 additional analyses out of which three show K/Ca (30.5) and 40Ar* (99.3%) values attributable to sanidine and provide realistic ages. Based on the corresponding probability density distribution, two of the three experiments that cluster best were selected for calculation of the weighted mean age and related parameters. On the basis of these few data points a weighted mean age of 17.63±0.18 Ma for the Lučane 3 sanidine can be calculated. Additional age information is provided by 10 additional experiments with multiple grain fusions of selected biotite crystals from the same sample. For these crystals, the average radiogenic 40Ar* amounts to 66% which is relatively low for biotite and might indicate slight weathering to chlorite. The weighted mean age of all 10 biotite fusion analysis yields 17.67±0.20 Ma with a relatively high MWSD value of 6.7. But since these results hint at minor alteration of the biotite crystals it was decided to perform two incremental heating experiments on the same separate.

The results of the incremental heating experiments are presented as age spectra diagrams (Fig. 1.5). An age is accepted as an accurate estimate of the crystallization age when the following criteria are fulfilled. There should be a well-defined, high temperature plateau for more than 50% of the 39Ar released formed by three or more concordant, contiguous steps. A well-defined isochron should be obtained from the results of the gas fractions on the plateau, while also the 40Ar/36Ar intercept for the trapped argon derived from the isochron should not be significantly different from the atmospheric

29


ratio of 295.5 (McDougall and Harrison, 1999). Indeed, the age spectra diagrams for the first as well as the second incremental heating experiment show minor weathering along grain boundaries. After several heating increments, however, a well defined plateau appears. For both experiments it comprises around 80% of the cumulative 39Ar release and consists of 8 and 9 contiguous concordant steps. The respective weighted mean ages are 17.90±0.18 Ma for the first and 17.93±0.18 Ma for the second replication. The isochron ages correspond to the weighted mean ages (Table 1.1) and the trapped 40Ar/36Ar component is atmospheric. The total crystallization ages calculated using all incremental heating experiments indeed correspond to those found in the total fusion experiments. Since the incremental heating age spectra diagrams are well defined and the use of the weighted mean plateau ages circumvents bias due to minor alterations, we take 17.92±0.18 to represent the crystallization age of the Lučane 3 biotite crystals.

1.5.2 Palaeomagnetism

Thermal demagnetization diagrams indicate that the total NRM of the samples is generally composed of two components. Blocking spectra slightly overlap, as can be observed in figure 1.6 a-g. The low temperature component is mostly removed at 220 degrees and is interpreted to be an overprint. The NRM intensities after heating up to 220 °C typically range between 10-2 and 1.5 mAm-1, as is common for limestones with a low detrital component. ChRM directions are generally established above this temperature and the component under consideration mostly decays straight to the origin. At 285 ºC on average around 75% of the NRM is removed.

Samples subjected to AF demagnetization can be divided into two groups on the basis of the intensity of their NRM and corresponding ChRM. For the first group, in which samples generally have a higher intensity NRM, ChRM directions can be reliably established and have a starting intensity over 100 µA/m (Fig. 1.6 h-j). For this group, the total NRM consists of two components. The first component, mostly removed at 20 mT, is interpreted to be an overprint. ChRM directions are established above 20 mT. For this group demagnetization diagrams display characteristics very similar to the thermal results and ChRM intensity also typically ranges between 10-2 and 1.5 mAm-1. The second group of samples has a generally lower intensity NRM and there is a large scatter observable in the demagnetization diagrams (Fig. 1.6 l). If the NRM decayed below 100 µA/m after application of a 25 mT field, reliable ChRM directions could usually not be established.

No gyroremanence is observed even when applying alternating fields as large as 100mT. At this field strength the remaining NRM of the majority of samples is negligible. The absence of gyroremanence and almost complete removal of the NRM at 80-100 mT suggests the most likely magnetic carrier is

0.000

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0.030

0.040

0.050

0.060

0.070

0.080

0.090

Age (Ma)

Prob

abili

ty

Luca

ne 1

n=7

Luca

ne 2

n=6

Luca

ne 3

, sa

nidi

nen=

3Lu

cane

3, b

iotit

e

n=10

15.43 16.2417.68

17.63

15

16

17

18

19 a)

Cumulative 39Ar Released (%) A

ge (M

a)

17.93 ± 0.18 Ma

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

0 10 20 30 40 50 60 70 80 90 100

LUCANE 3 biotite VU78-11 IH 1

b)

17.90 ± 0.18 Ma

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

0 10 20 30 40 50 60 70 80 90 100

c)

LUCANE 3 biotite VU78-11 IH 2

Cumulative 39Ar Released (%)

Age

(Ma)

Figure 1.5. Overview of the 40Ar/39Ar results. a) Probability density distribution for the multiple single fusion experiments on Lučane 1, 2 and 3 sanidine and Lučane 3 biotite. b) First incremental heating spectrum for Lučane 3 biotite. The chosen age plateau and corresponding weighted mean age are indicated. c) Second incremental heating spectrum for Lučane 3 biotite with chosen plateau and corresponding weighted mean age

30


magnetite. The directions isolated by means of principle component analysis as described above as well as

the intensity of the ChRM are plotted against stratigraphic level in figure 1.4 in order to establish a magnetostratigraphy for the Lučane Section. NRM intensity does not correlate with lithology on a macroscopic scale. Potential correlation on a bed to bed scale was not investigated.

1.6 Discussion

1.6.1 Age model

The palaeomagnetic data gathered along the Lučane section allow establishing a reliable magnetostratigraphy for the infill of the Sinj Basin. Correlation of this magnetostratigraphy to the Astronomically Tuned Neogene Timescale (ATNTS) (Lourens et al., 2004), using the absolute ages of the three dated ash layers as tie points, is straightforward.

The age of the very top of the Sinj section is constrained by correlation of the uppermost normal interval to chron C5Bn.2n. with an astronomical age of 15.160 Ma (Lourens et al. 2004). While a single site suggests a transition from normal to reversed at the very top of the section, the large amount of organic material in that part obscures the palaeomagnetic signal. However, given the relatively thick upper normal magnetozone we can presume that the top of the section may well include, or be very close to, the C5Bn.2n to C5Bn.2r reversal boundary. In this way, we arrive at an age of 15.0 My for the top of the Lučane section.

The age of the base of the section is constrained by the 40Ar/39Ar age for the Lučane 3 tuff layer. Bad outcrop exposure resulted in a poor magnetostratigraphy for this part of the section and the age of 17.92±0.18 Ma remains the only tie point. Accordingly, we arrive at an age of approximately 18 Ma for the base of the Lučane section.

The Lučane section was thus deposited from 18.0 to 15.0 Ma. It correlates to the upper Burdigalian and lower Langhian Mediterranean stages and Ottnangian, Karpatian and lowermost Badenian Paratethys stages. This period, except for its initial part, coincides largely with the Middle Miocene Climatic Optimum. Formation of Lake Sinj and accumulation of its thick limestone series might thus have been stimulated by the favourable climatic conditions.

It should be kept in mind that the ages of the reversal boundaries between C5Dr1r and C5Bn2n, to which the major part of the section is correlated, were determined on the basis of spreading rates in the ATNTS (Lourens, 2004). The ages of the reversal boundaries can easily turn out to be up to 40 ka off, once an astronomical tuning for this part of the Miocene will be established.

The misfit in the reversal pattern between polarity interval D and H most likely arises due to difficulties retrieving the original magnetic signal. It might, on the other hand, indicate the presence of a hiatus. Then the major part of chron C5Cn.2n could be lacking in the Lučane record. Direct field evidence for the presence of such hiatus was, however, not found. When compared to the astronomical time scale of Billups et al (2004) the missfit within C5Cn is much smaller. It also shows a better fit with the magnetostratigraphic record of chron C5Cn in the Ebro Basin (Larrasoaña et al., 2006; Pérez-Rivarés et al., 2004 ) as well as deep sea IODP records (Lanci et al., 2004), where magnetozones corresponding to C5Cn.2r are always thicker than one would expect from its estimated duration in Lourens et al. (2004).

In the largest part of the section, the constructed magnetostratigraphy provides a good control on the sedimentation rate (Fig. 1.7). The lower half of the section has a sedimentation rate of approximately 10 cm/kyr while the upper half was deposited at the double rate of 20 cm/kyr. The sedimentation rates at the very top and bottom of the section do not deviate excessively from rates in the rest of the section. Incipient tectonism related to the start as well as the end of sedimentation in the Sinj basin might however have caused the sedimentation rate to be temporarily higher.

1.6.2 Lithostratigraphy, fossil flora and climate change

Lithostratigraphic subdivision of the western part of the Sinj Basin by Kerner (1905b) is also based

31


sn49.1 th / tc

N

up/W

S32.1a th / tc

N

up/W

sn70.1 af / tc

Z

up/W

115.2 m

472.67 m

170.05 m

S26.1a th / tc

N

up/W 449.67 m

SN_08_70.1 af / tc

N

up/W SN_08_43.1 af / tc

N

up/W

A) B) C)

D) E) F)

G) H) I)

J) K) L)

279.58 m 267.94 m

interval C

interval I

interval J

20

175 200

295

20

175200

260

20

175260

0

25

50

10

0

2550

10

sn93.1af / tc

N

up/Wsn87.2af / tc

N

up/W

interval A interval B

interval C interval H interval H

sn35.1th / tc

N

up/W

interval D

sn28.1th / tc

N

up/W

interval E

sn25.1th / tc

N

up/W

interval F

sn03.1th / tc

N

up/W

interval G

26.6 m 62.6 m

192.1 m 218.4 m

226.5 m 233.5 m

20

175200285

20

175295

20

200285

100

175200

285

0

50

20

100

10

15

40100

Figure 1.6. Thermal (th) and alternating field(af) tectonically corrected (tc) and tectonically non-corrected (notc) demagnetization diagrams for samples from the Lučane section. For an elaborate description we refer to the results section.

32


on changes in plant assemblages (Fig. 1.3). Equipped with the new age model we are now able to calibrate the lithostratigraphic units and biostratigraphic markers to the geological time scale. This allows correlations of the Lake Sinj plant assemblages with related occurrences outside the DLS.

The lower part of the studied succession (zone 2 according to Kerner, 1905b) is characterized by the common occurrence of conspicuous Doderleinia fruits (a tentative tape-grass representative, Jiménez-Moreno et al. 2008). This interval is now dated at ~17 Ma and thus correlates to the Late Burdigalian (Fig. 1.8). This suggests that this Lake Sinj occurrence is slightly younger than the corresponding Late Ottnangian horizon in the Alpine-Carpathian foredeep (Bůžek, 1982), which is approximately 17.5 Ma old according to Piller et al. (2007). These two Doderleinia horizons are thus apparently non-coeval, contrary to the hypothesis of Meller & Bergen (2003).

In the middle part of the investigated succession the tape-grass plant assemblage is suddenly replaced by a fossil star-fruit (Damasonium) assemblage (Fig. 1.8). Star-fruit remains are especially common in the upper part of that interval, corresponding to Zone 7 of the Kerner (1905b) lithostratigraphic division (Fig. 1.3) and now dated at ~15.5 Ma (Fig. 1.8). Based on recent star-fruit distribution and behavior, their mass occurrence in Lake Sinj has been tentatively related to intermittent acidification events bound to lake level rises inducing increased reworking of shore face material (Mandic et al., 2009).

The topmost part of the section, corresponding to Zone 8 of Kerner (1905b), is marked by a distinct palaeoenvironmental shift triggering enhanced production and preservation of organic matter as indicated by numerous lignite intercalations. This interval is now shown to represent the period between ~15.4 and ~15 Ma. The corresponding pollen spectrum (Jimenez-Moreno et al. 2008) is dominated by warm taxa with Engelhardia (walnut related tree plant), indicating a subtropical humid climate. The deposition of the lignites and its associated warm floral assemblage were probably induced by the advance of the Miocene Climatic Optimum (figure 1.8).

1.6.3 Age of the large mammal sites Lučane and Ruduša

The first evidence of long-term emersion of Lake Sinj is the 2 m thick coal seam in the uppermost part of the section. Its petrology (Mandic et al., 2009) suggests that the coal seam formed in a mire environment, which is corroborated by the recovery of several large land mammal fossils. Amongst these are the elephant-like proboscidean Gomphoterium angustidens (Cuvier) and the extinct pig Conohyus olujici (Olujić, 1999; Bernor et al., 2004). Remains of the extinct rhinoceros Brachypotherium brachypus (Latret) (Olujić, 1999) were excavated at a slightly higher stratigraphic level within the same coal bearing lithostratigraphic unit, exposed in the abandoned coal mine of Ruduša (Fig. 1.2). Both large mammal sites, previously tentatively estimated to be of late Early Miocene age (Bernor et al. 2004), are now shown to be ~15 Ma old. This age is in agreement with the age ranges of the represented vertebrate species that were, except for Conohyus olujici, widely distributed throughout Europe and Asia Minor (Rössner & Heissig, 1999).

Cladistic analysis of Conohyus olujici by Bernor et al. (2004) showed it to be a very early member of the Conohyus clade, occurring prior to the clades striking radiation in Europe and South Asia. Its age was therefore estimated to range between 16-17 Ma. Van der Made and Moralis (2003) provided a review on Conohyus occurrences in Europe. According to their work, the earliest Conohyus remains (C. simorrensis) were found in at the localities Puente de Vallecas, Somosaguas and Montejo de la Vega in Spain, Hommes and Channay in France, Göriach, Au, St. Oswald and Rosenthal in Austria, Mala Miliva in Serbia, and Bâlâ in Turkey. They placed all these sites in zone E of the Iberian regional mammal zonation, which correlates to the topmost of MN5 of the European Mammal Zonation. Because, the base of zone E correlates with the base of the chron C5ACr (Krijgsman et al., 1994), the maximum numerical age for these C. simorrensis occurrences is 14.1 Ma. Our age model confirms the hypothesis of Bernor et al. (2004) that the ~15 Ma old extant pig of Lučane is the oldest Conohyus representative in Europe, even though the age for the Lučane site is more than 1 Myr younger than previously estimated,

The new age for Conohyus olujici does not support the inference by Bernor et al. (2004) that it is more primitive, i.e. older, than the earliest South Asian Conohyus (C. sindiensis). This could, however,

33


be an artifact of conflicting stratigraphic data with regards to the first occurrence (FO) of Conohyus in South Asia. In the magnetostratigraphically dated Chita Parwela section (Johnson, 1985) of the Siwaliks (Pothohar / Potwar Plateau, Pakistan) the FO of Conohyuis recorded at 14.5 Ma (Barry in Bernor et al., 2004). In the Sehwan Section (Sind, Pakistan), on the other hand, Conohyus sindiensis was recoverd from a horizon that correlates biochronologically to a much older level (16.1 Ma) of the Chita Parwela section (Bruijn and Hussain, 1984, Bernor at al., 1988). That correlation might on the other hand be ambiguous because Khan et al. (1984) assign the base of the Sehwan section to chron C5Bn.1r on the basis of their palaeomagnetic data. The whole succession would then be younger than 15 Ma (Huesing et al., 2008). Flynn et al. (1995), in turn, report the FO of Conohyus in the Siwaliks at 16.3 Ma, which is distinctly older than the occurrence date at Lučane. We conclude that a thorough revision of the stratigraphic data of Pakistan is needed to solve these discrepancies.

1.6.4 Patterns in the mollusk fauna

A thorough literature investigation (Harzhauser & Mandic, 2008, 2010 and own data) shows that approximately 200 species and subspecies have been described from the Dinaride Lake System. About 110 are also reported to be present in the deposits of Lake Sinj. Lake Sinj and Lake Drniš (with its famous locality Miočić - Neumayr, 1869) together represent the classical malacological fossil site complex called the Dalmatian freshwater Neogene (e.g. Brusina, 1897). Despite its long tradition and apparent importance in palaeontological investigation no reliable age information was obtained so far. Numerous correlations resulted in multiple age inferences ranging from Early Miocene (Bernor et al., 2004) through Middle and Late Miocene (Kochansky-Devidé & Slišković, 1978; Olujić 1999) to Pliocene (Ivanović et al., 1977). Our study shows that the Lake Sinj sediments accumulated between 18 and 15 Ma.

The taxonomic richness of Lake Sinj and the DLS has no other Miocene counterpart in Europe, except for Lake Pannon (Harzhauser & Mandic, 2008). The DLS and Lake Pannon both show higher diversities than the modern ancient Lake Baikal, Lake Tanganyika or Lake Ohrid. For Lake Pannon this has been explained by its size and longevity that facilitated accumulation of species through iterative

Early Miocene Middle Miocene

C5Dr2r C5Dr1r

?

Lucane 1

Lucane 2

Lucane 3

0

50

100

150

200

250

300

350

400

450

500 15.0 15.5 16.0 16.5 17.0 17.5 18.0

Age (My)

Stra

tigra

phic

Lev

el

A

B

C

D

EF

I

J

G

H

C5Dn C5Cr C5Cn3n C5Cn2n C5Cn1n C5Br

Figure 1.7. Sedimentation rate along the Lučane section as derived from the constructed time-frame. In the lower half of the section the average rate is around 10 cm/ky while in upper half rates increase to about 20 cm/ky.

34


radiations and extinction events (Harzhauser & Mandic, 2010). It is now clear that Lake Sinj, with its extraordinary taxonomic richness and conspicuous endemic faunal character with up to 98% species elsewhere unknown (Harzhauser & Mandic, 2008) (Fig. 1.8) was a perennial aquatic environment that persisted for 3 Myr. Therefore, it can be inferred once more, that longevity enhanced the accumulation of taxa and resulted in striking patterns of autochthonous evolution.

Insight in the mechanism by which and especially the rate at which species diversity can increase arises from the uppermost part of the studied section, which corresponds to the time interval between 15.25 and 15 Ma. Here Olujić (1936, 1999) documents radiation events in two aquatic gastropod genera Melanopsis and Prososthenia (Fig. 1.8). These striking morphological disparity events are now shown to take place in ~100 kyr. Over this short course of time, two Melanopsis and six Prososthenia clades produced 28 transitional morphospecies. At the end of the radiation pulse, of both genera only one single species remained present in the lake. Which physical factor forced this course of evolutionary change is still a matter of investigation. The collection of additional data and geochemical proxies in particular is necessary to resolve this issue.

The original uncertainty about the age of the DLS did not allow an accurate interpretation of its palaeobiogeographic relation to Lake Pannon. Our study shows that deposition in Lake Sinj had ended 3 Myr before deposition in Lake Pannon commenced. Whereas some endemic genera such as Orygoceras managed to survive in the region until the rise of the Lake Pannon, the lack of similarity at species level already suggested a certain disconnection between these two domains (Harzhauser & Mandic, 2008).

One important aspect of the present study was the calibration of the evolutionary trends in mollusk fauna to the global timescale for the purpose of regional biostratigraphic correlation. Figure 1.8 displays the occurrences and local acmes of mollusk species over the course of time as recorded in the studied section. Dreissenid bivalves (Mytilopsis) are especially important for regional correlation since they show striking evolutionary changes and are common throughout the Dinaride Lake System (Kochansky-Devidé and Slišković, 1978, 1980). In the Lučane section, the occurrence of one small and primitive Mytilopsis (M. hercegovinensis) is restricted to the lower and middle part, whereas the larger and more advanced M. drvarensis and M. aletici are traced troughout the middle and the upper part. According to our new results, the morphological transition between M. drvarensis and M. aletici, characterized by an increase in size and flattening of the shell (cf. Mandic et al., 2009), occurred between 15.25 and 15.7 Ma and took less than 450 kyr (Fig. 1.8).

When combined with the previously established age calibration of the occurrence of M. kucici and clivunellid gastropods in Lake Pag (Jiménez-Moreno et al., 2009; Bulić & Jurišić-Polšak, 2009) (Fig. 1.8) these results prove the hypothesis of Kochansky-Devidé & Slišković (1978) that the younger strata of the DLS are characterized by the presence of highly evolved endemics such as M. aletici, whereas the older parts bear clivunellid gastropods and significantly more primitive dreissenids. This allows us to correlate the stratigraphic boundary between the early and late DLS phase, which slightly precedes the first occurrence of M. drvarensis, with the base of chron C5Br.

In the north-eastern part of the DLS, lacustrine strata lack the highly evolved mollusk fauna with M. drvarensis and M. aletici so characteristic for Lake Sinj, and are superposed by marine Badenian Paratethys sediments. Kochansky-Devidé & Slišković (1978) therefore hypothesized that the region wide marine incursion occurred before M. drvarensis and M. aletici evolved. The lowermost marine strata have, however, lately been dated at 14.8 Ma by means of marine plankton stratigraphy (Ćorić et al, 2009). Lacustrine sedimentation in the Sinj basin thus preceded the flooding of the northern parts of the DLS by the Paratethys Sea and the hypothesis of Kochansky-Devidé & Slišković (1978) must be abandoned. A different explanation for the exclusive occurrence of the higher evolved mollusk assemblage in the southern DLS has thus to be found. Such evaluation, which must be based on the complete DLS record, is beyond the scope of the present study.

35


1.7 Conclusions

The combined palaeomagnetic and 40Ar/39Ar data presented in this paper allow correlation of the lacustrine succession deposited in Lake Sinj to the global timescale. Thereby, Lake Sinj becomes the first of the Miocene Dinaride Lakes to be accurately dated. It is now clear that its deposits accumulated from 18 to 15.0 Ma, which correlates to the upper Burdigalian and lower Langhian Mediterranean stages and Ottnangian, Karpatian and lowermost Badenian Paratethys stages.

The evolutionary model for dreissenid radiation across the Dinaride Lake System has been significantly improved since the timing of several acme occurrences in the studied section could be established. We confirm the relative stratigraphic positions previously derived from regional stratigraphic patterns and evolutionary relationships inferred from shell morphologies (Kochansky-Devidé and Slišković 1978, 1980). Moreover, an exact biochronology of acme ranges for three marker species is established: the primitive long-lived universalist Mytilopsis hercegovinensis from 17.4 to 15.6 Ma, the intermediate M. drvarensis from 15.9 to 15.7 Ma and finally the crown of Dinaride Lake autochthonous evolution, the highly progressive specialist M. aletici from 15.3 to 15.0 Ma.

According to our results, the Conohyus olujici mandibles preserved at the very top of the investigated Lučane section have an absolute age of 15.0 Ma. Although this is much younger than the age estimate of 16-17 Ma by Bernor et al. (2004) it still makes Conohyus olujici the oldest Conohyus in Europe.

It is evident that the integration of palaeomagnetic and 40Ar/39Ar research provides a powerful method to date sedimentary successions, like those of Lake Sinj, plagued by endemism. It furthermore presents a straightforward method to improve palaeogeographic reconstructions for the Dinaride Lake System as a whole.

Figure 1.8. New geochronology for the Lake Sinj deposits and fossil record correlated to an updated DLS stratigraphic model (geological time scale after Lourens et al., 2004; correlation of Central Paratethys stages after Piller et al., 2008; global climate record after Zachos et al., 2001).

36


Acknowledgements

We are grateful to the Austrian FWF Project P18519-B17: “Mollusk Evolution of the Neogene Dinaride Lake System”, and we acknowledge support by the Netherlands Research Centre for Integrated Solid Earth Sciences (ISES) and by the Netherlands Organization for Scientific Research (NWO). In addition, we would like to thank Roel van Elsas for his help with the mineral separation and Tom Mullender and Mark Dekkers for their help with the palaeomagnetic measurements. Finally, we would like to thank both Miguel Garcés and an anonymous reviewer for their constructive comments.

37


Tabl

e 1

.1.

Sum

mar

y of

the

40Ar

/39Ar

res

ults

. M

SWD

is M

ean

Squa

re W

eigh

ted

Dev

iate

s, N

is t

he t

otal

num

ber

of r

epet

ition

s in

the

sin

gle

fusi

on e

xper

imen

ts a

nd t

he

tota

l num

ber

of s

teps

in t

he in

crem

enta

l hea

ting

expe

rimen

ts.

In b

rack

ets

the

num

ber

of e

xper

imen

ts u

sed

to c

alcu

late

the

wei

ghte

d m

ean

(WM

) or

pea

k pr

obab

ility

de

nsity

(PP

D)

age.

39Ar

K is

the

per

cent

age

of 39

ArK

rele

ased

by

plat

eau

step

s. 40

Ar*

is t

he r

adio

geni

c am

ount

of

40Ar

. Tot

al fu

sion

and

isoc

hron

age

s an

d in

vers

e is

ochr

on

inte

rcep

ts a

re g

iven

for

refe

renc

e. E

rror

s ar

e gi

ven

at 9

5 pe

r ce

nt c

onfid

ence

leve

l. M

SWD

, 40Ar

* (%

), 39

Arκ

(%),

K/C

a, I

soch

ron

age

(Ma)

and

Inv

erse

isoc

hron

inte

rcep

t w

ere

dete

rmin

ed b

ased

on

the

expe

rimen

ts s

elec

ted

for

calc

ulat

ion

of t

he w

eigh

ted

mea

n ag

e..

Irrad

iatio

n ID

Sam

ple

Min

eral

Met

hod

J-va

lue

PPD

age

(M

a)W

M a

ge (M

a)M

SWD

N40

Ar*

(%)

39A

rκ (%

)K

/Ca

Tota

l fus

ion

age

(Ma)

Isoc

hron

ag

e (M

a)

Inve

rse

isoc

hron

in

terc

ept

VU

69-C

11Lu

cane

1sa

nidi

nefu

sion

0.00

2609

315

.43

15.4

3 ±

0.05

0.81

10(7

)93

.18

10.0

915

.52

± 0.

0515

.39

± 0.

0930

5.5

± 18

.7

VU

78-3

Luca

ne 2

sani

dine

fusi

on0.

0026

6216

.24

16.2

4 ±

0.05

0.05

10(6

)98

.00

25.6

516

.27

± 0.

0516

.24

± 0.

0728

7.6

± 43

.1

VU

78-1

Luca

ne 3

sani

dine

fusi

on0.

0026

617

.62

17.6

3 ±

0.06

0.23

5(2)

99.2

460

.25

17.9

2 ±

0.06

--

VU

78-1

1Lu

cane

3bi

otite

fusi

on0.

0026

682

17.6

317

.67

± 0.

116.

6910

(10)

65.9

818

.13

17.6

9 ±

0.7

17.0

5 ±

0.31

315.

9 ±

10.2

VU

78-1

1-IH

1Lu

cane

3bi

otite

IH0.

0026

682

17.9

3 ±

0.18

0.78

18(8

)82

.38

74.9

516

.468

17.6

9 ±

0.18

17.9

2 ±

0.18

297.

1 ±

2.7

VU

78-1

1-IH

2Lu

cane

3bi

otite

IH0.

0026

682

17.9

0 ±

0.18

3.04

18(9

)85

.02

87.2

66.

712

17.6

5 ±

0.19

17.8

7 ±

0.18

298.

2 ±

3.7

Fresh snow covered the Livno Valley over night during a field trip in April

Chapter 2A chronostratigraphy for the Dinaride Lake System deposits of the Livno-Tomislavgrad Basin: the rise and fall of a long-lived lacustrine environment

Arjan de Leeuw, Oleg Mandic, Wout Krijgsman, Klaudia Kuiper, and Hazim Hrvatović

The Dinarides are an integral part of the Alpine orogenic belt and stretch over large parts of Slovenia, Croatia, Bosnia-Herzegovina, Monte Negro, and Serbia. A great number of intra-montane basins formed in the interior of this Late Eocene to Early Oligocene orogen during the Miocene. These basins harbored a suite of long-lived lakes, collectively called the Dinaride Lake System (DLS). Lake Livno was with its 600 m2 of preserved surface the second largest of these Dinaride Lakes. At present its deposits are divided over the Livno and Tomislavgrad Basins that were part of a single basin when Lake Livno first formed. High resolution age constraints for the over 2 km basin infill have been lacking up to now, partly due to the endemic nature of its lacustrine fauna. This severely hampered geodynamic as well as paleoenvironmental reconstructions. Here, we present a chronostratigraphy based on radio-isotopic and magnetostratigraphic data. 40Ar/39Ar measurements reveal that the Tušnica volcanic ash, found in between the Gomphoterium bearing coal seams at the base of the basin infill, is 17.00±0.17 Ma old. 40Ar/39Ar dating of the Mandek ash, correlative to the uppermost sedimentary unit, provides an age of 14.68±0.16 Ma. Correlation of the composite magnetostratigraphy for the main lacustrine depositional phase to the Astronomically Tuned Neogene Time Scale is straightforward and reveals that the majority of the deposits of Lake Livno accumulated between 17 and approximately 13 Ma. The life-time of the paleo-lake thus coincided with the Miocene Climatic Optimum. The disappearance of Lake Livno is most likely attributable to a change in tectonic regime. Calcarenites and breccias, derived from the basin margins, first enter the lake around 14.8 Ma and subsequently coarsen and thicken upwards. The basin margins were apparently gradually uplifted before subsidence stalled. Comparison with chronostratigraphic data for other constituents of the DLS leads to the conclusion that their lifetimes largely coincide. Finally, we calibrate the most important marker fossils of the various Dinaride basins to the geological time scale and we present a new biochronological scheme for the DLS.

This chapter is based on: de Leeuw A., Mandic O., Krijgsman W., Kuiper K.F., and Hrvatović H., A chronostratigraphy for the Dinaride Lake System deposits of the Livno-Tomislavgrad Basin: the rise and fall of a long-lived lacustrine environment. Submitted to Stratigraphy.

40


2.1 Introduction

In the Miocene, the Dinaride-Anatolian land separated the epicontinental Paratethys Sea from the Mediterranean and connected the Middle East with Central Europe. A great number of intra-montane basins developed within the Dinaride Mountain chain, in which a multitude of long-lived lakes settled (Fig. 2.1). The longevity, thick sedimentary successions and large geographic extent of this so called Dinaride Lake System make it a key archive for the faunal and floral evolution of southeastern Europe (Krstić 2003; Mandic et al. 2008; Jimenez-Moreno et al. 2008, 2009). Good age control on these sediments is lacking, since strong endemism characterizes the lacustrine fauna. The regional biostratigraphic scheme is furthermore far from complete and hardly time-indicative due to a lack in high resolution age data. This hampers regional geodynamic as well as paleobiogeographic reconstructions and several key questions remain unanswered. It is for example not clear if all basins formed simultaneously or if they were generated in different phases. Their significance with regards to the geodynamic evolution of the orogen thus remains unclear. In addition, it remains obscure if the establishment and eventual disappearance of the lakes should be attributed to a tectonic or climatic cause.

To solve these questions, a large chronostratigraphic study with special emphasis on long continuous sections suitable for multi-disciplinary dating techniques was set up. This study resulted in high resolution age constraints on the Pag, Sinj, and Gacko basins of the DLS (Jimenez-Moreno et al. 2009; De Leeuw et al. 2010; Mandic et al. 2010, 2011). In this paper, we investigate the sedimentary succession of the Livno and Tomislavgrad basins in Bosnia and Herzegovina (Fig. 2.1). These basins, with their almost 2 km thick infill, harbored one of the main constituent lakes of the Dinaride Lake System. We use magnetostratigraphic and radio-isotopic dating techniques to construct a detailed (bio)-chronology for the evolution of Lake Livno.

2.2 Geological setting

The deposits of Lake Livno are at present distributed over four intra-montane basins situated in the High Karst area of the external Dinarides (Fig. 2.2). This study concentrates on the two main basins; one surrounding the town of Livno, and the other the town of Tomislavgrad (formerly Duvno and Županjac). Although these basins are currently disunited and lie at different altitudes (700–710 m and 860–890 m) they formerly formed a single basin. Their Miocene infill consists of up to 2.6 km of lacustrine sediments, which represent two consecutive phases of lake formation.

The deposits of the first lacustrine phase (Fig. 2.2) attain up to 2000 m and consist for the larger part of limestone. The mollusk assemblages of this phase indicate that Lake Livno was part of the widespread Dinaride Lake System. At the very base of the sedimentary succession there is an approximately 10 m thick coal interval. In the main coal seam, remains of Gomphoterium angustidens, an ancient elephant, were found and Malez and Slišković (1976) therefore considered it to be of Middle Miocene age. The same level was, in contrast, correlated to the late Early Miocene based on its pollen content (Pantić 1961). The coal interval is overlain by a thick package of limestones. Calcarenite layers, with carbonate grains, likely derived from basement rocks, intercalate in the middle part of the limestone succession. These layers coarsen and thicken upwards and eventually turn into carbonate clast breccias. These breccias also coarsen and thicken upwards. The uppermost part of the deposits of this first lake phase does not crop out, since it is covered by those of the second lake phase.

The deposits of the second lacustrine phase overlie those of the first phase discordantly and attain up to 640 m (Fig. 2.2). Based on its pollen (Pantić 1961; Pantić and Bešlagić 1964), mollusk (Jurišić-Polšak and Slišković 1988) and ostracod record (Milojević 1961; Milojević and Sunarić 1962) this second phase has been correlated with the Upper Miocene of Lake Pannon, which at that time covered large parts of adjacent Central and Eastern Europe. According to Milojević and Sunarić (1964), and Papeš (1977) the Livno and Tomislavgrad basins had already been tectonically disconnected when the younger lake settled.

41

Chapter 2: A chronostratigraphy for the Dinaride Lake System deposits of the Livno-Tomislavgrad Basin

2.3 Sections, lithology and depositional history

The deposits of the primary lake phase, on which we concentrate, are subdivided in four lithostratigraphical units, M1 to M4, on the Livno sheet of the geological map 1:100.000 of former Yugoslavia (Papeš 1972, 1975). Basal unit M1 refl ects a period of coal deposition and is exposed in the Drage Quarry of the Tušnica mine 10 km SE of Livno (Saracevic et al. 2009). This outcrop also exposes the lowermost part of unit M2, which consists of gray limestone with clivunellid gastropods. The remainder of M2 is badly exposed and available only in scattered outcrops, for example the Drage W. and Mokronoge sites. Unit M3 is composed of yellow limestones with dreissenid bivalves, and M4 of calcarenites and breccias intercalating with lacustrine limestones. Together, they comprise a ~1500 m thick succession, the majority of which is well exposed along the Ostrožac stream 3 km west of Tomislavgrad.

2.3.1 The Tušnica Section

The 45 m thick Tušnica section is located in the Drage opencast coalmine situated at the foot of the Tušnica Mountain near the eastern boundary of the Livno Basin (Fig. 2.2). It was logged and sampled

AlpsDinarid

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es

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political boundary

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Sarajevo

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SplitMiljevina

Ljubljana

TOMISLAVGRADBASIN

Figure 2.1. Map indicating the location of the Livno and Tomislavgrad Basins in the interior of the Dinarides. Lake Livno was part of the Dinaride Lake System that stretched out from the island of Pag in the west to the Sava and Drava depressions in the north and the Gacko basin in the south. Modifi ed after Pavelić (2002).

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along the SSE-NNW striking wall of the quarry. The base of the section is situated at N43°44’17.3’’ E17°05’35.0’’ (WGS84) and the top at N43°44’19.2’’ E17°05’32.9’’.

The lower 10.5 m of the section are dominated by coal subdivided in three seams. The lower two seams are separated by a ~1 m thick organic material rich sandy limestone interval with lymneid snails and plant remains. A volcanic ash bed separates the middle and the upper coal seam (Fig. 2.3a). The ash layer is ~20 cm thick, laterally continuous, grayish-whitish in color, and consists of sandy and silty clay with dark mica flakes. The transition from coal to ash and vise versa is very sharp. It was sampled for 40Ar/39Ar dating at N43°44’25.8’’ E017°05’28.2’’. A transitional zone with clayey and coaly inter-layers starts above the upper coal seam. It is followed by dominantly dark brown and grayish well bedded limestones rich in organic matter that dominate the remaining 30 m of the section (Fig. 2.3b). These beds belong to interval M2 (Milojević and Sunarić 1964) and contain scattered fish and plant remains.

2.3.2 The Ostrožac section

The 1700 m thick Ostrožac section (Fig. 2.3) is situated near the NW margin of the Tomislavgrad basin. It follows the Ostrožac brook, running down the eastern slope of the Tušnica Mountain, for about 2 km. The brook strikes N-S, subnormal to the bedding that is ~150°/30° in the stratigraphically lower part of the section and ~170°/55° in the upper part of the section. The base (N43°43’40.5’’ E17°10’59.6’’) is located about 150 m N of the place where the path to Eminovo Selo meets the road to the abandoned Vučipolje coal mine. The top (N43°43’40.5’’ E17°10’59.6’’) reaches the Jošanica village where a large megabreccia crops out just north of a large curve in the main road to Tomislavgrad.

Milojević and Sunarić (1964) describe coal seams and organic material rich limestone beds with the same character and thickness as those in the Tušnica section, from the Vučipolje coal mine at 1 km distance of the base of the Ostrožac section. They indicate that the overlying part of the M2 clivunellid limestone interval is about 300 m thick. The stratigraphic distance between the Tušnica and Ostrožac section is estimated in accordance.

The Ostrožac section starts with light colored lacustrine limestones intercalated by clay layers and volcanic ash beds (Unit 1: 8.2 m, Fig. 2.3). These already pertain to stratigraphic interval M3 (Fig. 2.2 and 2.3). The well bedded limestone, light beige, gray, yellowish or brownish in color, is dominantly fine-grained, although some coarser-grained intervals are present as well. The beds are ~10 cm thick. The clay intercalations are at maximum 20 cm thick and are predominantly found in the lower part of the unit. An interval of brownish limestone with roots of water plants follows (Unit 2: 49.6 m; Fig 2.3c). The limestone is somewhat softer than before. These are overlain by a thick interval of planar bedded limestones with a bed thickness of 5-20 cm (Unit 3: 215.9 m; Fig. 2.3d). In this interval, plant remains disappear completely. A single bentonite layer is located in its middle part. In the next unit (Unit 4, 211.2 m; Fig 2.3e, 2.3f), ripple bedding dominates although in some intervals plain beds return. The ripples are commonly about 1 cm thick and there’s about 10 cm of distance between consecutive crests. Some intercalations of plant remains occur in association with horizons of coarser grained limestone. The next unit (Unit 5, 111.2 m) starts with an about 5 cm thick calcarenite layer situated within laminated limestones with plant-stems preserved on its bedding plains. Ripple and trough cross beds characterize this unit. Slump and channel structures occur in combination with more calcarenite intercalations. These are red or brownish in color and consist of badly sorted, angular lithoclasts. Above follow intervals of well bedded limestone (Unit 6, 54.0 m; Fig. 2.3g) and ripple bedded limestone (Unit 7, 24.4 m) succeeded by a thinning upward succession of more well bedded limestone (Unit 8, 72.2 m). The first breccia recorded in the Ostrožac section occurs at the base of the following interval (Unit 9, 42.0 m), which is dominantly composed of alternating cross-bedded and massive limestones. The breccia is about 2 m thick and consists of angular, badly sorted carbonate lithoclasts. Two, up to 0.5 thick, volcanic ash layers occur in this unit as well. Both the tephra and breccia beds are laterally continuous and can be traced for at least several 100 m. The breccia thickens in westward direction. Thick bedded (Unit 10, 97.2 m; Fig. 2.3h) and thin bedded limestones (Unit 11, 73.9 m; Fig. 2.3i) with intercalated calcarenite layers and inversely graded breccia follow. Unit 12 displays a unique carbonate facies with frequent micro-breccia

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intercalations and ooid intervals (Unit 12, 48.3 m). The topmost interval (Unit 13, 394.9 m; Fig. 2.3j) is badly exposed and partly covered by an artificial lake. It is consists of marly limestones with recurrent breccia intercalations. The topmost breccia is a 26.3 m thick megabed with limestone blocks of up to 2 m in diameter. More marly limestones follow until we reach the discordant contact (Milojević and Sunarić 1964; Papeš 1975) with the deposits of the younger lacustrine cycle (Fig. 2.2). The measured thicknesses and the succession of lithologic units correspond very well with data presented by Milojević and Sunarić (1964).

2.3.4 The Mandek section

The Mandek section (Fig. 2.3l) is located 10 km south of Livno, just E of the Mandek town on the shore of artificial lake Mandek (Fig. 2.3k) and belongs to stratigraphic unit M4. The section was logged from North to South along the gulley of the Vojvodinac brook which runs normal to the bedding. The outcrop was

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Quaternary sediments, glacial, fluvial and glacio-lacustrine

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White limestoneLimestone with intercalating calcarenitesand upwards thickening conglomerates with limestone clasts, volcanic ash, thin benthonites

Yellow limestones with dreissenid bivalvesGray limestone with clivunellid gastropods

Conglomerates, dark organic matter rich limestone with Pisidium, coal, volcanic ash

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Sys

tem

)2n

d la

cust

rine

cycl

e

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Sampled sections

Drage W.

Mokronoge

Figure 2.2. Generalized stratigraphic column and geological map of the Livno, Tomislavgrad and Roško-Polje Basins with the location of the investigated sections and sites. The extent of the sampled sections is marked in the stratigraphic column by blue bars. The blue triangle indicates the approximate position of the Tušnica Gomphotherium site (Kochansky-Devidé and Slišković, 1972; Malez and Slišković, 1976). The geological map was compiled and adapted from the 1:100.000 geological maps of former Yugoslavia – sheets Livno, Sinj, Imotski and Glamoč (Papeš 1972; Papeš 1975; Ahac et al. 1976; Marinčić et al. 1976; Raić et al. 1976; Marinčić et al. 1977; Papeš and Ahac 1978; Raić and Papeš 1978; Papeš et al. 1982; Raić et al. 1984). Stratigraphic column was drawn according to Milojević and Sunarić (1964) and Papeš (1972, 1975).

44


already mentioned by Luburić (1963). It exposes a volcanic ash, weakly lithified and whitish in color. At its top, it grades into lacustrine carbonates, dominated by fine-bedded limestone. The exposed interval is about 6 m thick. A prominent 8 m thick breccia horizon crops out 14 m stratigraphically below the ash. The separating interval is completely covered by vegetation. Below the breccia, there’s an additional 15 m of lacustrine limestone in which a second 30 cm thick bentonitic tephra occurs. Both ash horizons are also indicated on the geological map of Papeš (1972). A sample of the main ash layer was collected in a small pit about 100 m E of the gulley at N43°43’49.9’’ E017°01’21.6’’.

2.3.5 The Drage West and Mokronoge sites

Although the clivunellid bearing interval M2 is in general badly exposed in the Livno as well as the Tomislavgrad basin, fragments crop out at the Drage West and Mokronoge sites (Fig. 2.2). Drage West (N43°44’29.0’’ E17°04’55.6’’) is located about 300 m W of the main entrance to the Tušnica coalmine. The narrow gulley displays light and dark brownish bedded limestones with Clivunella katzeri (Fig. 2.4A), small dreissenid bivalves and plant remains such as Taxodium-related Glyptostrobus europaeus and pertains to the upper part of interval M2. The Mokronoge site (N43°45’20.5’’ E17°13’58.2’’) is located near an Islamic graveyard just S of the Mokronoge town, alongside the road to Tomislavgrad (Kochansky-Devidé and Slišković 1978). It exposes well bedded light beige limestones with abundant dreissenid bivalves and plants remains. The bivalves include rather small specimens of Mytilopsis drvarensis, about 30 mm in diameter (Fig. 2.4B), and remarkably large specimens of M. kucici (Fig. 2.4C), about 50 mm in length.

2.3.6 Depositional history

The coals exposed in the Tušnica and Vučipolje mines testify of swamp conditions during the initial phase of basin formation. Coal formation is terminated when the basin floods and a long-lived lake establishes. As the lake deepened suboxic bottom conditions developed, as indicated by the organic-rich limestones of the Tušnica mine and overlying clivunellid limestones (Unit M2, Fig. 2.3). The strata exposed in the lower part of the Ostrožac section are indicative of predominantly shallow water conditions. The rippled structure of these limestone beds points out that they accumulated in the shallow littoral zone. The calcarenites and breccias that characterize interval M4 are badly sorted, which implies short transport. We interpret them as debris-flows indicative of local seismic activity in conjunction with uplift of the basin margins from which the material originates. The coarsening upwards sequence, which these calcarenites and breccias built, preludes the end of the first lacustrine phase. The angular discordance between the

Figure 2.3. Lithological column for the Dinaride Lake System sediments exposed along the Tušnica and Ostrožac sections of the Livno and Tomislavgrad Basins placed in a composite section for the main lacustrine cycle of the palaeo-lake (Fig. 2.2). Stratigraphic interval: correlation of the lithostratigraphic units from Fig. 2.2 to the studied section. Units M2 and M4 are subdivided according to lithostratigraphic scheme by Milojević and Sunarić (1964) differentiating lower M2 with dark, organic matter rich limestone with Pisidium, upper M2 with gray limestone and clivunellid gastropods, lower M4 with sandstone intercalations and upper M4 with upward thickening breccia intercalations. The M4 lower boundary was detected at the stratigraphic height indicated by Milojević and Sunarić (1964) that was distinctly higher than the position indicated on 1:100.000 geological map by Papeš (1972). Units: Lithostratigraphic units identified by detailed logging in the field. Units (A) - Ostrožac section: 1) Limestone / clay / marl alternation with tephra intercalations. 2) Brownish limestone with plant remains. 3) Brownish well bedded limestone. 4) Brownish ripple bedded limestone. 5) Ripple through bedded limestone with channel and slump structures and sandstone intercalations. 6) Well bedded limestone. 7) Ripple bedded limestone. 8) Bedded limestones with thinning upwards sequences. 9) Partly cross bedded limestone with marl interbedding and a 2 m thick breccia superposed by a 0.5 m thick ash layer. 10) Thick bedded limestone. 11) Thin bedded limestone with 1 m thick breccia. 12) Limestone intercalated by ooliths and red sandstone lenses and beds. 13) Marly limestone with breccia intercalations. Units (B) - Tušnica section: 1) Coal with clay and marl intercalations and a volcanic ash layer. 2) Well bedded organic rich limestone bearing Pisidium, Glyptostrobus and fish bone remains. For a description of the pictures we refer to the main text.

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46


first and second lacustrine unit provides additional evidence for tectonic activity at the end of the first phase. This interpretation is in line with that of Milojević and Sunarić (1964) and Papeš (1975), who suggested that uplift of the Tušnica Mountain began during deposition of Unit M4. They postulate that this divided the Livno-Tomislavgrad basin in two independent lakes during the second lacustrine phase.

2.4 Radio isotopic dating

2.4.1 Sampling and methods

Two major volcanic ash deposits are intercalated in the basin infill (Fig. 2.2). These are the tuff intercalated into the main coal of the Tušnica section and the thick tephra layer of the Mandek section. Both ashes were sampled and subsequently processed at the Department of Isotope Geochemistry (VU Amsterdam). The bulk samples were crushed, disintegrated in a calgon solution, washed and sieved over a set of sieves between 63 and 250 μm. The residue was subjected to standard heavy liquid as well as magnetic mineral separation techniques. The fraction of grains larger than 150 μm of both samples contained ample feldspar crystals. These were handpicked and leached with a 1:5 HF solution in an ultrasonic bath during 5 min. The mineral separates were then loaded in a 6mm ID quartz vial together with Fish Canyon Tuff (FC-2) and Drachenfels (Dra-1, f250-500) sanidine. The vial was irradiated at the Oregon State University TRIGA reactor in the cadmium shielded CLICIT facility for 10 hours.

All age calculations use the decay constants of Steiger and Jäger (1977). The age for the Fish Canyon Tuff sanidine flux monitor used in age calculations is 28.201 ± 0.03 My (Kuiper et al. 2008). The age for the Drachenfels sanidine flux monitor is 25.42 ± 0.03 My (Kuiper et al. in prep). Correction factors for neutron interference reactions are 2.64±0.017×10−4 for (36Ar/37Ar)Ca, 6.73±0.037×10−4 for (39Ar/37Ar)

Ca, 1.211 ± 0.003 × 10−2 for (38Ar/39Ar)K and 8.6 ± 0.7×10−4 for (40Ar/39Ar)K. Errors are quoted at the 1σ level and include the analytical error and the error in J. All relevant analytical data as well as error determination can be found in the online supplementary material.

2.4.2 40Ar/39Ar ages

10 multiple grain fusion experiments have been analyzed for the Mandek and Tušnica tuffs, in order to determine the 40Ar/39Ar age. These provided two almost homogenous age populations (Fig. 2.5). The full analytical data are displayed in the online supplementary material. For the Tušnica tuff, the average percentage of radiogenic 40Ar* is 89.5% and its K/Ca ratio is on average 3.3. The isochron ages are concordant with the weighted mean age and the trapped 40Ar/36Ar component is atmospheric. This provides an age of 17.00±0.17 Ma for the Tušnica tuff, in exact agreement with the age indicated by the probability density

Figure 2.4. Dinaride Lake System endemic lacustrine mollusks from the W Drage (A) and Mokronoge (B and C) sites. A. Clivunella katzeri. B. Mytilopsis drvarensis. C. Mytilopsis kucici.

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weighted mean age and uncertainty

single experiment with analytical uncertainty

probability density distribution

0.002

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0.000

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13 14 15 16 17 18 Age (Ma)

Pro

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Figure 2.5. Probability density diagrams for the multiple total fusion experiments for the Mandek and Tušnica tuffs.

distribution calculated for the 10 duplicate experiments (Fig. 2.5). This age is thus assumed to reflect the tuffs crystallization age. For the Mandek tuff, the average percentage of radiogenic 40Ar* is 59.5% and its K/Ca ratio is on average 1.51. Also for this tuff, the isochron ages are concordant with the weighted mean age and the trapped 40Ar/36Ar component is atmospheric. This provides an age of 14.68±0.16 Ma for the Mandek tuff. This age is in good agreement with the age indicated by the probability density distribution (Fig. 2.5) and reflects its crystallization age. For both ash samples the MSWD of the calculated weighted mean ages is smaller than the T-student distribution at the 95% confidence level (online supplementary material).

Beside the Mandek and Tušnica tuffs, several volcanic ash layers in the Ostrožac section were sampled and subjected to mineral separation techniques. However, all except one were benthonized and too fine grained to provide suitable grains for 40Ar/39Ar dating. Results for sample VU1, taken from the only coarser grained ash, situated at the base of the Ostrožac section, were disturbed by a large reworked component. Thus its crystallization age could not be determined.

2.5 Magnetostratigraphy

2.5.1 Sampling and methods

Hand samples were gathered from the Tušnica and Ostrožac sections in order to construct a magnetostratigraphy. The orientation of all samples and bedding planes was measured by means of a magnetic compass, and corrected for the local magnetic declination. In the laboratory, standard paleomagnetic cores were drilled and subsequently divided into several specimens. Thermal as well as alternating field demagnetization techniques were applied in order to isolate the characteristic remanent magnetization (ChRM). The natural remanent magnetization (NRM) of the samples was measured after each demagnetization step on a 2G Enterprises DC Squid cryogenic magnetometer (noise level 3·10-12Am2). Heating occurred in a laboratory-built, magnetically shielded furnace employing 10-30 °C temperature increments up to 280-350 °C. AF demagnetization was accomplished by a laboratory-built automated measuring device applying 5-20mT increments up to 100mT by means of an AF coil interfaced with the magnetometer. The ChRM was identified through assessment of decay-curves and vector end-point diagrams (Zijderveld 1967). ChRM directions were calculated by principal component analysis (Kirschvink 1980) and are always based on at least four consecutive temperature or field steps.

2.5.2 Demagnetization results

The samples of the clayey limestones of the Tušnica section were subjected to stepwise AF demagnetization. Their NRM generally consists of two components (Fig. 2.6). A normal overprint, especially observable in non-tilt corrected diagrams, is removed in fields up to 25 mT. Subsequently, between 25 and 60 mT

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approximately 95% of the NRM disappears. This second component decays straight to the origin and we interpret it as the ChRM. ChRM intensities range between 1 and 12*10-4 A/m. We do not observe any gyroremanence. All ChRM directions have a MAD<15. Since all reliable samples show reversed directions, we interpret the whole Tušnica section to be of reversed polarity (Fig. 2.7).

Thermal demagnetization diagrams (Fig. 2.6) of the limestones of the Ostrožac section indicate that their NRM is generally composed of two components. A low temperature component, mostly removed below 220°C, displays only normal polarities and is interpreted to be a recent overprint. The high temperature component is demagnetized above 220°C and displays normal and reversed directions. At 400 °C typically 80-95% of the NRM had been removed. Taking into account the very low intensity signal, thermal demagnetization diagrams are of surprisingly good quality and mean angular deviation remained below 20° for 90% of the derived directions. For only five samples, Zijderveld diagrams were regarded as non-interpretable due to high scatter in the vector endpoints. For the other 54 samples directions and polarities were established.

Two and sometimes three components appear during AF demagnetization. A low field component, demagnetized between 0 and 15 mT, displays only normal directions and is assumed to be a present day field overprint. The second component, demagnetized between 20 and 40 mT, displays normal as well as reversed directions. At 40 mT mostly over 90% of the NRM had been removed. For a minority of the samples an even higher field component appears. It is typically demagnetized between 40 and 80 mT. The established AF directions were subdivided based on their respective field interval and MAD. In figure 2.7 we thus distinguish directions of the high-field (40-80mT) component, and directions of the intermediate component (20-40mT). The latter are subdivided into three groups with MAD<10, 10<MAD<20, and 20<MAD<42 respectively. All directions were established without forcing the resultant vector through the origin, except when vector endpoints between 40 and 80 mT clustered. Directions of the intermediate and high field component are generally very similar and are interpreted to reflect the geomagnetic field at the time of deposition.

Figure 2.7 displays the declination and inclination of the established thermal as well as AF directions versus stratigraphic level. Directions resulting from thermal demagnetization were regarded as most reliable for construction of the magnetostratigraphy and thus taken into account first. Subsequently, the directions resulting from AF demagnetization were considered. The 40-80mT component was regarded most reliable followed by the 20-40mT components ordered from low to high MAD. The resulting polarity pattern has been subdivided into parts A-L, which will each be discussed in detail.

In interval A, thermal as well as AF demagnetization diagrams indicate a normal polarity. Zijderveld diagrams of interval B are of high quality and thermal and AF directions clearly indicate a reversed polarity. Interval C is covered with alluvium and therefore no polarity can be established. Interval D is again characterized by a reversed polarity, as indicated by the thermally derived directions. Alternating field demagnetization diagrams are of a very low quality for this interval and often provide ambiguous directions. On the boundary of interval D and E, we observe down-working of the normal polarity magnetic field of interval E into the sediment column. We thus interpret Interval E to start where only normal components are observed. A reversed polarity characterizes interval F. Striking is, that heating of samples from this interval above 200°C results in a sharp increase in intensity. The generated components have random directions, and thermal demagnetization diagrams are severely disturbed. Apparently, new magnetic minerals are being created. Thermal demagnetization diagrams of interval G, demonstrate normal polarities. AF results of G are generally of very low quality and can not be reliably interpreted. Interval H carries a reversed polarity as demonstrated by thermal demagnetization diagrams. Some signs of alterations appear above 200°C. AF results are of low quality but generally corroborate the reversed polarity. Both AF and thermal results are of high quality in intervals I, J, and L, and they are clearly of normal polarity. In intervals J, and M, intensities often sharply increase when samples are heated above 250-330°C. This temperature interval indicates that the rise in intensity might be attributable to the alteration of iron sulfides. The directions of the ChRM component have been determined forcing a vector from the 220-280°C temperature steps through the origin. Interval K is relatively short and bears

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Livn

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42.20 m

N

up/W

VU07.1a th / tc

N

up/W

20

175

400

NRM int = 250 * 10-6 A/m

0 20 40 60 80 100 120 0

0.2

0.4

0.6

0.8

1

field (mT)

TU15 af / no tc

N

up/W af / tc

N

up/W

33.46 m 0

15

25

50

80

0 NRM int = 1782 * 10-6 A/m

0 100 200 300 400 5000

0.2

0.4

0.6

0.8

1

T(º)

th / no tc

21.70 m

N

up/W VU05.1a th / tc

N

up/W

150

400

NRM int = 354 * 10-6 A/m 20

150

400

220

0 20 40 60 80 100 1200

0.2

0.4

0.6

0.8

1

field (mT) 42.20 m

VU07.1aaf / tc

N

up/Waf / no tc

up/W

N

NRM int = 435 * 10-6 A/m

0

25

10

32 50

100

field (mT)

field (mT)

T(º)

T(º) field (mT)

NRM int = 465 * 10-6 A/m

NRM int = 43 * 10-6 A/m

NRM int = 106 * 10-6 A/m

NRM int = 123 * 10-6 A/m

NRM int = 55 * 10-6 A/m

18.03 m field (mT) NRM int =

2512 * 10-6 A/m b)

c) d)

e) f)

g) h)

a)

i) j)

af / tc

Nup/W

VU05.1aaf / no tc

21.70

N

up/W

0 20 40 60 80 100 1200

0.2

0.4

0.6

0.8

1

th / tc

N

up/WVU42.1ath / no tc

287.10

N

up/W

0 100 200 300 400 5000.2

0.4

0.6

0.8

1VU42.1aaf / tc

287.10

N

up/W

0 20 40 60 80 100 1200

0.2

0.4

0.6

0.8

1af / no tc

N

up/W

VU127.1ath / tc

943.30

N

up/Wth / no tc

N

up/W

0 100 200 300 4000.2

0.4

0.6

0.8

1VU127.1aaf / tc

943.30

Nup/W

0 20 40 60 800

0.2

0.4

0.6

0.8

1af / no tc

N

up/W

20

200 280

350

400

0

15 20

0

15 25

380

280 250

175

20

0

25

4050 100

TU08af / tc

N

up/W

af / no tc

N

up/W

0 20 40 60 80 1000

0.2

0.4

0.6

0.8

1

Figure 2.6. Demagnetization diagrams for samples from the investigated Tušnica and Ostrožac sections in the Livno and Tomislavgrad basins. Diagrams for thermally demagnetized samples are marked ‘th’ while those for alternating field demagnetized samples are marked ‘af’. In diagrams marked ‘no tc’, meaning they are not corrected for the tilt of the strata, the overprint direction is indicated. In diagrams marked ‘tc’ the ChRM direction is indicated. The number in the lower left corner of each diagram identifies the stratigraphic position (m) of the demagnetized sample in the respective section. Decay diagrams indicate absolute (solid line) and vectorial (dashed line) decay.

a reversed polarity as indicated by both thermal and alternating field demagnetization diagrams.

2.6 Chronostratigraphic framework

Correlation of the magnetostratigraphy for the Livno Basin to the geological timescale is facilitated by the acquired 40Ar/39Ar ages for the intercalating Tušnica and Mandek tuffs. Since the former has an age of 17.00±0.17 Ma, the reversed polarity interval retrieved in the Tušnica - Drage Quarry must correlate to chron C5Cr (Fig. 2.7). The large reversed interval in the lower part of the Ostrožac section most likely represents chron C5Br. This implies that the unexposed interval between these two sections, comprising approximately 350 m (Milojević and Sunarić 1964), covers the time span of C5Cn. The topmost part of this chron straddles the very base of the Ostrožac section (Fig. 2.7). Interval E most likely corresponds to C5Bn.2n, interval F to C5Bn.1r, interval G to C5Bn.1n, and interval H to C5ADr. Intervals I and J together

50


correlate to chron C5ADn. We cannot exclude, however, a short reversed interval in the top of interval I or the base of interval J. Reversed interval K might thus represent either C5ACr or C5ABr. This results in a correlation of interval L to either C5ACn or C5ABn. This correlation suggests an age of approximately 16 Ma for the base, and an age of either 14 Ma or 13.5 Ma for the top of the Ostrožac section. The first option (option 1) implies the sedimentation rate strongly increases in the breccia bearing part of the section (Fig. 2.8). Adop tion of the second option (option 2) means sedimentation rates increase only slightly. The 14.68±0.16 Ma Mandek ash correlates to stratigraphic interval M4 of the subdivision of the 1:100,000 geological map of the Livno basin. The age of the Mandek ash suggests it is time-equivalent to part of interval H of the Ostrožac section. Just below the Mandek ash, a 6 m thick breccia crops out. Since the first breccia enters the Ostrožac section in the top part of interval I, we conclude that breccias apparently started to accumulate asynchronously in different parts of the basin.

Since the Tušnica section is located at the very base of the infill, sedimentation in the Livno basin started around 17 Ma. In order to estimate when deposition ended, an additional 300 m of poorly exposed sediments that overlie the section sampled along the Ostrožac stream have to be taken into account (Fig. 2.7). Extrapolating the calculated sedimentation rates for the topmost sampled interval, we estimate a minimum age between 12.6 and 13.2 Ma for the mega-breccia that concludes the Ostrožac section (Fig. 2.8). Since the breccias coarsen and thicken upwards, the amount of clastic input gradually increases along the sampled section. In the interval above, this effect is more strongly pronounced and the sedimentation rate might thus increase significantly. The majority of the infill of the Livno basin, attributed to the first lacustrine cycle, thus accumulated between 17 and approximately 13 Ma.

2.7 Discussion

2.7.1 Comparison with the other lakes of the Dinaride Lake System

For a long time, accurate age control on the lacustrine deposits of the intra-montane Dinaride basins remained an outstanding problem. Recently, however, new chronostratigraphic data have become available. The DLS sediments in the Sinj basin accumulated between 18 and 15 Ma (de Leeuw et al. 2010), while deposition in the Gacko basin lasted from 15.8 to approximately 15.0 Ma (Mandic et al. 2010). The sediments exposed along the shores of the island of Pag are between 17.1 and 16.7 Ma old (Jimenez-Moreno et al. 2008). The lacustrine phase of the Sava and Drava depressions, situated along the northern rim of the Dinarides, lasted from 18 Ma to at least 16 Ma (chapter 5). The DLS sediments in the Livno basin have now been shown to have accumulated between 17 and approximately 13 Ma. We conclude that deposition in all these Dinaride Lakes concentrates in a period stretching from the Early to Middle Miocene (18-13 Ma), which correlates to the Burdigalian and Langhian Stages of the Geological Time Scale and to the regional Ottnangian, Karpatian and Badenian stages of the Paratethys.

A profound phase of basin formation thus struck the Dinarides in the Early to Middle Miocene. At this time, the DLS, which occupied the resulting depressions, stretched out from the Island of Pag in the west, to the Gacko basin in the south, and the Sava and Drava basins in the north. Subsidence in the Livno basin was fairly rapid compared to the Sinj, Gacko and Pag basins. The sedimentation rate in the Sinj basin on average amounts 17 cm/kyr (de Leeuw et al. 2010), similar to the sedimentation rate of 22-30 cm/kyr on the Island of Pag (Jimenez-Moreno et al. 2008). In Gacko, the average rate of deposition

Figure 2.7. Lithological columns, obtained magnetic data and resulting polarity pattern for the Tušnica and Ostrožac sections in the Livno and Tomislavgrad basins, placed in a composite section for the main lacustrine cycle of the palaeo-lake (Fig. 2.2). Stratigraphic interval: Correlation of lithostratigraphic units from Figure 2.2 to the studied section. Units: Lithostratigraphic units identified by sedimentological logging. For a detailed explanation of the stratigraphic intervals and sedimentary units see Figure 2.3. Polarity column: In the polarity column, black indicates normal polarity, white reversed polarity and grey intervals for which no reliable polarity could be established. Black arrows: Samples for which demagnetization and decay diagrams are shown in Figure 2.6. Letters A – L: Polarity intervals discussed in the text. ATNTS (Astronomically Tuned Neogene Time Scale) after Lourens et al. (2004). Column A: Mediterranean stages, Column B: Central Paratethys stages.

51


180 360 -90 -30 30 90

220-380˚C40-80 mT20-40 mT, MAD<10

20-40 mT, 10<MAD<2020-40 mT, 20<MAD<42

1

13

12

11

10

9

8

7

6

5

4

3

2

1

2

E

F

H

I

G

J

A

B

C

D

L

K

top megabreccia bedof the Ostrožac succession

mea

sure

ddi

stan

cedi

stan

ce a

fter

Milo

jević

& S

unar

ić (1

964)

500

1500

1000

0

uppe

r lo

wer

up

per

lower

0

5

10

15

20

25

30

35

40

180 360 -90 -30 30 90

17.00 ± 0.17 Ma

Declination (º) Inclination (º) PolarityTušnica Section

ProboscideanSite

M1

M2 (m)

Lithology

Bur

diga

lian

Lang

hian

O

ttnan

g.

Kar

patia

n B

aden

ian

Ser

rava

llian

ATNTS*

17.6

17.4

17.2

16.8

16.6

16.4

16.2

15.8

15.6

15.4

15.2

14.8

14.6

14.4

14.2

13.8

17.0

16.0

15.0

14.0

13.0

C5Dn

C5Cr

C5Cn3n

C5Cn2r

C5Cn2n

C5Cn1n

C5Br

C5Bn1n

C5Bn1r

C5Bn2n

C5ADr

C5ADn

C5ACr

C5ACn

C5ABr

C5AAr

C5ABn

C5AAn

13.6

13.4

13.2

A B

Entrance of Proboscidea in Europe

Ma

Declination (º) Inclination (º)Lithology Polarity

M3

M4

Pap

eš (1

972)

0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

850

900

950

1000

(m)

Units(A)

Units(B)

Ostrožac Section

breccia

benthonite

breccia

breccia

calcarenite

benthonite

benthonitebenthonite

10

calcarenite

calcarenitecalcarenite

Strat.Level (m)

Tušnica ash

StratigraphicInterval

0

20

40

60

Mandek Section

M4

(m)

14.68 ± 0.16 Ma

Mandek ash

+ + +

breccia

benthonite

option 1

option 2

Milo

jevi

ć &

Sun

arić

(196

4) a

nd th

is s

tudy

52


was estimated to be between 10-30 cm/kyr, based on assumed astronomical forcing of sedimentary cycles (Mandic et al. 2010). In the Livno basin the sedimentation rate was significantly higher; around 57-60 cm/kyr. Since all of the investigated sediments accumulated in, or close to, the photic zone, the accumulation rate directly reflects basin subsidence.

2.7.2 The rise and fall of Lake Livno: interplay of climate and tectonics

Both persistent basin subsidence and a long-term beneficial climate are necessary to guarantee accumulation of a 1.7 km thick pile of lacustrine sediments. Since the appearance of Lake Livno as well as Lake Sinj broadly coincides with a strong rise in global temperature due to the onset of the Miocene Climatic Optimum, a link is readily made. Their appearance furthermore coincides with a phase of profound extension in the Pannonian Basin which might have triggered or enhanced the formation of the Dinaride intra-montane basins (Ilić and Neubauer 2005).

At 14.8 Ma, shortly after deposition in the Sinj and Gacko basin ends, the first calcarenites appear in the Livno Basin. Between 14.4 and 14.2 Ma the first breccia appears in the Ostrožac section. In other locations in the basin, breccias started to accumulate even earlier, as becomes clear from the Mandek section where a breccia underlies the 14.68±0.16 Ma volcanic ash. From this moment onwards, breccias thicken and coarsen until a mega-breccia eventually concludes the Miocene lacustrine phase. These breccias contain clasts of Mesozoic age derived from the basin margins and reveal that these were prone to erosion. This suggests the margins became uplifted in response to enhanced tectonic activity. Subsidence apparently stalled or was outpaced by the enhanced input of erosional material. A change in tectonic regime is thus the most likely cause for the disappearance of Lake Livno which slightly postdates 13 Ma. During the second lake phase separate lakes develop in the then disunited Livno and Tomislav basins. An angular unconformity separates the deposits of the first and second lake phase. This supports a tectonic cause for the end of the first lake phase. A contrasting explanation might involve the severe climatic deterioration that set in around 14 Ma, and transformed parts of the Central Paratethys into a hypersaline sea during the Badenian salinity crisis (de Leeuw et al. 2010). Since the intra-montane lakes of the Dinarides have been shown to be extremely sensitive to climatic change (Mandic et al. 2010; Jimenez-Moreno et al. 2008, 2009), the coincidence of the disappearance of Lake Livno with the termination of the so-called Middle Miocene Climatic Crisis might not be accidental.

2.7.3 Towards a time-indicative endemic mollusk biostratigraphy

Our new chronostratigraphy for the Dinaride basins pinpoints the age of a number of mollusk occurrences (Fig. 2.9), important for the regional biostratigraphic scheme (Kochansky-Devidé and Slišković 1972, 1978, 1980). The first is the occurrence of the primitive dreissenid bivalve Mytilopsis hercegovinensis (18-17.2 Ma), documented in the lower part of the Sinj section. This type of Mytilopsis is morphologically related to the larger sized Mytilopsis kucici, which occurs above a horizon with clivunellid gastropods (~17 Ma) in the upper part (~16.7 Ma) of the section on Pag (Bulić and Jurišić-Polšak 2009, Jimenez-Moreno et al 2009).

In the Livno and Tomislavgrad basins, clivunellids are restricted to stratigraphic unit M2 with an approximate age range of 16.1–16.9 Ma (Fig. 2.7). It contains fossils of Clivunella katzeri, a primitive morphotype of Mytilopsis drvarensis, and a progressive morphotype of M. kucici (Fig. 2.4). This is the oldest registered occurrence of M. drvarensis and its presence in unit M2 demonstrates that it already appeared in the Lower Miocene. The encountered M. kucici and Clivunella katzeri specimens, on the other hand, are the youngest ones known and these species might become extinct at the Early to Middle Miocene boundary. The occurrence of either clivunellids or M. kucici in combination with M. drvarensis is thus an important marker for the uppermost Lower Miocene in the DLS. In contrast to M. kucici and C. katzeri, the stratigraphic range of Mytilopsis drvarensis extends into the Middle Miocene and a striking radiation of the species occurs in chron C5Br (16-15.2 Ma) (de Leeuw et al. 2010).

Two more first occurrences complete our current knowledge of the DLS biochronostratigraphic scheme. These are the Mytilopsis frici FO at 15.6-15.4 Ma in the Gacko basin (Mandic et al. 2010), and

53


the Mytilopsis aletici FO at 15.4 Ma in the Sinj basin (de Leeuw et al. 2010). The establishment of age ranges for these endemic mollusks allows their use as time indicators and might enable future dating of fossil bearing successions not suitable for paleomagnetic and radio-isotopic techniques.

2.8 Conclusions

A detailed chronostratigraphic framework based on integrated magnetostratigraphic and radio-isotopic data demonstrates that the majority of the DLS sediments in the intra-montane Livno and Tomislavgrad basins accumulated between 17 and approximately 13 Ma. The very top of the DLS sediments is overlain discordantly by deposits of a younger lake phase and do not crop out. The end of the fi rst lake phase is therefore badly constrained.

When subsidence initially set in, swamp conditions prevailed and several coal seams formed.

Figure 2.8. Reconstruction of the sedimentation rate for the Livno and Tomislavgrad Basin and comparison with the rate of sedimentation in the Sinj Basin (de Leeuw et al. 2010).

breccia

benthonite

breccia

breccia

calcarenite

benthonite

benthonitebenthonite

calcarenite

calcarenitecalcarenite

EARLY MIOCENE MIDDLE MIOCENE

0

200

400

600

800

1000

1200

1400

1600

?

Lučane 1

Lučane 2

Lučane 3Tušnica

Sinj B

asin

Livn

o &

Tom

isla

vgra

d

Bas

in

~10 cm/ky

0

100

200

300

400

500

Age (My)

Str

atig

rap

hic

Le

vel (

m)

Str

atig

rap

hic

Le

vel

(m)

13.0 18.0 17.0 16.0 15.0 14.0

C5Dr2r C5Dn C5Cr C5Br C5ADn C5ACn

~20 cm/ky

~37

cm/k

y

~55

cm

/ky

Mega-breccia: end of the Miocene lake phase

~39

cm/k

y

optio

n 1

optio

n 2

54


Subsequently the basin, then still undisected, was flooded and a long-lived lake established. Around 1700 m of lacustrine limestones with several intercalating volcanic ash layers accumulated. Around 14.8 Ma calcarenites and breccias, derived from the basin margin started to enter Lake Livno. They coarsen and thicken over time. This suggests an inversion of the tectonic regime invoked gradual uplift of the basin margins and eventually caused Lake Livno to disappear. During the second lake phase, two separate lakes formed in the then disunited Livno and Tomislavgrad basins.

Comparison with other recently dated Dinaride Lakes demonstrates that their life-times coincide and points out that a profound phase of basin formation struck the orogen in the Early to Middle Miocene. The coincidence of the expanse of the Dinaride Lake System with the Miocene Climatic Optimum suggests the concomitant climatic conditions were beneficial for lake formation. The occurrence of either clivunellids or M. kucici in combination with M. drvarensis is shown to be time-indicative of the uppermost Lower Miocene and is thus considered an important chronological marker in the DLS. Integrated radio-isotopic and magnetostratigraphic dating techniques once more prove to be a powerful chronostratigraphic tool in settings plagued by endemism.

Acknowledgments

We are highly indebted to Tvrtko Ćubela (Coalmine Tušnica, Livno), Jeronim Bulić (HPM Zagreb) and Stjepan Ćorić (GBA Vienna) - this study would not be possible without their support and logistic help. We thank Roel van Elsas for assistance with mineral separation and Jan Wijbrans and Cor Langereis for discussion. The study contributes the Austrian FWF Project P18519-B17: ‘Mollusk Evolution of the Neogene Dinaride Lake System” and was supported by the Netherlands Research Centre for Integrated Solid Earth Sciences (ISES) and by the Netherlands Organization for Scientific Research (NWO/ALW).

55


Figure 2.9. Chronology for key biostratigraphic and depositional events of the Dinaride Lake System.

Fossil mollusks preserved in a coal layer

Chapter 3Palaeoenvironmental evolution of Lake Gacko (Southern Bosnia and Herzegovina): impact of the Middle Miocene Climatic Optimum on the Dinaride Lake System

Oleg Mandic, Arjan de Leeuw, Boško Vuković, Wout Krijgsman, Mathias Harzhauser and Klaudia Kuiper

In the Early to Middle Miocene, a series of lakes, collectively termed the Dinaride Lake System (DLS), spread out across the north-western part of the Dinaride-Anatolian continental block. Its deposits, preserved in numerous intra-montane basins, allow a glimpse into the palaeoenvironmental, palaeobiogeographic and geodynamic evolution of the region. Lake Gacko, situated in southern Bosnia and Herzegovina, is one of the constituent lakes of the DLS, and its deposits are excellently exposed in the Gračanica open-cast coal-mine. A detailed study of the sedimentary succession that addresses facies, sediment petrography, geophysical properties, and fossil mollusc palaeoecology reveals repetitive changes in lake level. These are interpreted to reflect changes in the regional water budget. First-order chronologic constraints arise from the integration of radio-isotopic and palaeomagnetic data. 40Ar/39Ar measurements on feldspar crystals from a tephra bed in the upper part of the sedimentary succession indicate a 15.31±0.16 Ma age for this level. The reversed magnetic polarity signal that characterises the larger part of the investigated section correlates to chron C5Br of the Astronomically Tuned Neogene Timescale. Guided by these chronologic data and a detailed cyclostratigraphic analysis, the observed variations in lake-level, evident as two ~40-m and seven ~10-m scale transgression-regression cycles, are tuned to ~400-kyr and ~100-kyr eccentricity cycles. From the tuning, it can be inferred that the sediments in the Gacko Basin accumulated between ~15.8 and ~15.2 Ma. The economically valuable lignite accumulations in the lower part of the succession are interpreted to indicate the development of swamp forests in conjunction with lake-level falls corresponding to ~100-kyr eccentricity minima. Pedogenesis, rhizoliths and palustrine carbonate breccias in the upper part of the section reveal long-term aridity coinciding with a ~400-kyr eccentricity minimum. Eccentricity maxima are interpreted to trigger lake-level high-stands. These are accompanied by eutrophication events caused by enhanced denudation of the surrounding basement and increased detrital input into the basin. The presented age model proves that Lake Gacko arose during the Middle Miocene Climatic Optimum and that the optimum climatic conditions triggered the formation of this long-lived lake.

This chapter is based on: Mandic O., de Leeuw A., Vuković B., Krijgsman W., Harzhauser M. and Kuiper K.F. (2010) Palaeoenvironmental evolution of Lake Gacko (Bosnia and Herzegovina): impact of the Middle Miocene Climatic Optimum on the Dinaride Lake System. Palaeogeography, Palaeoclimatology, Palaeoecology, 299: 475-492.

58


3.1 Introduction

The Dinaride-Anatolian land formed a major barrier between Paratethys and the proto-Mediterranean Sea during the Early and Middle Miocene (Fig. 3.1A). It harboured a widespread and long-lived array of lakes, collectively known as the Dinaride Lake System (DLS). Deposits of the individual lakes have been preserved in numerous intra-montane basins (Fig. 3.1B) throughout the Dinaric Alps from Slovenia to Serbia and Montenegro (Krstić et al., 2003; Rasser et al., 2008; Harzhauser and Mandic, 2009). They form an extraordinary archive for the paleoenvironmental, paleobiogeographic and geodynamic evolution of the region.

Lake Gacko was the southernmost constituent of the DLS (Fig. 3.1B). Excellent outcrop conditions in the Gračanica open-cast coal-mine (Thomas and Frankland, 2004) provide a superb opportunity to study the development of this lake. Clear vertical changes in facies as well as faunal assemblages characterise the exposed sedimentary succession. The archives of this highly sensitive lake registered subtle environmental variations as alternations of coal, marl, lacustrine- and palustrine limestone. The presence of one thick and several thinner ash layers testifies to concurrent volcanic activity in Southeastern Europe and facilitates the construction of a time-frame through 40Ar/39Ar dating. The simple synclinal structure of the basin and the readily available basic geologic, lithostratigraphic and palaeontologic data provided by the Gračanica coal-mine Exploration and Production Authority and a number of past scientific investigations (Neumayr, 1880; Brusina, 1897; Katzer, 1921; Milojević, 1966, 1976; Mirković, 1980; Thomas and Frankland, 2004) provides a suitable framework for this high-resolution paleoenvironmental study.

Kochansky-Devidé and Slišković (1978), subdivide the evolution of the DLS in two subsequent stages, each characterised by different molluscs. Based on its phylogenetically advanced mollusc assemblage, Lake Gacko pertains to the younger stage. Its lifetime should therefore coincide with the late stage evolution of Lake Sinj in Croatia, another constituent lake of the DLS. For the latter, a high-resolution description of the depositional history, a reconstruction of the vegetation and climate dynamics based on pollen records, and a solid chronostratigraphic framework were recently published (Jimenez-Moreno et al., 2008; Mandic et al., 2009; De Leeuw et al., 2010). Newly obtained results from Lake Gacko are thus readily comparable with data from Lake Sinj and provide a better insight into the dynamic faunal as well as paleoenvironmental evolution of the DLS as a whole.


The Gacko Basin (Fig. 3.1C) is a typical intra-montane karst polje (large flat-floored valley) situated at approximately 930 m above sea-level. The 40 km², strongly elongated tectonic depression is oriented in a NW-SE direction, parallel to the strike of the Dinarides (Muftić, 1964; Mojićević and Laušević, 1965, 1973; Milojević, 1966, 1976; Mirković et al., 1974; Mirković, 1980). Its margins are defined by normal faults (Thomas and Frankland, 2004). The karstic basement consists of predominantly shallow-water deposits that accumulated on the Dinaric/Adriatic carbonate platform in the Mesozoic. In the Late Cretaceous, the platform disintegrated and flysch deposition started in the north-eastern tectonic unit (Fig. 3.1C). Intensive thrusting subsequently affected the south-eastern tectonic unit and, during the Palaeocene and Eocene, flysch accumulated there as well. Late Eocene clastic molasse-type sediments, termed the Promina Formation, are the last marine deposits in the region. The Miocene lacustrine sediments form a simple but slightly asymmetric syncline structure, with its NW-SE oriented axis slightly offset to the southwest. Post-depositional tectonics shifted the north-western part of the syncline southward and divided it from the rest of the basin by S-N striking anticlinal folds (Thomas and Frankland, 2004) (Fig. 3.1C and 3.1D).

The Miocene sedimentary infill of the Gacko Basin comprises about 360 m exclusively lacustrine sediments (Milojević, 1976; Mirković, 1980; unpublished exploration and drilling data by Gacko Mine and Thermal Power Plant company; Figs. 3.1D and 3.2). The architecture of the sedimentary succession can be interpreted as a single transgression-regression cycle of the lake. The initial flooding of the basin

59

Chapter 3: Palaeoenvironmental evolution of Lake Gacko: impact of the Middle Miocene Climatic Optimum on the DLS

resulted in an about 20-m-thick basal conglomerate with a matrix of sand and clay (Fig. 3.1D – Unit 1). Three marl-to-coal sequences follow on top of this conglomerate, each about 50 m thick in the central part of the basin (Fig. 3.1D – Units 2 to 4). The coals bear taxodiacean stems and trunks that are indicative of swamp conditions at the lake margin. Slightly upward in the section, the predominance of marls indicates drowning of the swamp environment and installation of a perennial lake (Fig. 3.1D – Unit 5). A thick volcanic tuff marks the top of this marl unit. Above the tuff, a second coal interval points to returning swamp and mire conditions due to a drop in water level. This is the terminal phase of Lake Gacko (Fig. 3.1D – Unit 6). The volcanic tuff and second coal interval are restricted to the central part of the basin and are outside of the scope of the present study.

3.3 Material and methods

The stratigraphic succession of Lake Gacko was logged and sampled in the abandoned SW part of open-pit B of the Gračanica mine in the NW part of the basin (Thomas and Frankland, 2004) (Fig. 3.1C, 3.1D and 3.2). It is located between GPS WGS1984 datum points N 43° 10’ 37.5”, E 18° 29’ 17.7” and N 43° 10’ 33.0”, E 18° 29’ 7.4”. The section has a stratigraphic thickness of 76.8 m. It starts with a 2.8-m-thick coal seam which is the lowermost lignite layer of the basin infill. This coal seam, buried under mine debris during the field study, overlies the clayey marls of the “footwall deposits of coal II” unit of Milojević (1966). The top of the section corresponds to the level of the Gacko Polje plane (Fig. 3.2).

Detailed sedimentological logging was carried out in the field and allowed first-hand analysis of the

Figure 3.1. The studied area. (A) Palaeogeographic position between the Paratethys and the Mediterranean Tethys during the Langhian (after Rögl, 1998). (B) Geographic position showing distribution of other major DLS basins (after Mandic et al., 2009). (C) Geological setting of the Gacko Basin with indicated position of two studied localities (after Mirković et al., 1974, Mojićević et al., 1965). The numbering and colour code of Miocene lithostratigraphic units coincide with illustration D. (D) Detailed geological map of the western part of the Gacko Basin (after Thomas and Frankland, 2004) with indicated position of the Gračanica section. The relative position of represented lithostratigraphic units is indicated by the column on the left. For colour codes see Figure 3.2.

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represented facies types and their architecture. Thin sections of selected carbonate facies types were prepared for detailed microfacies analysis in the laboratory. The fossil record was studied both in the field and in the lab.

3.3.1 Mollusc palaeontology

Well-preserved molluscs are rare in the section and are concentrated in its lower and middle part. Aragonite leaching in the upper, carbonate-dominated part is demonstrated by rare gastropod moulds. Only three samples (Mollusc samples 1, 3 and 4, Fig. 3.2) were suitable for quantification because, for other samples, shell decalcification or a strongly cemented rock matrix prevented washing and sieving. The quantified samples were sieved at fraction >250 μm. One additional mollusc sample (2), taken from the abandoned Vrbica open-cast mine, was investigated (GPS WGS1984 Position: N43° 08’ 42.7’’ E18° 33’ 35.8’’; Figs 3.1C and 3.2). This sample is situated in the adjoining hanging wall of the main coal seam, above a single 10-cm-thick coal layer intercalated in dark brown clayey and silty marls bearing Mytilopsis frici. The presence of the latter marker fossil in combination with the lithology and lithostratigraphic position of the site allows straightforward correlation to the lower part of the Mytilopsis-marl unit of the Gračanica section (Fig. 3.2). The abandoned Vrbica opencast mine is located beside the road, halfway between Avtovac and Gacko (=Metohija) and therefore corresponds to the classical fossil mollusc site of Neumayr (1880) and Brusina (1897).

3.3.2 Geophysical logging and spectral analysis

To better understand the fine-scale vertical variations in the detrital rock component, gamma ray and magnetic susceptibility measurements were taken with a hand-held “Compact Gamma Surveyor” scintillation gamma radiometer and “SM-20” magnetic susceptibility meter with a sensitivity of 10-6 SI units (GF Instruments, Brno, Czech Republic). The distances between the geophysical point measurements were 10 cm (GR) and 5 cm (MS), respectively (Fig. 3.2).

Power spectra are calculated for the magnetic susceptibility and natural gamma radioactivity in the depth domain using the PAST program (Hammer et al., 2001) and the Lomb periodogram algorithm for unevenly sampled data. The data were detrended prior to spectral analysis by subtraction of the linear regression line to improve resolution at low frequencies. Gaussian band-pass filtering was conducted using the AnalySeries time series analysis tool developed by Didier Paillard (Paillard et al., 1996).

3.3.3 Magnetostratigraphy

In order to construct a magnetostratigraphy for the Gacko Basin, 40 sites with two standard palaeomagnetic cores each were drilled (Fig. 3.2). The resolution achieved in the section stretching from the coal beds at the base to the prominent volcanic ash at the top was around 1 site per 2 m stratigraphically. Samples were collected with a gasoline-powered hand-held drill. The orientation of all samples was measured using a magnetic compass. Bedding planes were similarly determined at regular intervals. Subsequently, both bedding planes and sample orientations were corrected for the local magnetic declination, adding 4° east.

Figure 3.2. The results of deposition and facies analysis and lake-level-change. Positions of lithological units defined for the studied section and their correlation with the general Gacko Basin lithostratigraphic division are shown (Milojević, 1976, Mirković, 1980, Thomas and Frankland, 2004 and unpublished exploration and drilling data by Gacko Mine and Thermal Power Plant company). The colour code of the lithological log reflects the natural sediment colour; for detailed lithological description see text. Position of discussed thin-sections and mollusc samples are indicated, and gamma ray and magnetic susceptibility logs are reproduced. Results of mollusc palaeoecological analysis and positions of inferred transgression-regression cycles and possible intensities of relative lake-level rises are indicated as well.

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Palaeomagnetic analysis was carried out at the Palaeomagnetic Laboratory of Utrecht University. The cores were sliced into multiple specimens of which one per site was subsequently subjected to Alternating Field (AF) demagnetization. After each demagnetization step, the natural remanent magnetization (NRM) of the samples was measured on a 2G Enterprises DC Squid cryogenic magnetometer (noise-level 3·10-12 Am2). AF demagnetization was performed by a laboratory-built automated measuring device applying 5–20 mT increments up to 100 mT by means of an AF-coil interfaced with the magnetometer. The characteristic remanent magnetization (ChRM) was identified by inspection of decay-curves and vector end-point diagrammes (Zijderveld, 1967). ChRM directions were calculated by principal component analysis (Kirschvink, 1980).

3.3.4 Geochronology

An approximately 1-m-thick greenish and clayey volcanic ash in the uppermost part of the section was sampled twice for 40Ar/39Ar dating (Fig. 3.2). Both samples were taken from the same part of the ash-layer but with a lateral distance of ~10 m. The samples were processed in the Department of Isotope Geochemistry (VU Amsterdam). The bulk samples were crushed, disintegrated in a calgon solution, washed and sieved over a set of sieves between 63 and 250 μm. The residue was subjected to standard heavy liquid as well as magnetic mineral separation techniques. The fraction of grains larger than 150 μm of both samples contained abundant feldspar crystals. These were handpicked and leached with a 1:5 HF solution in an ultrasonic bath during 5 min. The mineral separates were then loaded in a 10 mm ID quartz vial together with Fish Canyon Tuff (FC-2), Drachenfels one (f250–500) and Drachenfels two (f>500) sanidine. The vial was irradiated at the Oregon State University TRIGA reactor in the cadmium-shielded CLICIT facility for 10 h.

After return to the VU Amsterdam laboratory, ten splits of both samples were loaded into a copper sample-tray together with Drachenfels as well as Fish-Canyon sanidine standards. The tray was pre-heated under vacuum using a heating stage to remove undesirable atmospheric argon. Thereafter, samples were placed in the UHV sample chamber and degassed overnight. The samples were then fused using a Synrad CO2 laser in combination with a Raylase scan-head as a beam delivery and beam diffuser system. After purification, the resulting gas was measured with a Mass Analyzer Products LTD 215-50 noble gas mass spectrometer. Beam intensities were measured in peak-jumping mode in 0.5 mass intervals over the mass range 40–35.5 on a Balzers 217 secondary electron multiplier. System blanks were measured every three to four steps. Mass discrimination was monitored by frequent analysis of aliquots of air. The irradiation parameter J for each unknown was determined by interpolation using a linear fit between the individually measured standards.

All 40Ar/39Ar ages were calculated with the in-house developed ArArCalc software (Koppers, 2002), applying the decay constants of Steiger and Jäger (1977). The age for Fish Canyon Tuff sanidine flux monitor used in age calculations is 28.201±0.046 Ma (Kuiper et al., 2008). The age for the Drachenfels sanidine flux monitor is 25.42±0.05 Ma (Kuiper et al., in prep). Correction factors for neutron interference reactions are 2.64±0.017×10−4 for (36Ar/37Ar)Ca, 6.73±0.037×10−4 for (39Ar/37Ar)Ca, 1.211±0.003×10−2 for (38Ar/39Ar)K and 8.6±0.7×10−4 for (40Ar/39Ar)K. Errors are quoted at the 1σ level.

3.4 Depositional facies analysis and lake-level change

3.4.1 Lithology

A detailed description of the lithological record is essential in order to document the character of fine facies shifts observed in the section. It forms the backbone of the paleoenvironmental interpretation, guides the reconstruction of lake-level changes, and provides a solid framework for the cyclostratigraphic model put forward.

In the investigated section, 6 lithostratigraphic units designated with the letters A to F are recognised. The lower three units are dominated by coal, the upper three by carbonate deposits. Coal is represented by a soft-brown coal (lignite) with the following average quality values at the studied site (after Thomas

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and Frankland, 2004): total moisture 37.4 % a.r., ash 15.1 % a.r., total sulphur 1.22 % a.r. and net caloric value 9.623 kJ/kg.

The present lithostratigraphic classification follows, with two exceptions, the classification established by Milojević (1966 and 1976) (Fig. 3.2). Unit B includes, however, beside “footwall coal I” also two adjoining units, namely “Fossarulus-” and “Melanopsis-marl”. The latter two are not well developed at studied locality and cannot be clearly differentiated from the “footwall coal I”. This coincides with the observation by Milojević (1966) that the two marl units, attaining up to 50 m in the basin centre, reduce toward the basin margin to meter scale. Furthermore, “hanging wall marls” in the studied section comprise two distinct lithological units termed Unit E and F. The top of the “hanging wall marls”, together with overlying “tephra” and “coal units”, could not be investigated in the studied section because they were present exclusively in the central part of the basin (Figs 3.1D and 3.2).

3.4.1.1 Unit A (“footwall coal II”)

Unit A (9.8 m) is three-folded, comprising a 3-m-thick coal seam in the lower part, a 2.8-m-thick marl in the middle part and a 4-m-thick coal seam on top. It overlies the whitish-grey clayey marl. The lower coal seam is intercalated by marl lenses and inter-layers and incorporates weakly coalified remains of tree stumps and branches. The marl interval bears common mollusc remains and is portioned by a 10 cm coal seam into s lower, laminated, wood fragments-bearing, beige part and an upper, coal laminae-bearing, greyish part.

3.4.1.2 Unit B (“footwall coal I”, “Fossarulus-” and “Melanopsis-marl”)

Unit B (13 m) consists of alternating coal seams and dark greyish limestone. The coal layers show rough cyclic architecture by a thickening upward trend up to the 3-m-thick coal seam in the middle part of the unit, followed by a thinning upward series. The lower boundary is transitional, characterised by coal-limestone alternation at a 10 cm scale. In the lowermost 2.7 m the limestone is dark greenish-greyish with brownish dots and abundant impurities (coalfield wood fragments) and common mollusc remains. White carbonate crust accumulations can also occur in transitional zones between coal and limestone. The bed transitions in this part of the unit are still gradual. At the next coal seam, the lower coal boundaries become sharp and planed, resulting in lithologically very clear limestone differentiation (Fig. 3.3.1). The upper coal boundaries are sharp as well, but commonly undulated due to original surface relief made by taxodiacean tree trunks and branches deposited on top of coal seams and covered subsequently by limy deposits. Five laterally traceable grey limestone beds are present. The coal seams are characterised by larger and smaller limestone lenses and interlayers of restricted lateral extension. They include common, horizontally oriented, weakly coalfield remains of tree stumps, trunks and branches at the meter-scale.

3.4.1.3 Unit C (“main coal”)

This unit (<10.2 m) comprises the topmost coal seam interval. It includes the best-quality coal because of the lowest relative volume of intercalated limestone beds. That volume, however, as already observed by Thomas and Frankland (2004), increases westwards. Also, the thickness increases in that direction. The unit starts with a 1.4-m-thick homogeneous coal seam with sharp and plane lower boundary (Fig. 3.3.1). The overlying, 3.4-m-thick interval shows intensive alternation of coal and limestone beds. At the studied site, coal predominates in the interval (Fig. 3.3.1). To the west, however, limestone replaces it progressively, becoming there thicker than the coal. On top, a homogeneous, up to 3.4 m thick, high-quality coal seam occurs with only sporadic limestone lenses. Finally, the intensive alternation of coal and limestone beds (maximum thickness 1.6 m) marks the very top of the Unit C. Eastwards, both latter subunits decrease in thickness by up to 50%.

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3.4.1.4 Unit D (“Mytilopsis-marl”)

Unit D (14.3 m) is characterised by thick intervals of grey, fossil-poor limestone. Still including coal seam intercalations, it represents a transitional unit between the coal- and the limestone-dominated section parts. It comprises two subunits bounded by an erosional hiatus. The lower subunit (8.2 m) comprises, in the lower part, dark grey limestone (equal to limestones of previous units) passing upward into dark grey limestone. Therein, intercalations of a few up to 10-cm-thick coal seams and dark greyish clayey marl beds occur. Overlying this is a 1.2-m-thick dark greyish clay bed, topped by beige limestone bearing coalified wood fragments; it passes upwards into light greyish, intensively fractured limestone. The upper subunit overlies an erosional surface (Figs. 3.2, 3.3.2 and 3.3.3). The low-grade relief is planed by up to 30-cm-thick mollusc-bearing sandstone filling up the 30-cm-deep fissures in the underlying limestone; above it is rich in dark, coalified organic matter. This is followed by an about 4-m-thick prominent, light grey, initially banded then homogeneous limestone interval, which lacks fossils. Its upper half bears dark nodules introduced after a thin (1 cm) coal intercalation persisting laterally for more than 100 m. A dark limestone (2.2 m) follows upward. Its lower part is composed of sand-infilled bioturbations, the middle part of an enhanced sandy component, and the upper part of thin coal seams alternating with bivalve coquinas (Figs. 3.3.2, 3.3.4 and 3.3.5). The unit ends with beige limestone comprising still thin coal and coquina inter-layers.

3.4.1.5 Unit E (“hanging wall marls” - breccia unit)

The lower boundary of this unit, introducing an abrupt change of depositional style in the succession, is marked by an erosional hiatus (Fig. 3.2). Unit E is 16.2 m thick, but its thickness enlarges significantly in westward direction, where an intercalation of up to 2-m-thick cross-bedded carbonate sands is present. In the studied section the unit starts with beige breccia (0.8 m) composed of intraclasts and palustrine limestone fragments overlying the erosional surface (Figs. 3.3.2 and 3.3.6). The next 0.9 m interval first shows a dark brown laminated sandy limestone bed, then a characean oogoniae-bearing, beige sandy limestone bed, and finally a dark greenish carbonate breccia layer. This is followed above by a 3.5-m-thick, dark grey to dark brown, microtube-bearing limestone. Gastropods, still rare in the lower part, become common in the upper part, accumulated to coquinas. This interval bears grey laminated limestone in its lower part and breccia intercalations in its upper part. The characean oogoniae that are already present in the previous interval become a common constituent of limestones within the following 1-m-thick, light greyish bed. The thickest breccia horizon follows; it is beige, attaining 3.5 m. A single 20-cm-thick dark limestone bed is intercalated in its upper part. The upper 7.2 m of the unit is represented through alteration of characean limestone with maximally 0.9-m-thick breccia intervals. Rare gastropod remains can be present in the characean limestones. The top of the unit is marked by the section’s uppermost breccia bed (Fig. 3.2).

3.4.1.6 Unit F (“hanging wall marls” – tephra-clay unit)

Unit F (11.5 m) is lithologically very variable (Fig. 3.2). The base is marked by a ripple-bedded characean limestone followed by a series of light-coloured, beige and grey characean and ostracod massive limestones. In the central part of the unit the limestone becomes intercalated by up to 0.9-m-

Figure 3.3. Field views of the most typical depositional facies. (1) Alternation of coal and grey limestone around the boundary of Units B and C. (2) Section interval including Units D2 and E1 with grey limestone, thin, coal seam-related dreissenid shell accumulations, tufa-lithoclast breccia and rhizolith limestone. (3) View of the brecciation surface (arrow) at base of Unit D2. (4–5) Gray limestone of Unit D2 with accumulated shells of Mytilopsis frici. (6) Tufa-lithoclast breccia at base of Unit E. (7) View of the suc cession of the lower part of Unit F with indicated position of 0.5-m-thick, greenish tephra layer. (8) The very top of the section (Unit F) with organic-rich clay and marl-bearing, thin coal seam intercalations.

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thick, greenish sandy and clayey tephra layers (Fig. 3.3.7). Deposition of organic-rich brownish clays is restricted to the uppermost 3.9 m of the section. Initially, clay alternates with limestone. In the middle part of the interval, brownish clay bears gastropod fragments. Within the final 1.1 m the gastropod-bearing clay becomes dark brown, finally bearing thin coal inter-layers at the very top of the section (Fig. 3.3.8).

3.4.2 Carbonate depositional facies

Four main limestone facies found in the studied section were investigated additionally in thin sections to more precisely interpret the depositional facies. Special emphasis was given to the microtubuli-bearing limestone and the intraclast breccia in respect to the character of included components. For sample positions see Fig. 3.2.

3.4.2.1 Greyish limestone

This limestone type dominates the lower part of the section. It is present in the units B, C and D and is associated there with coal deposits. These limestones are always in direct stratigraphic contact with latter, representing the coal seam intercalations or alternating with thicker coal layers (Fig. 3.2). Such packstones commonly bear molluscs and coalified plant remains (Figs. 3.4.1 and 3.4.2). Gacko Basin corresponds, by its geological setting, to the synchronous Sinj Basin. The latter likely accommodates DLS palaeo-lake deposits and comprises, within its topmost infill part, similar coal-limestone alternations (Mandic et al., 2009). There, the limestones intervals are regarded to represent the lake-level rises, whereas the coal phases are correlated with the lake-level falls. The same depositional model applies for the Gacko Basin.

3.4.2.2 Rhizolith limestone

This limestone bears no fossil remains except for common, up to 3-mm-long, black, calcitic microtubuli. It is dark greyish and dark brownish, restricted to the lower part of Unit E. There, it dominates one 3.7-m-thick interval (Fig. 3.2). The thin sections (Figs. 3.4.3 and 3.4.4) show packstone with size-sorted, non-oriented microtubuli floating in a micritic matrix. Tubuli attain maximally 1 mm in radius and are commonly fragmented. Their walls are thin and composed of dark, globular-micrite. Subordinated are microtubuli with white, calcititic, non-micritised walls. These walls are multi-tubular in cross-section, supporting the classification into calcified root cortices. Following examples by Froede (2002) and Košir (2004), the dark microtubuli are furthermore interpreted as fossilised root casts. Consequently, all microtubuli can be termed rhizoliths. Fragmentation and size-sorting of the rhizoliths point to their transport from the place of origin, usually positioned at a pedogenic, vegetated lake fringe. Rhizolith production is bound to the type of pedogenesis that is characteristic for arid and semi-arid climate conditions (Flügel, 2004).

3.4.2.3 Tufa-lithoclast breccia

The breccia intercalations are restricted to Unit E, occurring from its base to the top, with the thickest bed (2.5 m) in the middle part of the unit (Fig. 3.2). Breccia beds usually have sharp lower boundaries that can be clearly erosional; the upper boundaries are transitional. Layers are dominantly beige, but thin inter-beds of dark brownish or greenish matrix are present as well. They are associated with rhizolith and characean limestone. The lithoclast composition is dominated by up to 10-cm-large calcareous tufa, although coalified wood fragments and lacustrine limestone clasts are also present. The latter show, in thin section, fringe cement crusts of plant stems (Figs. 3.4.5 and 3.4.6). The stems usually attain about 4 mm in radius and belonged to aquatic macrophytes. The limestone can therefore be classified, after

Figure 3.4. Photomicrographs (optical microscope) of selected lacustrine carbonate facies. (1–2) Organic matter- and mollusc-bearing grey limestone, sample 1, Unit B. (3–4) Rhizolith limestone, sample 2, Unit E1. (5–6). Tufa-lithoclast from tufa-lithoclast breccia, sample 3, Unit E1. (7–8) Characean limestone, sample 4, Unit F.

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Ford and Pedley (1996), as paludal tufa. Its origin was bound to ephemeral ponds developed along the lake fringe. The breccia is therefore related to erosional processes of marginal environments that could develop during the lake-level low stands as marked by an erosional hiatus or be transported into the lake during storm events.

3.4.2.4 Characean limestone

This limestone facies is restricted to Units E and F, alternating there with tufa-lithoclast breccia, rhizolith limestone and organic-rich clay (Fig. 3.2). Its colour is dominantly beige, sometimes, light or dark grey. Except for a single ripple-bedded interlayer, it is massive. Sporadically occurring massive micritic limestone with absent or rare characean oogonia or the ostracod-bearing limestone of the Unit F are closely associated with this facies type. In thin section, whole and fragmented oogonia are visible, floating in the micritic matrix (Figs. 3.4.7 and 3.4.8). Based on density variation, it represents wackestone to packstone. The presence of characean limestones points to the presence of open lacustrine conditions (Platt and Wright, 1991).

3.4.3 Mollusc palaeontology

Fossil molluscs are very good indicators of palaeoenvironmental changes in lacustrine settings (Cohen, 2003). They are highly sensitive to hydro-climate fluctuations and record abiotic factors such as water chemistry, depth, and turbulence. Hence, the evaluation of their taxonomy, species-richness, and proportional abundances helps reconstruct changes in depositional environment and is an essential element in assessing lake-level variations.

The investigated samples comprise 18 species-level taxa of exclusively lacustrine molluscs (Figs. 3.5 and 3.6). Their taxonomic identifications are based on Neumayer (1869, 1880), Brusina (1874, 1897, 1902), Kochansky-Devide and Slišković (1978) and Olujić (1999). All but Mytilopsis frici (Figs. 3.3.4 and 3.3.5) are represented in the quantified samples. M. frici is an endemic dreissenid bivalve that belongs to the M. drvarensis clade. It is rounded and flattened, attaining a size of 30 to 50 mm. The flattened morphology is partly secondarily induced by sediment compaction and therefore the specimens could originally have had somewhat more inflated morphologies. The specimens show a dorso-ventral sinus, which distinguishes them from their predecessor - the very similar M. drvarensis. Their identification as M. frici is in agreement with previous results of Kochansky-Devide and Slišković (1978).

3.4.3.1 Distribution

On top of the lower coal seam of Unit A, shell accumulations with the pulmonate snails Gyraulus and Lymnaea are present. In the overlying beige limestone, rissooidean gastropods have been additionally observed. A quantified sample from that layer (Mollusc sample 1 in Fig. 3.2) comprises 8 species (Fig. 3.6). The pulmonates are still frequent therein, represented by Gyraulus pulici (23.6%) and Radix hyaloleuca (17.9%), but rissooideans are slightly more commonly represented with the stenothyrid Bania prototypica (19.8%) and the hydrobiid Fossarulus with two species, F. fuchsi (13.2%) and F. buzolici (13.2%) (Fig. 3.5).

The pulmonate snail accumulations are still present in Unit B. Its lowermost 4 m bear abundant mollusc remains concentrated in transitional zones between coal and limestone beds. Present therein are Gyraulus pulici, Ferrissia illyrica and Lymnaea klaici (Fig. 3.5). Additionally present but rare are the rissooidean snails Fossarulus cf. buzolici and Stalioa cf. parvula. In the upper part of the unit, molluscs are absent except for the 1.3-m-thick, grey limestone interval, which bears rare Radix cf. hyaloleuca. Upward, in Unit C, no molluscs have been observed (Fig. 3.2).

In the lower part of Unit D, molluscs are absent in the studied section. In contrast, in the Vrbica open-cast mine (Fig. 3.1C) molluscs are very common at this particular stratigraphic position (Mollusc sample 2 in Fig. 3.2). The quantified sample from there comprised the highest recorded species richness of 12 species (Fig. 3.6). Its composition is dominated by the rissooidean Prososthenia neutra (30.4%) and Fossarulus bulici (23.5%) followed by the previously completely absent Melanopsis lyrata (15.7%).

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A small content of pulmonate snails known already from the underlying coal-bearing succession is additionally present. The latter decrease even further in the middle part of Unit D, which is characterised by the maximum dominance of Melanopsis lyrata (32.2%) (Mollusc sample 3, Figs. 3.2, 3.5 and 3.6).

Figure 3.5. Mollusc taxa identified in quantified samples. (1–4) Melanopsis lyrata Neumayr, Mollusc sample (MS) 3. (5) Radix hyaloleuca (Brusina), MS 1. 6. Fossarulus bulici Brusina, MS 1. (7) Fossarulus buzolici Brusina, MS 1. (8) Fossarulus fuchsi Brusina, MS 2. (9–10) Prososthenia neutra Brusina, MS 3. 11. Gyraulus pulici (Brusina), MS 1. (12) Planorbarius sp., MS 2. (13) Pseudamnicola stosiciana Brusina, MS 1. (14) Bania prototypica (Brusina), MS 1. (15) Gyraulus pulici (Brusina), MS 3. (16) Ferrissia illyrica (Neumayr), MS 2. (17) Orygoceras dentaliforme Brusina, MS 2.

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Taxonomic richness remains high with 9 identified species. Prososthenia neutra (25.6%) and Fossarulus fuchsi (15.6%) are additionally frequent therein. In the topmost part of the unit, monospecific shell accumulations of Mytilopsis frici are strikingly associated with thin coal seams.

Molluscs are still present throughout Unit E, but in its upper part commonly leached, preserved only as moulds. Mollusc diversity decreases in the lower, organic-matter-richer interval. Hence, Mollusc sample 4 (Fig. 3.2) from the base of the brownish rhizolith limestone bears only 3 species; the dominant species here are Fossarulus bulici (67.9%) followed by Radix hyaloleuca (17.9%) and Gyraulus pulici (14.3%) (Figs. 3.5 and 3.6). Most of the mollusc remains present up to the top of the section belong to Fossarulus. With the reappearance of organic-matter-rich sediments at the top of Unit F, large lymnaeid snails known from the base of the Unit B occur again, together with F. bulici and F. buzolici.

3.4.3.2 Palaeoecology

The recorded succession of mollusc assemblages supports the interpretation of the studied interval for a large-scale transgression-regression series. Whereas its lower and upper parts are characterised by ephemeral pond assemblages dominated by pulmonate snails such as Radix and Gyraulus, its middle part shows a completely different, highly diversified fauna dominated by Melanopsis and Prososthenia.

The dominance of Melanopsis and Prososthenia is reminiscent of the Sinj Basin, where both genera are abundant especially in the upper part of the basinal infill (Olujić, 1999; Mandic et al., 2009). The very diverse and fairly well-preserved mollusc assemblage suggests a deposition in a long-lived palaeo-lake environment. The Melanopsis-Prososthenia assemblage indicates agitated shallow-water, open-lake conditions. The assemblage with Radix and Gyraulus is, in contrast, characteristic for wetland and marsh faunas inhabiting small temporal lakes and ponds. It marks the marginal position of the site during low-stand conditions.

The additional species present in the lower part of the succession, such as Unio or Pisidium, on the other hand fit well into the picture of a rather marginal and temporal facies. Nevertheless, the extremely low species richness in Units E and F points to the introduction of strong environmental stress, coinciding with the occurrence of lowermost paludal tufa breccia. The long-lived lake fauna probably became extinct in the lake and did not recover until the end of the studied section. In contrast, Fossarulus, common throughout the section, is highly abundant particularly in Unit E and F. This demonstrates its presence in a pioneer guild of that paleo-lake.

The dominance of Fossarulus and Melanopsis in two separate lithostratigraphic units of the lower part of the basinal infill was already recognised by Milojević (1966) and since than used for its ecostratigraphic division (Fig. 3.2). The present study refines this division in recognising the reoccurrence of Fossarulus and the decease of Melanopsis domination in the upper part of the succession, starting with the Unit E. As discussed above, that division must reflect the change of general palaeoenvironmental conditions from a dominantly low lake-level and arid climate during the Fossarulus-assemblage phase in the lower and upper part of the succession to a dominantly high lake-level humid climate during the Melanopsis-assemblage phase in the middle part of the section (Fig. 3.2).

3.4.4 Geophysical logging

Natural gamma radioactivity as well as magnetic susceptibility is correlated to the amount of detrital material such as clay minerals (Emery and Myers, 1996) in the rock. It allows the evaluation of changes in detrital sediment input into the basin as a function of land denudation. However, volcanic matter (Hardardottir et al., 2001) and radioactive elements potentially bound to organic matter (Jiménez-Moreno et al., 2009; Saracevic et al., 2009) may influence the signal. Usually, however, these disturbances of the detrital signal can easily be recognized due to their point-wise distribution and distinctly higher values than those available from the terrigenous clay component.

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3.4.4.1 Magnetic susceptibility

The change in magnetic susceptibility reflects well the lithological change along the section (Fig. 3.2). The periods of high magnetic susceptibility (MS) are bound preferably to grey limestone intervals. In contrast, coal, tufa-lithoclast breccia and characean limestone (but also volcanic ash intercalations) at the top of the section generally show low MS values. Furthermore, the MS log shows striking correlation with the lithological units described in chapter 4.1. The Units A, C, E and F are characterised by decreased, B and D by increased MS values. In Unit B, the increased values are restricted to the lowermost and topmost part. Unit D, with generally increased MS, has minimum values concentrated at its top, its base and the erosional boundary between the two subunits. Unit F also shows a slight MS increase in the middle part, interrupted by the ash intercalation, and decrease in marginal parts.

The MS increase can be interpreted as reflecting the increased detritus input in the basin during dominantly wetter climate conditions and during lake-level high-stands. Such an interpretation fits well with the MS pattern recorded in the section. The three dominantly coal-producing periods (Unit A, middle part of Unit B and Unit C), representing a marginal lake environment, show decreased MS. In contrast, their boundary intervals (lower and upper part of Unit B), represented by grey limestone intercalations and marking the lake-level rise, show increased MS. As pointed out in Chapter 4.1, these intervals represent condensed marginal facies of transgressional limestone units reaching up to 50 m thickness in the basinal centre. Also, in Unit D, the decreased MS is bound to coal intercalations and to erosional

Figure 3.6. Diagramme showing mollusc distribution in four quantified samples. The major supra-generic taxonomic categories mentioned in the text are indicated.

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surfaces, proving the lake-level fall and emersion there. The lake-level low-stand at the base of Unit F, suggested by the decreased MS, is demonstrated there by the presence of ripple-bedded limestone overlying the tufa-lithoclast breccia at the top of Unit E.

3.4.4.2 Natural gamma radioactivity

The gamma ray (GR) log shows generally higher values in the coal-bearing units (A, B and C), the lower part of Unit D and in Unit F (Fig. 3.2). The strong increase in the latter unit is clearly bounded to ash intercalation, the slight increase above it, in contrast, to organic matter-rich clay. In the lower part, coal shows generally somewhat lower values than the limestone-dominated intervals (lower and upper part of Unit B and lower part of Unit D). The exception is the peak in the coal seam of the middle part of Unit B. The carbonate-dominated interval in the upper part of the section (upper part of Unit D and Unit E) shows generally decreased GR with a minimum in the thickest tufa-intraclast breccia.

The exceptionally increased GR in the coal seam in mid Unit B most probably reflects radioactive matter input to the basin due to a single volcanic event. The capability of organic matter to bind the radioactive isotopes, and occasional tephra fall events, distort the GR in a manner that is difficult to extract from the signal provided by the siliciclastic input to the basin and the lake-level rise events. Nevertheless, the pattern observed in the MS log for the lower part of the section is also present in the GR log. There, the base and the top of Unit B and the lower part of Unit D show an increased GR, suggesting lake-level rise events there. The generally low GR throughout Unit E suggests a relative lake-level low-stand with maximum reached by the thick tufa-lithoclast breccia.

3.4.5 Transgression-regression cycles

The studied succession shows a number of larger and smaller-scale lake-level oscillations represented by striking depositional facies shifts along the section. In a lacustrine setting such oscillations reflect changes of the regional water budget in response to variations in precipitation, evaporation and groundwater level (Cohen, 2003). Consequently, the depositional history of lakes is highly sensitive to the interplay between humid (high-stand) and arid (low-stand) climate conditions.

Integrated data on lithology, carbonate depositional facies, mollusc paleoecology and geophysical logging support the recognition of seven transgression-regression cycles (TRC) for the investigated succession at the ~10-m scale (Fig. 3.2). Especially the relative lake-level low-stands can be pointed out with accuracy. In the lower part of the section they are bounded to coal-dominated intervals (Unit A, middle part of B and C), representing the basin-ward progression of the vegetated lake-fringe. In Unit D they are represented by emersion horizons and erosional boundaries. In the upper part of the section the shallowest depositional environments are indicated by a thick breccia interval in mid Unit E, by ripple-bedded limestone overlying breccia at the base of Unit E and by the coal intercalations at its very top.

Superposed to that basic TRC pattern, a larger, ~40-m scale lake-level fluctuation is present. It shows minimum lake-levels during TRC 1 and TRC 5 and maximum values during TRC 3 and TRC 7 (Fig. 3.2). The minimum lake level in TRC5 starts with the base of Unit E and is marked by an abrupt change of lithology and mollusc content. All point to a severe lake-level fall at its lower boundary. The massive occurrence of rhizoliths is in accordance with that interpretation, indicating semi-arid climate conditions during TRC 5 (Flügel, 2004). Their abrupt disappearance with the onset of TRC 6 is followed upsection by the gradual disappearance of tufa-lithoclast breccia and ever more dominant characean limestones. This points to increasingly open-lake conditions. The reoccurrence of lignite deposition in TRC7 also points to a renewed increase in regional humidity.

3.4.5.1 TRC 1 (10 m)

Its lower boundary is marked by the beige limestone in mid Unit A between two thick coal seams. Its ephemeral-pool mollusc assemblage suggests the lake retreat as a cause for the short-term restriction of coal deposition. Minimum values in both GR and MS logs agree with that interpretation.

The maximum flooding is correlated with the interval dominated by grey limestone located in the

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lower parts of Unit B. Deposition of grey limestone marks the drowning of the vegetated fringe due to the lake-level rise. The increased GR and MS values at this interval agree with this interpretation of carbonate depositional facies. The prolonged open lake deposition is furthermore indicated by thick synchronous limestone deposited in the basin centre.

3.4.5.2 TRC 2 (15 m)

Its lower boundary is marked by the thickest coal seam in the middle of Unit B. This seam represents the longest persistence of the organic matter-producing depositional environment at the lake margin. The maximal flooding is marked by the grey limestone at the top of Unit B. Based on its position immediately below the Main Coal Unit of the lithostratigraphic Gacko Basin division by Milojević (1976) (=Unit C), it correlates with the lake deepening event, which produced thick limestone deposits in the basinal centre. The increased MS and GR also reflect that correlation. The introduction of a Melanopsis-assemblage demonstrates the onset of perennial-lake conditions.

3.4.5.3 TRC 3 (9 m)

The lower boundary coincides with the thickest coal seam of Unit C. This is the most massive and most homogeneous coal layer of the whole investigated section, reflecting possibly a mire or peat depositional environment.

The maximum flooding and installation of high-stand conditions are indicated by the change of the limestone colour from grey to brown. This marks the start of the deposition under suboxic bottom conditions in the period of highest humidity, enhanced detritus input and deepest depositional conditions. The preservation of organic matter at the lake bottom can be explained by a thermocline and/or enhanced primary production in the lake. The intercalated brownish clay layers possibly mark the concentrated detrital material typical for deep lake deposition.

This particular TRC probably marks the deepest depositional conditions in the whole section. This conclusion is strongly supported, in addition to by the previous observations, by highest MS and GR, pointing to highest detrital input into the basin. Furthermore, the strongly diversified mollusc assemblage at this particular interval proves the establishment of a long-lived palaeo-lake in the basin.

3.4.5.4 TRC 4 (8.1 m)

The lower boundary is marked by the erosional surface developed on brecciated grey marls (Fig. 3.3.6). This surface marks the emersion through a lake-level fall. The internal brecciation of the surface was probably enhanced by root action. The maximal flooding surface (MFS) is correlated similarly to TRC 3 with the colour change from light to dark grey limestone. The strong lake deepening, resulting in partial suboxic bottom conditions, is indicated already by the dark nodules below that surface. The placement of the MFS is additionally supported by the increased values of MS and GR.

TRC 4 is characterised by the presence of an extraordinarily thick, monotonous grey limestone interval. Its occurrence up to the basinal margin reflects the strong lake-level rise combined with persistent and stable open-lake conditions within that interval. The persistence of the highly diversified mollusc assemblage also reflects the long-lasting, stable lake conditions.

3.4.5.5 TRC 5 (8.2 m)

The lower boundary coincides with the boundary between Units D and E. It is marked by the erosional surface developed above the coal seam-bearing series. The surface is covered by the tufa-intraclast breccia. This marks the retreat of the open-lake conditions and installation of marginal depositional settings comprising a palustrine environment of temporary, macrophyte-vegetated and carbonate-enriched ephemeral ponds. This coincides with the renewed development of an ephemeral pool mollusc fauna (Fossarrulus-assemblage), which replaces completely the diversified fauna of TRC 3 and 4.

The MFS is again correlated with the change from dark grey into dark brown colour now occurring

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within the rhizolith limestone. As pointed out in chapter 4.2.2 the limestone represents a shallow open-lake setting, with rhizoliths redeposited from the pedogenic, vegetated lake-fringe during the lake-level rise. The increase in organic matter within the limestone presumably corresponds with lake eutrophication as a result of enhanced input of denudation material. The rhizolith abundance in the sediment points to a short transport. Their massive production, restricted moreover to this particular interval, can be interpreted as a maximal aridification event within the studied section.

3.4.5.6 TRC 6 (9.1 m)

The thick tufa-lithoclast breccia marks the lower boundary of this TRC. It represents a long-lasting lake-level low-stand that produced the thickest lithoclast accumulation recorded in the section. The MFS is correlated with the thickest characean limestone-bed, indicating the long-term persistence of open-lake conditions. Absent hypereutrophication of the lake during this cycle reflects the very low primary production in the lake, possibly due to relatively dry climate conditions lasting throughout the interval. Nevertheless, the absence of rhizoliths indicates generally wetter conditions than those characterising TRC5.

3.4.5.7 TRC 6 (11.5 m)

This TRC has its lower boundary in a ripple-bedded characean limestone. Its upper boundary is settled with the intercalated lignite laminae at the top of the section (Fig. 3.3.4), marking the reoccurrence of marginal lake depositional conditions. The MFS is correlated with the thickest limestone interval. It bears characean oogonia and ostracods, indicating a long-term, open lake depositional phase for this interval. The dark brownish, organic matter-rich clays on top mark the termination of the carbonate production and the renewed general humidity increase in this interval. The depositional cycle is disturbed by massive volcanic tuff intercalations in its middle part.

3.5 Chronology

3.5.1 Magnetostratigraphy

The intensities of this characteristic component range between 1×101 and 1.5×103 mAm−1. No gyroremanence is observed and, after application of a 100 mT field, the remaining NRM of most samples is negligible. In the upper half of the section a normal low-field overprint is removed between 0 and 32 mT, and a reversed high-field component is demagnetised at higher fields. In the lower, coal-dominated part of the section, all samples reveal normal polarities. The inclinations of the normal polarity component above 32 mT correspond to the inclination of the present-day geomagnetic field at the Gacko site. These directions are therefore interpreted as a secondary overprint. The inclinations of the reversed components are strikingly lower, but in very good agreement with the results from the Sinj Basin (Leeuw et al., 2010). These reversed directions are interpreted to be of primary origin, which implies that the upper part of the Gacko succession was deposited during a reversed polarity chron (Fig. 3.7).

Figure 3.7. Palaeomagnetic results for the Gacko section plotted in equal area diagrammes as well as in stratigraphic order. Between 40 and 67 m the magnetic signal is characterised by reversed directions, interpreted to be of primary origin. The tilt corrected (tc) and non tilt corrected (ntc) Zijderveld diagrammes for Ga18 are typical demagnetisation diagrammes for samples from this part of the section. The lower, coal-dominated part of the section generally shows normal directions. The Zijderveld diagrammes of sample Ga3 are typical for this interval. The tilt corrected (tc) and non tilt corrected (ntc) display both the normal and reversed directions. The red circle represents the present-day field direction at the location of the section. The normal directions clearly have a higher inclination than the reversed directions and are moreover statistically indistinguishable from the present-day field. Therefore, the normal directions are interpreted to be an overprint.

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The results of the 40Ar/39Ar total fusion experiments of the Gacko samples 1 and 2 are given in the online supplementary material. The weighted mean plateau age of 15.36±0.03 Ma for Gacko 1 is equivalent to the weighted mean plateau age of 15.37±0.02 Ma for Gacko 2 (analytical errors only). However, the inverse isochron intercepts deviate from the atmospheric argon ratio of 295.5 and are indicative for excess argon. Therefore, the inverse isochron age are regarded as the best age estimate of the ash layer. A combined inverse isochron (Fig. 3.8) yields an age of 15.31±0.03 Ma and trapped 40Ar/36Ar component of 338±12. The uncertainty increases to ±0.16 Ma when uncertainties in J, the age of the primary standard and decay constants, as reported in Kuiper et al. (2008) and Steiger and Jäger (1977) respectively, are included.

3.5.3 Correlation to the GPTS

Potential options to correlate the observed interval of reversed polarity in the middle to upper part of the section are C5Bn.1r (14.877–15.032 Ma), C5Br (15.160–15.970 Ma) or C5Cn.1r (16.268–16.303 Ma: Lourens et al., 2004). When taking the 40Ar/39Ar age of the exposed volcanic ash layer into account, it is clear that it can be exclusively correlated to chron C5Br. This means that the deposits between 40 m and 67 m of the section must have accumulated between 15.974 and 15.160 Ma (Lourens et al., 2004), although the exact start and end of sedimentation cannot be determined based on that result. This implies that Lake Gacko developed during the early Langhian, corresponding to the final stages of the Miocene Climatic Optimum. In the stratigraphic terminology of the Central Paratethys, it correlates to the early Badenian (Piller et al., 2007).

3.6 Astronomical tuning

The inferred superposition of a ~10-m scale on a ~40-m scale transgression-regression depositional cycle suggests the changes in lake-level might be orbitally paced and are potentially controlled by ~100-kyr and ~400-kyr eccentricity cycles. To test if this assumption is robust, the MS and GS records were subjected to a spectral analysis in the depth domain, aiming to better visualise their rhythmicity and amplitude modulations (Fig. 3.9).

3.6.1 Spectral analysis

The Lomb periodogram for the MS data series shows a single significant power interval with two peaks corresponding to cycle thicknesses of 11.3 m and 8.6 m (Fig. 3.9). The significant power interval is even broader in the GR data and covers the 16.8 m to 5.6 m range. Within this range, a single high power-peak is detected at a cycle periodicity of 8.4 m. These spectral power distributions imply the presence of a ~10-m-scale sedimentary cyclicity in the section and corroborate the inferences based on the depositional facies analysis.

Gaussian band-pass filtering was then used to extract the frequency component of the most significant power-peaks in the Lomb periodogram. Band-pass filters were centered on the peaks and their band-width was defined according to the width of the intervals retrieved in the Lomb periodogram. Filtering of the cycle periodicities of the 8.4 m GR peak with a band-width of 16.8–5.6 m, and filtering of the 11.3 m MS peak with a band-width of 16.5–8.6 m, reveals a close correlation with the inferred TRC cycles (Fig. 3.10).

The filtered GR log shows two intervals in which it deviates from the TRC cyclicity (dashed line in Fig. 3.10), both related to extraordinarily strong natural gamma radioactivity input. As pointed out in chapter 4.4.1 these GR peaks are bounded to volcanic tuff fall into the basin. Since they coincide with minima in MS values they cannot be related to increased detrital input. Very low intensities in the MS signal cause a resolution problem in TRC cycles 5 and 6 (dashed line in Fig. 3.10). However, this low-intensity interval reflects a phase of minimum detrital input into the basin, which suggests a relatively low lake-level.

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3.6.2 Periodic changes in eccentricity as forcing factor of the observed lake-level variations

Prominent ~100-kyr a1nd ~400-kyr eccentricity-forced climate variability characterises the Miocene Climatic Optimum between 16.9 and 14.7 Ma (Holbourn et al., 2007). Continental lacustrine records in central Spain for the corresponding time interval provide clear evidence of eccentricity control on the depositional environment (Krijgsman et al., 1994; Abels et al., 2010), and in the central Mediterranean, climate variability recorded in marine records reflects the eccentricity cycle (Abels et al., 2005).

Adoption of eccentricity (~100-kyr and ~400-kyr) as the main forcing factor for the transgressive-regressive cycles in Lake Gacko implies a mean sedimentation rate of ~0.1 m/kyr for the studied section. Based on lithostratigraphic correlation with the units established by Milojević (1966), the studied succession can be correlated to a minimum interval of about 210 m in the centre of the basin. The minimum sediment accumulation rate for the central part of the Gacko Basin would thus amount to ~0.3 m/kyr. This is in good agreement with accumulation rates obtained in the Miocene Lake Sinj of the DLS (de Leeuw et al., 2010).

Several other lignite-bearing lacustrine successions in south-eastern Europe also express a sedimentary cyclicity indicative of orbitally controlled lake-level fluctuations (Van Vugt et al., 1998, 2001) Lignite-marl alternations in the Late Miocene to Early Pliocene Ptolemais coal mine in northern Greece were for example shown to predominantly reflect precessional forcing (Van Vugt et al., 1998). A ~21-kyr precessional forcing for the ~10m thick cyclicity of the Gacko Basin would, however, imply an

Figure 3.8. Inverse isochron diagramme of multiple grain 40Ar/39Ar fusion experiments on Gacko 1 and 2 feldspar. The inset shows the data in more detail. The solid line is the isochron line, the dashed line is the reference line representing the plateau age (15.37±0.16 Ma) and an 40Ar/36Ar intercept value equal to modern atmosphere (295.5), and the ellipses represent the 1σ analytical error.

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accumulation rate of ~1.3 m/kyr. Such a high accumulation rate is only probable in basins with a strong fluvial input, which Gacko is not.

Since strongly eccentricity-forced climate variability characterises the MCO and has already been shown to exert a strong influence on contemporary continental lacustrine environments in Spain, it is the most likely forcing factor for the transgression-regression cycles of Lake Gacko. The resulting sedimentation rates are in good agreement with previously published sedimentation rates elsewhere in the DLS.

3.6.3 Correlation to the astronomical curves

Detailed microfacies analyses of the Spanish Late Miocene successions showed that minima in the ~100-kyr and ~400-kyr eccentricity amplitudes correspond to prolonged dry climate periods and lake-level falls (Abels et al., 2008). This supports the correlation of humid regional climate phases to intervals with maximum eccentricity values in the astronomical target curve.

Despite its substantial uncertainty, the 40Ar/39Ar isotope age of 15.31±0.16 Ma provides a first-order correlation point that ties the upper part of the section to the increase of the contemporary ~400-kyr cycle (Fig. 3.10), in agreement with the inferred increase of humidity from TRC 5 to TRC 7. The ~10-m depositional cycles of presumed ~100-kyr eccentricity origin can consequently be correlated to the astronomical curve.

The amplitude modulation pattern of the band-pass filtered component of the GR curve for the interval between TRC 3 to TRC 6 shows a striking correlation to the modulation pattern of the ~100-kyr eccentricity curve between −15.261 and −15.648 Ma. Thereby, the aridification event (with extended caliche building and thick rhizolith accumulations constricted to TRC 5) corresponds precisely with the minimum of ~400-kyr eccentricity values. Consequently, the best fit correlation to the astronomical target curve indicates that the studied section has been deposited between 15.8 and 15.2 Ma.

3.7 Discussion

3.7.1 Dinaride Lake System mollusc phylostratigraphy revised

Based on the presence of a phylogenetically progressive endemic mollusc fauna, the Gacko Basin was regarded as belonging to the younger DLS stage by Kochansky-Devidé and Slišković (1978) (Fig. 3.11).

Figure 3.9. Spectral analysis in depth domain of magnetic susceptibility (MS) and gamma ray (GR) logs, calculated with PAST programme (Hammer et al., 2001). Vertical dashed line marks the non-significant part of the diagramme with frequencies beyond the four-cycle bandwidth. Horizontal dashed lines mark the 0.01 and 0.05 significance levels.

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Figure 3.10. Astronomical tuning of the depositional transgression-regression cycles inferred for the Gračanica section to the ~100-kyr and ~400-kyr eccentricity curves of La2004 (Laskar et al., 2004). The GR and MS records are additionally shown with their filtered components in depth domain (see text for discussion). Band-widths of the filtered records are according to their power spectrum illustrated in Figure 3.9.

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The presented biostratigraphic and cyclostratigraphic data confirm this hypothesis. Correlation of the studied section to the upper part of the Lučane section in the Sinj Basin (De Leeuw et al., 2010) indicates that the M. frici-bearing horizon in the Gacko Basin postdates the common last occurrence of M. drvarensis in the Sinj Basin and predates the first occurrence of the still more advanced M. aletici. Although the timing of the appearance of these three Mytilopsis species is now clear, the evolutionary mode in the clade remains uncertain because M. frici has not been found in the Sinj Basin yet. Both options – the iterative evolution of two species in different basins, as well as the gradual evolution of M. drvarensis into M. aletici through M. frici – are still possible.

3.7.2 Dinaride Lake System palaeo(bio)geography revised

As indicated above, the evolutionary history of the DLS is generally considered to comprise two successive stages (Kochansky-Devidé and Slišković, 1978). The older phase is characterised by the occurrence of phylogenetically primitive dreissenids, while during the younger phase more progressive dreissenids developed. Strata with primitive dreissenids are present in nearly all lacustrine basins of the Dinarides. It is thus thought that the DLS extended over the entire Dinarides and even into the southern part of the Pannonian Basin during its older phase. Evolutionarily progressive dreissenids are exclusively found in the south-western basins, whereas marine strata cover lacustrine sediments of the first DLS phase in the north-eastern basins. This led to the hypothesis that during the younger phase, named Lake Herzegovina by Krstić et al. (2003), the DLS retreated south-westwards in response to flooding of the northern-eastern lakes by the Paratethys Sea.

The currently available chronologic data, however, do not support this hypothesis. Flooding of the north-eastern lakes by the Paratethys occurs at, or slightly after, 14.9 Ma because the first marine sediments contain calcareous nannoplankton indicative of the NN5 zone (Ćorić et al., 2009). The new chronostratigraphic and cyclostratigraphic data for Lake Gacko and Lake Sinj, two of the main constituents of Lake Herzegovina, indicate that both had already disappeared by the time of marine flooding.

Therefore, the absence of evolutionarily progressive dreissenids such as Mytilopsis frici and Mytilopsis aletici north of the Gacko – Kupres – Šipovo line, i.e. from the part of the DLS stretching from the Sarajevo Basin to the Pannonian Basin, must be explained by other palaeogeographic processes. This might include an early presence of a watershed or the presence of a north-south climatic gradient. Such a gradient was already shown to exist in the Middle-Miocene Paratethys (Harzhauser et al., 2003). Future efforts, including stable isotope research, should point out the exact cause for the lack of an advanced assemblage in the northern DLS basins in the period between 16 and 15 Ma.

3.7.3 Impact of the MCO on the Dinaride Lake System

After the Mi1 Glaciation that struck the Earth in the earliest Miocene, global temperatures started to rise. They peaked during the Middle Miocene Climatic Optimum (MCO), which lasted from 16.9 to 14.7 Ma (Zachos et al., 2001) (Fig. 3.11). The rising temperatures had a large impact on European ecosystems. Coral reefs, crocodiles, taxodiacean and mangrove vegetation spread northwards into the Central European epicontinental Paratethys Sea (Harzhauser et al., 2003), and ectothermic vertrebrates thrived in the Alpine Foreland Basin (Böhme, 2003). Lake Gacko arose during the high times of the MCO (Fig. 3.11), characterised by minimum ice cap volume and increased greenhouse gas levels (Zachos et al., 2001; Holbourn et al., 2007). This suggests that the optimum climatic conditions stimulated lake formation in the Dinarides. Apparently, the global rise in temperature induced a critical change in the regional evaporation-precipitation balance. The strong expression of eccentricity forcing that accompanied the MCO (Holbourn et al., 2007) expressed itself through major fluctuations in the water budget of Lake Gacko and resulted in cyclic changes in lake-level.

The presented cyclostratigraphic results demonstrate that the palaeoenvironment of Lake Gacko was very sensitive to climatic fluctuations. The coincidence of the rise and disappearance of Lake Gacko with the MCO provides additional evidence of the climate dependence of the DLS. In contrast, the preservation of lacustrine deposits in the geological record depends on the creation of sufficient accommodation

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space. A severe extensional regime reigned in the Pannonian Basin during the Early and Middle Miocene and might have triggered the formation of basins in the Dinarides (Ilić and Neubauer, 2005). This beneficial geodynamic regime provided the depressions in which the DLS settled and provided sufficient accommodation space to preserve its deposits.

3.8 Conclusions

The optimum climatic conditions of the Middle Miocene stimulated formation of a perennial lake in the Gacko Basin. This palaeo-lake mirrored cyclic variations in the ~100-kyr and ~400-kyr eccentricity of the Earth’s orbit, known to have profoundly influenced the Middle Miocene climate. The resulting regional variation in hydro-climate induced recurrent changes in its sensitive palaeoenvironment. A detailed study of the lithological, palaeontological and geophysical information locked up in its lacustrine record documents these cyclic variations.

The lignites in the lower part of the basin infill indicate a vast swamp environment dominated by taxodiacean forests that extended across the whole basin. In response to the changing climate, lake-level rose, resulting in periodic swamp disintegration and marl deposition. Eventually, lignite deposition ceased and a perennial lake developed. This resulted in thick carbonate deposits in the middle part of the succession. Well-diversified endemic mollusc assemblages characterise this long-lived lacustrine environment.

The subsequent appearance of palustrine carbonate breccias and rhizolith limestones indicates the onset of a striking aridification phase with largely decreased lake-levels. Swashes caused reworking of the lake’s vegetated rim and temporary marginal ponds. Going upwards in the section, these facies types gradually vanish, and a thickening upward sequence of characean limestone beds characterises the uppermost part of the section. This indicates a lake-level rise inducing longer periods of open-lake conditions. The low-stand period with organic-rich, swamp-related sediments at the very top of the section suggests the return of a predominantly arid climate.

Figure 3.11. Stratigraphic position of the Lake Gacko within the Dinaride Lake System geochronology (see text for description) and its correlation with the global climate change (Zachos et al., 2001).

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Seven ~10-m-scale transgression-regression cycles characterise the studied section. Using the 15.31±0.16 Ma 40Ar/39Ar age for a volcanic ash layer in the upper part of the section as a tie-point, these small-scale cycles are tuned to the ~100-kyr cycles of the eccentricity curve. This correlation indicates the investigated sediments accumulated between ~15.8 and ~15.2 Ma, i.e. during the Early Langhian. The reversed palaeomagnetic polarity of the carbonate-dominated part of the section is correlated to chron C5Br.

The combined chronostratigraphic and cyclostratigraphic data of the present study indicate that the deposits of Lake Gacko are time-equivalent to the upper part of the sedimentary sequence in the Sinj Basin. This confirms previous biostratigraphic correlations based on the similarity of their highly evolved mollusc assemblages. This suggests that the evolutionary stage of the autochthonous molluscs is a powerful tool for regional biostratigraphic correlation.

Acknowledgements

We are highly indebted to Jovan Olujić (Geological Survey Zvornik), who first introduced us to the studied locality. This investigation highly profited from support by Stjepan Ćorić (Geological Survey Vienna), Dragan Mitrović (Geological Survey Zvornik) and Hazim Hrvatović (Geological Survey Sarajevo) and would have been not possible without their organisational help. We would like to thank Hemmo Abels and Fritz Hilgen (both Utrecht University) for fruitful discussions on astronomical tuning. We thank Alice Schumacher, Anton Englert, Franz Topka, Julia Topka and Thomas Neubauer (all Natural History Museum Vienna) and Roel van Elsas (VU University Amsterdam) for technical assistance. OM is indebted to his wife Medina Mandić for support and assistance with the field work. This study is funded by the Austrian FWF Project P18519-B17: “Mollusc Evolution of the Neogene Dinaride Lake System” and contributes to the NECLIME-project. It was furthermore supported by the Netherlands Research Centre for Integrated Solid Earth Sciences (ISES) and by the Netherlands Organization for Scientific Research (NWO/ALW). The article benefited from the critical remarks and suggestions by Frank Wesselingh (National Museum of Natural History Naturalis, Leiden) and an anonymous reviewer.

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Giant trucks carry load after load out of a coal mine in Bosnia and Herzegovina

Chapter 4Magnetostratigraphy and small mammals of the Late Oligocene Banovići Basin in NE Bosnia and Herzegovina

Arjan de Leeuw, Oleg Mandic, Hans de Bruijn, Zoran Marković, Jelle Reumer, Wilma Wessels, Enes Šišić, and Wout Krijgsman

A combined magnetostratigraphic and small mammal investigation was carried out to acquire better age control on the sedimentary infill of the Banovići basin in Bosnia-Herzegovina. Although the Dinarides occupy a crucial paleogeographic position bridging Central Europe and Anatolia no detailed records of its small mammal fauna have been published until now. A rich small-mammal assemblage with over 500 molars was excavated from a section exposing marls and clays just underlying the basins main coal layer. The fauna of this Turija small-mammal locality compares best with the uppermost Oligocene to lowermost Miocene localities from the European MP30/MN1 mammal zones and with Anatolian zone B from central Turkey. A 160 m thick series of lacustrine sediments, overlying the main coal layer in the nearby Grivice section, was sampled for paleomagnetic purposes. The magnetostratigraphic pattern of the Grivice section comprises a long reversed interval, a subsequent short normal interval, and another reversed interval. The most logical correlation of this pattern to the late Oligocene part of the Geomagnetic Polarity Time Scale (GPTS) is to chrons C6Cr, C6Cn.3n and C6Cn.2r (24 to 23.2 Ma). This correlation implies a sedimentation rate of ~20 cm/kyr for the Banovići basin, and an age of approximately 24 Ma for the Turija mammal site. The proposed correlation fits well with other magnetostratigraphically calibrated mammal records in western and central Europe as well as Anatolia, and with the recalibrated ages of 24.95±0.05 Ma and 24.72±0.04 Ma for two basalt flows bracketing the Enspel MP28 site in Southern Germany. The 24–23 Ma lifetime of Lake Banovići coincided with optimum climatic conditions and an elevated global temperature, in analogy with the middle Miocene lakes of the Dinaride Lake System. Our results indicate that a limited exchange of fauna from central Asia to western Europe existed in Oligo-Miocene times.

This chapter is based on: de Leeuw A., Mandic O., de Bruijn H., Marković Z., Reumer J., Wessels W., Šišić, E., and Krijgsman, W. Magnetostratigraphy and small mammals of the Late Oligocene Banovići basin in NE Bosnia and Herzegovina. Submitted to Palaeogeography, Palaeoclimatology, Palaeoecology.

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4.1 Introduction

A series of intra-montane basins formed within the Dinaride orogen after it emerged from the Tethys Sea in the late Eocene to early Oligocene. These basins were settled by lakes which deposits potentially represent a rich source of information about the paleobiogeographic evolution of the Balkan region. Good age constraints are, however, often lacking due to the severely endemic character of the lacustrine fauna and the paucity of magnetostratigraphic and radio-isotopic data. In comparable continental settings small mammal biostratigraphic data often provide the necessary age constraints. The small mammal record of the Dinarides, however, is poorly known, especially compared to other areas in western Europe (Daams et al., 1999; Van Dam et al., 2001; Barberá et al., 1994; Krijgsman et al., 1996a; Garcés et al., 1996) and central Anatolia (Ünay et al., 2003; Krijgsman et al., 1996b). This clearly hampers insight, not only in the biogeography, but also in the geodynamic evolution of the area.

Situated along the southern margin of the Paratethys and connecting Anatolia with the hart of Europe, the Dinarides occupy a paleogeographically very interesting position. Phases of strong endemism in the Paratethys marine record indicate that it was repeatedly disconnected from the Mediterranean by geodynamic or eustatic closure of gateways across the Dinarides-Hellenides and Pontides (Rögl, 1999). It is thus likely that terrestrial fauna was, at least periodically, able to migrate from Anatolia to Central Europe along the southern boundary of this epicontinental sea. Paucity of the small mammal record of former Yugoslavia (e.g. Maridet et al. 2007) currently prevents a detailed insight into the mammal exchange across the Dinaride Land during the Oligo- and Miocene.

We conducted a magnetostratigraphic and small-mammal study on outcrops exposed in the open-pit coal mine of the Banovići basin (for location see Fig. 2.1), aiming to better constrain the age of the lacustrine deposits in Bosnia-Herzegovina. The rich small mammal assemblage of the Banovići basin thus represents a vital contribution to the European paleontological record and forms a new stepping stone bridging the Oligo-Miocene records of Central Europe and Anatolia.


4.2.1 Regional stratigraphic and paleogeographic development

The Banovići basin is a ~12.5 km long and 6 km wide WNW-ESE directed elongated structure (Fig. 4.1). It belongs to a suite of largely isolated basins that were generated along the northeastern margin of the Dinarides after a late Eocene–early Oligocene tectonic phase (Tari and Pamić, 1998). Its basement rocks belong to the ophiolite massif of the Central Dinaridic Ophiolite Zone (Internal Dinarides) and consist of stratified massive peridotite, serpentinite, dolerites, amphibolite and amphibole schists (Sunarić-Pamić et al., 1971; Pamić et al., 1973a,b; Čičić et al., 1991a,b; Tomljenović et al., 2008). The western part of the basin exposes some middle Jurassic to lowermost Cretaceous limestones and siliciclastic rocks. The basin infill is of exclusively lacustrine origin and its basal part is marked as Oligo-Miocene on the geological map. Although a detailed structural geological framework is still absent, the Banovići basin presumably initially formed in the Oligocene in response to dextral transpressional movements on the Sava, Drava, Busovača and Vrbas faults (Hrvatović, 2006). These movements were caused by the oblique collision of the Dinarides-Southern Alps block with the ALCAPA block that also drove displacement on the Peri-Adriatic Fault (Tari and Pamić, 1998). A second phase of basin formation befell the Dinarides in the Miocene, when the Dinaride Lake System spread out across numerous intra-montane basins (De Leeuw et al., 2010; Mandic et al. 2011; chapters 2, 5 and 6).

4.2.2 Description of the basin infill

The infill of the Banovići basin consists of about 500 m of Oligocene to Miocene lacustrine deposits (Muftić and Luburić, 1963; Glišić et al., 1976) that currently extend over 37 km². It is partitioned into W-E elongated, generally southward tilted blocks (Fig. 4.2). The adjacent Seona (W) and Đurđevik basins (E) have a similar infill and might have originally been part of the same basin (Glišić et al., 1976).

87

Chapter 4: Magnetostratigraphy and small mammals of the Late Oligocene Banovići Basin

Ban

ovic

i Bas

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re 4

.1. G

eolo

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l map

(af

ter

Čići

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al.,

199

1a, b;

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arić

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., 19

71;

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73b)

and

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in.

88


The basin infill is divided into two parts on the geological map (Fig. 4.2). The lower part of the basin infill (Ol,M on Fig. 4.2) is about 80 m thick and starts with a fining upward series of conglomerates, sands and marls followed by a thin coal seam. This is overlain by sandy tufa limestone with lymneid snails in turn followed by an interval of greenish clay overlain by an alternation of coals and marls that grade upwards into the high quality, homogenous brown coal of the main coal seam (10–25 m). The top of the main coal layer is sharply defined and planar. The upper part of the basin infill (M1,2 on Fig. 4.2) comprises an up to 350 m thick series of lacustrine limestone, marl and marly sandstone that conformably overlies the main coal layer. Its lowermost 3 m contain abundant continental (Helicidae) as well as lacustrine (Planorbis sp., Bythinella sp., Unio sp., Pisidium sp.) mollusks. In the remaining part scarce fossil plant remains (Glyptostrobus sp.) as well as lacustrine mollusks (Velutinopsis sp., Pisidium sp.) and ostracods (Candona sp) are preserved. Locally up to 80 m of conglomerates, sands and clays with lignite intercalations follow above an erosional hiatus. These are correlated to the upper Miocene of the neighboring Tuzla basin on the basis of their pollen spectra (Muftić and Luburić, 1963).

4.2.3 The Turija Section

The Turija section (Fig. 4.3) is situated in the Turija open-pit mine located at the northern margin of the Banovići basin (GPS–WGS84: N44 25 32.7 E18 26 30.9). It exposes the very top of the strata underlying the main coal layer (Fig. 4.2). The outcrop reveals about 3 m of green clay with amphibolite lithoclasts in its lower part which are overlain by upward thickening coal seams alternating with greenish clay and grayish marls. A level with greenish clay four meters above the base of the section was extensively sampled for small mammal fossils.

4.2.4 The Grivice Section

The Grivice section was measured and sampled along the NNE-SSE striking wall of the quarry of the Grivice mine, situated along the basin margin 6 km east of the Turija section (Fig. 4.2). The base of the section is located at N44 26 04.8 E18 31 11.3 and the top at N44 25 58.6 E18 31 09.8. The section exposes the top part of the main coal layer (4 m) and a major part of the overlying lacustrine strata (163 m). These can be subdivided into three main lithological units: the Limestone Unit, The Limestone and Marl Unit and the Marl Unit (Fig. 4.3).

The Limestone Unit unit is 73 m thick and is dominated by limestones. It comprises 4 subunits with thicknesses ranging from 21–16 m. These start with massive or thickly bedded limestones in their lower part and subsequently grade into banded and thinly bedded limestones in their upper part. Subunit 1 is in direct contact with the main coal layer. It starts with 1.3 m of laminated dark brownish sandy limestone. The sand component diminishes upwards and sediment color changes to dark yellowish. Whitish limestones with dark intercalations follow and grade upwards into banded and bedded dark greenish organic rich limestone. The following two subunits have a similar built-up but lack the dark, organic rich interval characterizing the base of the first subunit. The final subunit bears clayey volcanic ash intercalations in its lower part. These are overlain by strongly laminated grayish limestones. The upper part is extensively banded and bears fossil leaves. Its topmost 4 m are dark in color which reflects a prominent increase in organic material.

The Limestone and Marl Unit comprises 49 m of limestones and marls and is divided into 2 subunits. The first is 23 m thick and limestone dominated, whereas the second is 26 m thick and dominated by marls. Their architecture is similar to the subunits of the Limestone unit with a more massive lower part and a well bedded and lightly banded upper part. This unit contains plant remains and ostracods, the latter mainly in the upper parts of both subunits. A dominantly yellowish sediment color characterizes this unit.

The Marl Unit is 41.5 m thick and predominantly consists of marls (Fig. 4.3) in which well-expressed bedding is absent. Three somewhat thinner subunits can be distinguished. The lower, middle and upper subunits are respectively 16 m, 10.5 m, and 15 m thick. Their lower part consists of marly limestones and limy marls whereas the upper part is dominated by marls. Carbonate content thus decreases from

89


the base to the top. The amount of preserved organic material and the density of ostracods, molluscs, fish and plant remains, on the other hand, increase in the same direction. A channel structure, present in the top part of the first subunit, is filled in with reddish sand bearing mud clasts and a monotypic bivalve shell accumulation with Pisidium sp.

4.3 Magnetostratigraphy of the Grivice Section

4.3.1 Sampling and laboratory methods

The Banovići - Grivice section, was sampled for paleomagnetic measurements in order to acquire a magnetostratigraphy (Fig. 4.3). Samples were collected with a hand-held gasoline-powered drill. In the field, the orientation of all samples and bedding planes was measured by means of a magnetic compass and corrected for the local magnetic declination (3°). The gross sampling resolution achieved was approximately one site every 3 meters. At each site 2 samples were taken. After the first demagnetization results were available a second sampling series covered the intervals 100–110 m and 134–142 m in more detail.

In the laboratory, the obtained cores were cut into several specimens that were subsequently stepwise demagnetized. The natural remanent magnetization (NRM) of the samples was measured after each demagnetization step on a 2G Enterprises DC Squid cryogenic magnetometer (noise level 3·10-12 Am2). Demagnetization was accomplished by a laboratory-built automated measuring device applying 16 5–20 mT increments up to 100 mT by means of an AF-coil interfaced with the magnetometer. The

correlation bed

section trace, lower terrace

section trace, upper terrace

main coal

Banovici - Grivice section Location: N44.435312, E18.520546 (WGS84)

Banovici - Turija mammal site Location: N44 25 32.7 E18 26 30.9 (WGS84)

Turija SectionHemicyoninae sp.

Figure 4.2. Pictures of the Grivice and Turija sections and a stratigraphic column of the Turija section with the mammal site indicated.

90


presence of iron sulfides in the studied samples was anticipated. In order to overcome the problem of gyroremanence during alternating field demagnetization the specifically designed per component demagnetization scheme of Dankers and Zijderveld (1981) was applied. In addition, small (2–5 mT) field steps were taken in the 20–40 mT range. The characteristic remanent magnetisation (ChRM) was identified through assessment of decay-curves and vector end-point diagrams (Zijderveld, 1967). ChRM directions were calculated by principal component analysis (Kirschvink, 1980).

To identify the magnetic carrier(s), high temperature thermomagnetic experiments were performed on a modified horizontal-translation type Curie Balance (Mullender et al., 1993). Bulk sediment samples of BA06 and BA50 were heated up to 700°C in air. At 200, 300, 350, 450, 500 and 600°C samples were cooled a 100°C in order to distinguish thermal behaviour from chemical alterations. Heating and cooling occurred at a rate of 10°C/min. The cycling field varied between 150 and 300 mT.

4.3.2 Rock magnetism

Curie Balance measurements for sample BA06.1, BA50.1, BA08-04.1, and BA08-09.1 are displayed in Fig. 4.5g, h, o, and p respectively. For sample BA06 heating and cooling segments are virtually reversible up to 300°C. Between 300 and 350°C the magnetization decreases and upon cooling it is not restored. This indicates the presence of greigite, which is remarkably s le and does not break down completely into a non-magnetic phase up to 400°C where a local minimum in magnetization is observed. Note that often this minimum is observed at ~350°C, i.e. 50°C lower. The rise in intensity upon heating above 400°C is characteristic for the transformation of pyrite (and of greigite that, however, is marginally present in comparison to pyrite) into magnetite. The creation of magnetite slows down considerably on cooling or is even halted entirely when the sample is cooled below 400°C. The magnetization increases on cooling because the spontaneous magnetization of magnetite increases with decreasing temperature. The formation of magnetite resumes again upon heating as indicated by a steady rise in magnetization between ~420 and 500°C. The increase in magnetization is much steeper on the 500–400°C cooling segment than on the 450–350°C segment which confirms an increased amount of magnetite. The small change in slope in the heating curve at ~580°C, coincides with the ordering temperature of magnetite. The distinctly lower magnetization along the 600-500°C cooling segment with respect to the 500–600°C heating segment indicates that some magnetite oxidized to haematite, which is much less magnetic. After heating up to 700°C, the final cooling curve is located even lower than the 600–500°C cooling segment, which indicates that even more magnetite was oxidized to haematite. However, some magnetite remained present, since the magnetization increases sharply between 570°C and 550°C. The next break in slope in the cooling curve slightly above 300°C is close to the ordering temperature of pyrrhotite and the strong increase in magnetization between 300 and 20°C should thus be attributed to the pyrrhotite created during the experiment. These results indicate the original presence of greigite and pyrite in sample BA06.1, in line with its lacustrine origin.

Greigite is also present in sample BA50.1 since the cooling segments of 300–200°C and 350–250°C are located below the corresponding heating curves, again indicative of the transformation of greigite into a non-magnetic (or less magnetic) phase. The increase in magnetization along the cooling segments of 450–350°C and 500–400°C is less steep than for BA06.1 indicating a lower amount of pyrite. In analogy with BA06.1, magnetite eventually oxidizes to haematite. The final cooling curve is located below the preceding heating and cooling curves since all thermally created magnetite and presumably most (if not all) of the original magnetite is oxidized to haematite. The lack of a clear rise in magnetization upon cooling below 300°C excludes the presence of a significant amount of pyrrhotite which concurs with a more complete oxidation to hematite. Curie Balance diagrams for sample BA08-04.1 and BA08-9.1 are very similar. Hardly any greigite is present. Significant amounts of magnetite are created from pyrite above 400°C and eventually a reasonably significant fraction of the magnetite oxidizes to haematite. No significant amounts of pyrrhotite are being formed during the thermal cycles.

91


0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

(m)

180 360

0 5 10 15 20 5 10 3 10 4 10

declination inclination intensity mad angle

polarity pattern lithology

top

tera

sse

botto

m

tera

sse

(in organic matter)

-90 0 90

Lim

esto

ne

Un

it

Lim

esto

ne

and

Mar

l Un

it

Mar

l Un

it

coal

Figure 4.3. Stratigraphic column and magnetic results for the Banovići Grivice section. The rightmost column shows the derived magnetostratigraphy.

92


4.3.3 Demagnetization and magnetostratigraphy

For all sites at least one, and for the upper two thirds of the section both of the two samples were demagnetized. Overall, demagnetization diagrams are of good quality. The NRM of the Banovići marls generally consists of two components. A low field overprint is typically demagnetized from 5 to 32 mT (e.g. Fig. 4.4b, e) and a well-defined high field component that is removed between 32 and 80 mT is interpreted to be the ChRM (e.g. Fig. 4.4a and 4.4d). NRM intensities typically range between 2 and 50 mAm-1. The low field component decays slightly quicker than the high field component. At 32 mT mostly 50% of the NRM had been removed. At 100 mT often c. 90% of the NRM had been removed. No gyroremanence is observed.

The steep bedding tilt of the investigated strata facilitates isolation of the ChRM. The low field component that appears in demagnetization diagrams typically carries directions that trend to the present day field direction in geographical coordinates and most likely represents a badly expressed recent overprint. The high field ChRM component is very clear and has a reversed polarity in the main part of the section (Fig. 4.3). It bears a normal signature between 132 and 152 m. The directions between 102 and 108 m do neither fit a normal nor reversed polarity. Curie balance measurements indicate these samples have a deviating mineralogy and do not contain the stable greigite that carries the ChRM in the rest of the section. After the first demagnetization results were available, the interval was re-sampled and carefully inspected. However, the newly acquired results confirmed the original measurements and no signs of profound alteration or disturbance were encountered in the field. We regard the directions from this interval as unreliable and not representative of the paleomagnetic field at the time of deposition. The section thus contains two reversed and one normal polarity intervals.

ChRM directions without tilt correction clearly do not cluster near the present day field (PDF) direction (Fig. 4.5a). This indicates they do not reflect a PDF overprint. Upon tectonic tilt corrections the directions from the combined reversed intervals cluster around dec=183.1° and inc=-52.9° using the vanDamme cutoff (Vandamme, 1994) to discard outliers (Fig. 4.5b, Table 4.1). The directions from the normal interval have an average declination of 349.2° and an average inclination of 55.6° (Fig. 4.5c, Table 4.1). When we combine the normal and reversed directions (Fig. 4.5d), we arrive at an average declination of 2.8°, and an inclination of 53.5° (Table 4.1).

4.4 Small mammal taxonomy and biostratigraphy

4.4.1 Material and methods

The fossils of the Turija mammal site were collected from a 20 centimeter thick lignitic clay bed which contains limestone nodules and is located just below the main coal layer (Fig. 4.2). Two tons of sediment were washed and sieved over 0.5 mm sieves in the field. The residue was treated with chemicals in the laboratory of Utrecht, washed and sieved and subsequently sorted, mounted and measured. This fossiliferous layer produced a collection of 295 rodent molars. Beside the rodents, 181 insectivore teeth and molars, 7 marsupial molars and 30 lagomorph molars (207 first and second rodent molars) have been found (Table. 2).

4.4.2 Taxonomy

Ten rodent species belonging to eight genera are recognized (Fig. 4.6). Among these the Muridae (mice and rats) are, with five species of four genera, dominating the assemblage in diversity as well as in number of specimens (80%). The Gliridae (dormice) are with three species of two genera the second group (18%) and the Sciuridae (squirrels) with two genera and two species are rare (2%). The Sciuridae are represented by a few cheek teeth of Palaeosciurus and two teeth of the enigmatic “Ratufa” obtusidens. The allocation of all the Palaeosciurus teeth to one species is questionable because of differences in size and morphology between the upper molars, but the number of specimens is too limited to allow the recognition of two species with confidence. Paleosciurus aff. feignouxi from Banovići

93


af /

tc N

up/W

d)

9.91

m

BA

06.1

af

/ no

tc

N

up/W

e)

0

25

100

32

0

25

100

32

020

4060

8010

00

0.2

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rial

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ied

alte

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field

(mT)

normalized intensityf)

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tc

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a)

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32 m

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af

/ no

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b)

0

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100

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c)

Magnetization ( Am2/kg )

BA

50.1

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ture

( C

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ture

(C

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06.1

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m

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0.00

0.03

300

400

500

600

700

Tem

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ture

( C

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9.1

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k)

appl

ied

alte

rnat

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field

(mT)


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rial

2040

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BA

8-04

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/ tc

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/Waf

/ no

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) n)

Abs

olut

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rial

appl

ied

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(mT)


020

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0.4

0.6

0.81

BA

8-09

.1af

/ no

tcN

up/W

af /

tc

107.

63 m

N

up/W

o)

0.00

0.02

Magnetization ( Am2/kg)

Tem

pera

ture

( C

elsi

us )

BA

08-0

4.1

104.

7530

040

050

060

070

030

040

050

060

070

030

040

050

060

070

0

Figu

re 4

.4. D

emag

netiz

atio

n, d

ecay

and

Cur

ie B

alan

ce r

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ts fo

r fo

ur s

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om t

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re n

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cted

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In

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ram

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he n

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ft c

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h di

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entifi

es th

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ratig

raph

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e in

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ion.

Fie

ld s

tren

gths

indi

cate

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ong

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ram

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ms

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e’ in

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tes

the

mea

sure

d in

tens

ity p

lott

ed a

s fu

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ivis

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indi

cate

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e no

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l dec

ay b

etw

een

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.

94


a) b)

n = 58 (71)dec = 183.1 inc = -52.9α95 = 2.1A95 = 2.6k = 77.9

c)

n = 19 (19)dec = 349.2 inc = 55.6α95 = 7.2A95 = 8.4k = 22.6

d)

n = 70 (77)dec = 2.8 inc = 53.5α95 = 1.9A95 = 2.3k = 79.8

Figure 4.5. Paleomagnetic directions for the Grivice section in equal area diagrams. For detailed description see text.

Banovici Grivice N n D I ΔDx ΔIx k α95 K A95R Directions 71 58 183.1 -52.9 3.1 2.7 77.9 2.1 53.3 2.6N Directions 19 19 349.2 55.6 10.4 8.2 22.6 7.2 16.9 8.4R & N Combined 77 70 2.8 53.5 2.8 2.4 79.8 1.9 54.7 2.3

Table. 4.1. Paleomagnetic results for the Grivice section.

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and those from southern Germany (Werner, 1994) represent a new species that is intermediate in size between the smaller P. goti and the larger P. feignouxi. Given the morphological resemblance of the teeth of all the species of this genus and the paucity of the material from Banovići we do not name this species formally, but allocate it to P. aff. feignouxi.

The Gliridae are represented by two species of Bransatoglis and a species of Microdyromys that shows an unusually wide range of variation in dental pattern. The lower molars of Bransatoglis n. sp. have three extra ridges between the mesolophid and the posterolophid. This clearly distinguishes the lower molars from those of B. concavidens and B. spectabilis. Bransatoglis fugax resembles the material from the late Oligocene type locality Coderet-Bransat better than the B. cf. fugax from the early Miocene of southern Germany (Werner, 1994). Microdyromys cf. monspeliensis has in some teeth extra ridges that do not occur in the type material of Microdyromys monspeliensis. In this respect the sample from Turija is very similar to Microdyromys monspeliensis from the early Miocene of southern Germany (MN2; Werner, 1994). The dental pattern of some teeth is reminiscent of those of Glirulus diremptus (Mayr, 1979), which suggests that the assemblages from Turija and southern Germany possibly document populations that are transitional between Microdyromys and Glirulus.

The Muridae are represented by one species of Deperetomys, one species of Mirrabella, one species of a new spalacid genus and two species of primitive Eumyarion. Deperetomys n. sp. is the largest and probably oldest member of the genus. Although there remains uncertainty about the identification of primitive and derived dental characteristics in Muridae, because of parallel and convergent evolution in this very diverse family, we interpret the large, unreduced third molars of this Deperetomys species as primitive and the incipient development of the X-pattern formed by the protoconid/entoconid and hypoconid/metaconid connections in the first lower molar, that is characteristic for younger species of Deperetomys, as derived. Mirrabella aff. anatolica is possibly the geologically oldest record of the genus Mirrabella. They differ from the type material of M. anatolica in a number of details that do not leave any doubt that it documents a different species. The material from Banovići shows a number of primitive murid characteristics that are modified in the species from Anatolia, yet these teeth are more robust and the upper third molar is more reduced. The material does therefore not fit the evolutionary trend reconstructed on the basis of the various species from Anatolia and Greece. We refrain from formally naming this species from Turija, because not all the tooth positions are represented in our collection and the variation in dental pattern is inadequately known.

The subfamily Eumyarioninae has the most diversified record among the murids from Turija. In particular Early Oligocene Atavocricetodon (Freudenthal, 1996) species from Spain have dentitions that are not essentially different from the Eumyarion from the earliest Miocene of Turkey (De Bruijn and Saraç, 1991) and Bosnia and Herzegovina. This suggests that the hiatus between Eucricetodon (Atavocricetodon) and the true Eumyarion, which is an immigrant into Europe (MN4) is filled in the eastern Mediterranean area. At the same time this raises the question of how to distinguish these two genera. We tentatively classify the two species from Turija in Eumyarion (E. n. sp. and E. microps). Among the primitive Eumyarion species, which have slender, “not inflated”, cusps these new Eumyarion species are on average somewhat larger than E. carbonicus and E. montanus, but there is overlap in size. The first upper and lower molars of Eumyarion n. sp. are shorter relative to the second upper and lower molars of the latter two species. The crescent-shaped anterocone, weak protoloph and usually strong paracone spur of the first upper molar makes this tooth readily distinguishable from this element of all other Eumyarion species. The second upper molar with its strong lingual branch of the anteroloph, often double protolophule and strong posterior spur of the paracone is quite characteristic also. The short posterior arm of the protoconid that is connected to the metaconid in the lower first molar is a characteristic that is shared with E. carbonicus and E. montanus, but these species have much stronger posterior arms of the hypoconid in all lower molars. Eumyarion microps from Turija has cheek teeth which are on average somewhat larger than the ones from the type locality (Harami 1, Turkey, De Bruijn and Saraç, 1991), but there is considerable overlap. There are some subtle differences in dental pattern between the two associations, but these are all a matter of degree and do not warrant the distinction of

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a different species.Finally Spalacinae n. gen. n.

sp. is readily distinguishable from Spalax, Pliospalax and Heramys by the presence of a large number of primitive murid dental characteristics. Most striking among these is that the cusps are not incorporated into the lophs in unworn molars. The dentition of this small new spalacid species shows a mixture of original and derived characteristics. The small bifid, labially situated, anterocone of the upper first molar and the peculiar metaconid/anteroconid complex of the lower first molar are interpreted as primitive for Spalacinae and make these teeth very different. Other primitive characteristics that distinguish this new genus and species are that the first and third lower molars have the same length and that the lingual sinus of the upper third molar is open. In contrast, the posteriorly directed metalophs of the majority of the upper first and second molars and the development of an s-pattern in the upper and lower second molars through wear as in geologically younger members of the subfamily are interpreted as derived. The evolutionary history of the Spalacinae is probably much more complex than has been suggested so far. This new species is the smallest, and probably geologically oldest, member of the Spalacinae so far, but its phylogenetic position within

the subfamily and the identity of the ancestor of the subfamily among the Paleogene Muridae remain unresolved.

4.4.3 Biostratigraphy

The biostratigraphical correlation of the rodent assemblage from Turija depends necessarily on comparison with faunas from central Europe and Anatolia, because our knowledge of the local succession is very limited. Moreover, the Oligocene/Miocene rodent fauna from Kazakhstan, situated along the east coast of the Paratethys (Bendukidze et al., 2009), appears to be very different from the fauna of the Dinarides, situated along the west coast. Surprisingly Eomyidae are absent in the assemblage from Turija. This hampers a straightforward correlation with the European sequence, but six out of the ten rodent species recognized in Turija are known from Europe and/or Anatolia. Fig. 4.7 gives the stratigraphical ranges of these species relative to the European MP/MN scheme and the preliminary Anatolian zonation (Ünay et al. 2003).

This convincingly shows that the best fit of the assemblage from Turija is with MP30 in Europe and

Order Rodentia Family Sciuridae Palaeosciurus aff. P. feignouxi Pomel, 1853 ?Ratufa obtusidens Dehm, 1950 Family Gliridae Sufamily: Bransatoglirinae Bransatoglis n. sp. Bransatoglis fugax Hugueney, 1967 Subfamily Dryomyinae Microdyromys cf. monspeliensis Aguilar, 1977 Family Muridae Subfamily incertae sedis Deperetomys n. sp. Subfamily: Eumyarioninae Ünay-Bayraktar, 1989 Mirrabella aff. anatolica (de Bruijn & Saraç, 1992) Eumyarion n. sp. Eumyarion microps Subfamily Spalacinae Gray, 1821 Spalacinae n. gen. n. sp. Order Insectivora Family Soricidae Subfamily Crocidosoricinae 2 Crocidosoricinae species Family Heterosoricidae sp. 1 (cf Quercysorex Engesser, 1975) sp. 2 (cf Heterosorex Gaillard, 1915) Family Talpidae 2 Talpidae species Family Erinaceidae sp. 1 (cf Amphechinus Aymard, 1850) Family Dimylidae 1 Dimylidae speciesOrder Lagomorpha Family Ochotonidae cf. Sinolagomys Bohlin, 1937 (?Bohlinotona de Muizon, 1977)Order Marsupialia Family Herpetotheriidae ? Amphiperatherium Filhol, 1879

Table. 4.2. Faunal list showing the complete small mammal record of the studied samples.

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Figure 4.6. Biostratigraphically relevant rodent taxa recorded from Banovići Turija samples. 1-3: Palaeosciurus aff. feignouxi, 1: D4, 2: M1-2, 3: M3. 4: Ratufa obtusidens, M3. 5-12: Bransatoglis fugax, 5: P4, 6: M1, 7: M2, 8: M3, 9: p4, 10: m1, 11: m2, 12: m3. 13-20: Microdyromys cf. monspeliensis, 13: P4, 14: M1-2, 15: M1-2, 16: M3, 17: p4, 18: m1, 19: m2, 20: m3. 21-23: Mirrabella aff. anatolica, 21: m1, 22: m2, 23: M2. 24-29: Eumyarion microps, 24: M1, 25: M2, 26: M3, 27: m1, 28: m2, 29 m3.

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zone B in Anatolia. This correlation is further supported by the presence of the new Bransatoglis species, which resembles B. concavidens (MP30), the new Deperetomys species which is more primitive than D. intermedius from zone C, Eumyarion n.sp. that resembles E. carbonicus from zone C and the new spalacid that is in many respects more primitive than the hitherto oldest spalacid Debruijnia arpati from zone D.

On the basis of the rodent fauna we conclude that the Turija locality is somewhat older than locality Harami 1 (Turkey, zone C), which has been correlated to chron C6Bn.2n of the magnetic polarity time scale by Krijgsman et al. (1996b). The rodent assemblage from Turija seems to be somewhat younger than that from Coderet (France), Late Oligocene (MP30) (Hugueney, 1969, Schmidt-Kittler, 1987). We thus interpret the

Turija assemblage as top MP30/base MN1, even though the scarcity of comparable faunas, the lack of Eomyidae and the presence of new species hamper exact correlation.

4.5 Chronology for the Banovići basin

The MP30/MN1 small mammal assemblage encountered in the lowermost part of the main coal sequence indicates a late Oligocene to early Miocene age. This contrasts former late Early Miocene age inferences based on pollen spectra and lithostratigraphic correlations with the Sarajevo and Livno basins (Muftić and Luburić, 1963; Glišić et al., 1976). The fauna encountered in the Turija section is older than that of the Harami 1 locality in Anatolia (Krijgsman et al., 1996b), and the mammal bearing level is thus older than chron C6Bn.2n. This provides a numerical minimum age of approximately 22.2 Ma. The acquired magnetic polarity pattern for the Grivice section, with a long reversed interval, a subsequent short normal interval, and another reversed interval, can best be correlated to the late Oligocene interval of chron C6Cr, C6Cn.3n, and C6Cn.2r of the GPTS (24–23.2 Ma) (Fig. 4.8). According to this correlation, the Turija mammal locality is around 24 Ma old.

The main lacustrine phase of Lake Banovići thus started around 24 Ma. Whereas in more central parts of the basin over 300 m of lacustrine deposits accumulated above the main coal horizon, their thickness decreases to about 200 m or less towards the basin margin (Glišić et al., 1976). Our correlation of the magnetostratigraphic pattern of the ~160 m long Grivice section to the 24–23.2 Ma interval of the GPTS implies a sedimentation rate of approximately 0.2 m/kyr. Extrapolation of this sedimentation rate to the remaining 40 m of sediments results in a 23 Ma age for the topmost lacustrine sediments. This suggests Lake Banovići disappeared at the onset of the Miocene, roughly coincident with global cooling due to the incipience of the Mi1 Glaciation (Zachos et al., 2001).

The strong affinities of the Turija fauna with assemblages from both Anatolia and Central Europe indicate that the intra-montane basins of the Dinarides comprised an important paleogeographic position for Asia-Europe mammal migration during the Oligo-Miocene period (Fig. 4.9). This is in good

Figure 4.7. The ranges of a number of small mammal species in the MP/MN mammal zones of Europe and local zones of Anatolia.

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100


agreement with earlier paleogeographical reconstructions that indicate the presence of a land-bridge along the southern margin of the Paratethys Sea in late Oligocene times (e.g. Rögl, 1999).

4.6 Discussion

4.6.1 The Turija Hemicyoninae (bear-dog) fossils

In the upper part of the Turija section, 1 m above the small mammal bearing layer, a lower jaw of a bear-like animal, identified as Hemicyon cf. stehlini (Vrabac et al. 2005), was found by mine-workers. Hemicyon is known from many European MN3–MN7/8 localities (Agusti et al., 2001). This identification thus suggests a Burdigalian-Langhian age for this level. If the determination of these bear-like remains is correct, a hiatus of over 4 Myr in the upper meter of the Turija section must be present. The magnetostratigraphic pattern of C6Cr-C6Cn.2r is not unique and the Grivice pattern can for instance also be correlated to younger intervals of chron C5Dr2r, C5Dr1n, and C5Dr1r (17.9–17.7 Ma), or C5Br, C5Bn2n, and C5Bn1r (16.0–14.9 Ma). There are, however, no indications for such a profound hiatus in the coal deposits in the Turija section. In combination with the stratigraphically lower small mammal fauna, we thus conclude that either Hemicyon has a larger biostatigraphical range or that this lower jaw belongs to a different taxon.

4.6.2 Implications for mammal chronology

Our correlation of the Turija MP30/MN1 site with the base of C6Cr (Fig. 4.8) follows the magnetostratigraphic correlation of the Anatolian Inkonak and Harami 1 sites (Krijgsman et al., 1996b). These fall in local zones A and C which correlate to MP30 and MN2 respectively. The Turija fauna is most similar to the Kargi 1 and Kilcak 0-3b of upper zone A, lower zone B and its age is thus in close correspondence with the age of similar fauna in Anatolia. In the Ebro basin, several mammal sites are magnetostratigraphically calibrated (Agusti et al., 2001; Gomis Coll et al., 1999). Correlation of the two MP30 sites in the Torrente de Cinca section in the Ebro basin by Agusti et al. (2001) to C6Cr is in good agreement with our age for the Turija site.

We are hesitant to adopt the age intervals listed by Mertz et al. (2007) for the MP29 and MP30 reference levels because they are based on interpolation of equal time-intervals between the top of MP28 and a disputed 24.0±0.1 Ma absolute age for the Oligocene-Miocene boundary by Wilson et al. (2002). The astronomically tuned age of this boundary in the Gradstein et al. (2004) timescale is 23.03 Ma. In addition, the MP30/MN0 zone is often mentioned to straddle the Oligocene-Miocene boundary which also excludes the age of this boundary as a good upper limit on the age of the MP30/MN0 zone.

The only absolute (40Ar/39Ar) ages, currently available for upper Oligocene mammal localities, come from the two basaltic flows that intercalate with the MP28 fauna of the Enspel fossil deposits in southern Germany (Mertz et al., 2007). We recalibrated the 24.79±0.05 Ma and 24.56±0.04 Ma ages for the Enspel Lower and Upper flow, according to the recently calculated astronomically tuned age of the Fish Canyon Tuff (Kuiper et al., 2008), and arrive at 24.95±0.05 Ma and 24.72±0.04 Ma, respectively. The flows that are interpreted to bracket the lacustrine sediments at the Enspel site that contain the eomyids relevant for the MP28 correlation are thus 0.16 Ma older than previously thought. The Enspel MP28 site is consequently between 24.95±0.05 Ma and 24.72±0.04 Ma old (Fig. 4.8).

The absolute ages for the Enspel basalts and our correlation of the Turija site indicate that, as earlier suggested by Agustí et al. (2001), the correlation of magnetostratigraphically investigated sections in the Swiss Foreland Basin (e.g. Schlunegger, 1999; Schlunegger et al., 1996, 1997a,b, 2007; Kempf et al., 1997, 1999; Kempf and Pross, 2005; Struck and Matter, 2002) has to be revised. These authors placed MP28 fauna in C7n and the lower half of C6Cr, and MP29 exclusively in C6Cr. This correlation introduces a large diachrony in the record, with ages that are about 1 Ma younger than Enspel radio-isotopic ages suggest. The 24 Ma age for the Turija fauna fits very well with the age of sites with a comparable faunal assemblage in the Formant-Findreuse section in the Alpine Foreland Basin in the Haute Savoie following the alternative magnetostratigraphic correlations by Sen (1997) and Agustí (2001) (Fig. 4.8).

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4.6.3 Regional implications

Precise age determination has long been the biggest outstanding problem for most intra-montane basins in the Dinarides (Pavelić, 2001). The here established 24–23 Ma age for the infi ll of the Banovići basin represents a major step forward for the regional stratigraphic scheme. It allows a more detailed evaluation of both tectonic and paleo-climatic factors affecting the post-orogenic development of the northern margin of the Dinarides. Some authors reason that the strike slip basins along the northern margin of the Dinarides, including the Sava and Drava depressions and the Banovići basin, initially arose due to movements on faults branching off the Peri-Adriatic Fault Zone (Tari and Pamić, 1998; Hrvatović, 2006). In our opinion, dextral strike slip might also have resulted from the northward translation and oblique docking of the Dinarides onto the Tisza-Dacia mega unit.

Although in the larger part of the Banovići basin sediments are heavily tectonized, our paleomagnetic results indicate that the basin did barely rotate since then. Comparison of the observed declination (2.8±2.8°) to the declination (5.9±4°) expected on the basis of the 20 Ma reference pole for Europe (Torsvik et al., 2008) shows these are statistically indistinguishable when the respective uncertainties are taken into account. The difference between the observed and expected inclination, 53.5° vs. 60.8°, most likely results from a fl attening effect due to compaction of the sediments (e.g. Krijgsman and Tauxe, 2004).

Paleo-vegetation data indicate that European continental climate in the late Oligocene is also marked by a distinct temperature peak (Mosbrugger et al., 2005). This late Oligocene Climatic Optimum is registered in marine isotopic reocords as well (Zachos et al., 2001; Villa et al., 2008). Correlation of the Grivice section to upper Oligocene chrons C6Cr, C6Cn.3n, and C6Cn.2r implies that the lifetime of Lake Banovići coincided with optimum climatic conditions and an elevated global temperature. The lake disappeared at the dawn of the Mi1 Glaciation. This is in analogy with middle Miocene lakes of the Dinaride Lake System that also developed during a period of optimum climatic conditions (de Leeuw et al., 2010; Mandic et al. 2011; chapters 2, 5, and 6). We conclude that the regional water budget was favorable for the formation of lakes in the Dinarides when global temperatures were relatively high.

Our new fossil fi ndings in the Banovići basin indicate that an at least limited exchange of fauna

Banovici

Paleogeography of Europe (~24 Ma)

?

?

Mediterranean Mediterranean Mediterranean Mediterranean Mediterranean Mediterranean Mediterranean Mediterranean Mediterranean Mediterranean Mediterranean

Paratethys Paratethys Paratethys Paratethys Paratethys Paratethys Paratethys Paratethys Paratethys Paratethys Paratethys Paratethys Paratethys Paratethys Dinaride-Anatolian Land Anatolia

Enspel

Ebro

Coderet

?

Molasse

Figure 4.9. Paleogeographic setting of Banovići and other mentioned mammal sites in the Late Oligocene (modifi ed after Rögl, 1999).

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between Central Anatolia and Europe took place around 24 Ma. Unfortunately, there is still a big gap in the mammal record of former Yugoslavia, with hampers a thorough understanding of mammal migration patterns between Asia and western Europe during the Oligo-Miocene. The excavation, description, and dating of more sites like Banovići is thus highly essential in order to understand the terrestrial paleobiogeographical relations of this crucial area.

4.7 Conclusions

The Banovići basin infill was dated to be of latest Oligocene age on the basis of small mammal biostratigraphic and magnetostratigraphic results. The assemblage of the Turija mammal site at the base of the basins main coal layer best compares with localities attributed to European MP30–MN1 small mammal zones and Anatolian zone B. The magnetostratigraphic pattern of the Grivice section, exposing 167 m of lacustrine marls and limestones overlying the main coal layer, consists of a long reversed interval at the base, a short normal in the upper part, and another reversed interval at the very top of the section. Using the small mammal assemblage as a rough age indicator correlation of the magnetostratigraphic pattern to the C6Cr to C6Cn.2n interval of the GPTS is preferred. This correlation indicates the main lacustrine phase of the Banovići basin started shortly after 24 Ma and lasted till ~23 Ma and implies deposition took place at a rate of 0.2 m/kyr. The presented results contribute to a better understanding of the mammal biostratigraphy and the paleogeographic and climatologic development of the north-eastern margin of the Dinarides.

Acknowledgements

We are indebted to Hazim Hrvatović (Geological Survey Sarajevo), Sejfudin Vrabac (University of Tuzla), and Stjepan Ćorić (Geological Survey Vienna) for their help in organizing the field work. Thanks go to Gudrun Höck (NHM Vienna) for discussions and help with the field work. This study is funded by the Austrian FWF Project P18519-B17: “Mollusk Evolution of the Neogene Dinaride Lake System”, by the Dutch Research Centre for Integrated Solid Earth Sciences (ISES), and by the Dutch Organization for Scientific Research (NWO/ALW).

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A geologist investigates two volcanic ash layers in a 15 m thick coal bed

Chapter 5Paleogeographic evolution of the Southern Pannonian Basin: 40Ar/39Ar age constraints on the Miocene continental series of northern Croatia

Oleg Mandic, Arjan de Leeuw, Jeronim Bulić, Klaudia Kuiper, Wout Krijgsman, Zlata Jurišić-Polšak

The Pannonian Basin, originating during Early Miocene, is a large extensional basin incorporated between Alpine, Carpathian and Dinaride fold-thrust belts. Back-arc extensional tectonics triggered deposition of up to 500-m-thick continental fluvio-lacustrine deposits distributed in numerous sub-basins of the Southern Pannonian Basin. Extensive andesitic and dacitic volcanism accompanied the syn-rift deposition and caused a number of pyroclastic intercalations. Here, we analyze two volcanic ash layers located at the base and top of the continental series. The lowermost ash from Mt. Kalnik yielded an 40Ar/39Ar age of 18.11±0.06 Ma. This indicates that the marine continental transition in the Slovenia-Zagorje Basin, coinciding with the onset of rifting tectonics in the Southern Pannonian Basin, occurs roughly at the Eggenburgian/Ottnangian boundary of the regional Paratethys time scale. This age proves the synchronicity of initial rifting in the Southern Pannonian Basin with the beginning of sedimentation in the Dinaride Lake System. Beside geodynamic evolution, the two regions also share a biotic evolutionary history: both belong to the same ecoregion, which we designate here as the Illyrian Bioprovince. The youngest volcanic ash level is sampled at the Glina and Karlovac sub-depressions, and both sites yield the same 16.00±0.06 Ma 40Ar/39Ar age. This indicates that lacustrine sedimentation in the Southern Pannonian Basin continued at least until the earliest Badenian. The present results provide not only important bench marks on duration of initial synrift in the Pannonian Basin System, but also deliver substantial backbone data for paleogeographic reconstructions in Central and Southeastern Europe around the Early–Middle Miocene transition.

This chapter is based on: Mandic O., de Leeuw A., Bulić J., Kuiper K.F., Krijgsman W., and Jurišić-Polšak Z., Paleogeographic evolution of the Southern Pannonian Basin: 40Ar/39Ar age constraints on the Miocene continental series of northern Croatia. Submitted to International Journal of Earth Science.

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5.1 Introduction

The Pannonian Basin is the largest extensional basin in a back-arc tectonic setting that formed during the Miocene in Central Europe (Fig.5.1). Its evolution was strongly determined by complex intra-European plate tectonic processes of continental collision and subduction, which also generated the Alpine-Carpathian-Dinaride mountain belt (Horvath and Tari 1999; Fodor et al. 1999; Schmid et al. 2008). The Southern Pannonian Basin (SPB: Fig. 5.2) comprises a series of depressions collectively termed the Northern Croatian Basin (Pavelić 2001; Ćorić et al. 2009). Their initiation started in the early-middle Miocene by passive continental rifting through lithosphere-thinning along with reactivation of large, Oligocene dextral transform faults (Pavelić 2001; Hrvatović 2006). This extensional phase was accompanied by strong andesitic and rhyolitic volcanic activity (Konečný et al. 2002; Kovacs et al. 2007).

Typical sedimentary successions of the initial SPB comprise continental, alluvial and lacustrine sediments unconformably overlying a strongly tectonized basement (Pavelić et al. 2003). The only exception is that stripe of the Slovenia-Zagorje Basin (Fig. 5.2), where alluvial series terminate the Oligocene to Lower Miocene marine deposition related to the Transtethyan Corridor (Rögl et al. 1998). The superposing lacustrine series contains characteristic faunal elements of high-similarity with the Dinaride Lake System to the south, thus belonging to the same bioprovince (Kochansky-Devidé and Slišković 1978; Bulić and Jurišić-Polšak 2009). The continental fluvio-lacustrine series are generally overlain by transgressive marine deposits representing a wide-spread ingression of the Paratethys Sea into the SPB.

Two volcanic ash-levels, one at the base and one the top of the continental series, were sampled here for Ar/Ar dating. The lower volcanic ash helps determine the age of the initial rifting tectonics of the SPB and the marine continental transition in the Slovenia-Zagorje Basin. The upper volcanic ash establishes the age of the lacustrine units and the maximum age of the Paratethys transgression. In combination, they provide new insights into the duration and evolution of the Dinaride Lake System (Krstić et al. 2003; Harzhauser and Mandic 2008).

5.2 Geological Setting

The series of depressions that form the SPB is located at the northern part of the Dinaride mountains (Fig. 5.1). These mountains formed in the Eocene by thrust sheet stacking related to subduction and collision with the Tisia microplate in the north and the Adriatic microplate in the south (Pamić et al. 1998; Tari and Pamić 1998; Tari 2002; Bennet et al. 2008; Tomljenović et al. 2008; Schmid et al. 2008). The largest basins are the Sava and Drava depressions, both representing through-like NW-SE-oriented structures, wherein the Sava and Drava rivers now flow in a southeastern direction (Fig. 5.2). These basins are mostly situated in northern Croatia, but extend partly into northern Bosnia and Herzegovina (Sava depression) and southern Hungary (Drava depression). The basement belongs to the Internal Dinarides, but shows the typical NW-SE Dinaride strike direction only in southern outcrops.

The sedimentary infill of the SPB is up to 6000 m thick and is divided into three megacycles reflecting the geodynamic history of the Pannonian Basin (Saftić et al. 2003). Our investigation deals with the lower part of the first cycle. It represents the initial syn-rift phase of the Pannonian Basin, comprises an upward succession of alluvial / lacustrine deposits, and terminates with the marine deposits of the Paratethys transgression. The Sava and Drava depressions accumulated thick clastic deposits plumbed by mudrocks that trapped significant oil accumulations from basement rocks (Lučić et al. 2001; Saftić et al. 2003; Dolton 2006). Our study area represents two opposing margins of the North Croatian Basins (Fig. 5.2), Mt. Kalnik in the north at the western margin of the Drava depression and the Karlovac-Glina sub-depressions at the southwestern margin of the Sava depression (Fig. 5.3).

5.2.1 Mt. Kalnik area

The inselberg Mt. Kalnik has a NE-SW strike and probably represents the rotated thorn of the westernmost

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Chapter 5: Paleogeographic evolution of the Southern Pannonian Basin

Internal Dinaride extension (Fig. 5.1). That block was displaced in a northeastern direction during the Oligocene along the Periadriatic lineament (Tomljenović et al. 2008; Fig. 5.2). In the Oligocene and early Miocene, the area of Mt. Kalnik formed the southeastern boundary of the Slovenia-Zagorje Basin representing the only marine environment of SPB during that time interval (Fig. 5.2). Related marginal marine sediments of the regional Egerian Stage, characterized by paralic coal deposition, are present in the northwestern area (Poljak 1942; Šikić and Jović 1968). They are overlain by transgressive, shallow marine, glauconitic sands of the Macelj Formation of the Eggenburgian Stage (Šimunić et al. 1981; Pavelić et al. 2001). The Eggenburgian deposits represent the last marine influx in the area prior to the much younger Middle Miocene (Badenian) transgression (Aničić et al. 2002). The deposition of Egerian and Eggenburgian sediments is accompanied by strong, Periadriatic line-related, andesitic volcanic activity (Šimunić and Pamić 1993; Altherr et al. 1995; Tibljaš et al. 2002). Freshwater deposition starts at Mt. Kalnik with fluvial siltstones, sandstones and conglomerates, passing upwards into lacustrine marls and sandstones (Pavelić et al. 2001). Based on microfloristic evidence and on the regional stratigraphic context, the fluvial deposits are attributed to the lower Ottnangian, the lake deposits to the upper Ottnangian (Pavelić et al. 2001).

Figure 5.1. Geotectonic map of the Pannonian Basin System and adjacent fold-trust belts (modified after Schmid et al. 2008).

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5.2.2 Karlovac and Glina sub-depressions

The southwestern margin of the Sava depression is marked by two sub-depressions (Karlovac and Glina) divided by the SSE-NNE striking Paleozoic and Mesozoic basement rocks of Kamešnica hill (Fig. 5.2). These sub-depressions are delineated in the SE by Paleozoic rocks, Jurassic ophiolithes and Eocene flysch deposits (Schmid et al. 2008; Tomljenović et al. 2008). The margin in the NW is determined by Triassic platform carbonates and Cretaceous flysch. The lacustrine series, rich in mollusks (Kochansky-Devide and Slišković 1979 and reference therein), bears coal deposits that were exploited until the 2nd world war (Jurković 1993). According to Saftić et al. (2003) the Karlovac sub-depression comprises more than 1.5 km thick infill, the Glina sub-depression more than 0.5 km thick infill of Neogene sediments. Based on facies distribution inferred from well data, Pletikapić (1960) argued that, during lacustrine deposition, the Sava and Glina depressions were disconnected through a longitudinal ridge made of Eocene rocks. In the Karlovac sub-depression the basal lacustrine deposits are outcropping only at the southern part of Kamešnica hill (Fig. 5.2). These deposits are more common in the Glina sub-depression and can be found along its entire margin. Ash intercalations in the lacustrine sediments of Glina were described earlier by Mutić (1979). According to that author they comprise andesitic-dacitic crystallvitroclastic and vitroclastic tuffs dominated by quartz, plagioclases and biotite.

5.3 40Ar/39Ar geochronology

5.3.1 Sections and materials

The volcanic ash at the base of the continental series was sampled at the eastern part of Mt. Kalnik, in the Glogovnica brook (GPS WGS 1984: 46.15858, 16.525669; about 20 km SE of Varaždin; Fig. 5.2). This is the easternmost of three localities collectively termed Knežev jarak by Tibljaš et al. (2002; pers. comm.). The tephra layer at Glogovnica brook represents a massive bed of 2 m gray, compacted, clayey biotite-bearing ash superposed by 6.7 m of a whitish to light grayish, non-consolidated, strongly altered silty ash bed. This suggests intensive volcanic activity close to the site. The section starts with 0.5 m coarse to fine-grained conglomerates overlain by 0.5 m coarse to middle-grained grayish brown sandstones. This is followed, up to the ash base, by 0.7 m of sandy, micaceous clay. The ash is finally superposed by 1 m of clayey sand (Fig. 5.3). The sampled sedimentary succession at Mt. Kalnik lacks fossils.

The volcanic ash at the upper part of the lacustrine sediments was sampled at two localities (30 km apart) in the southern peri-Sava sub-depressions (Fig. 5.2). The western locality is in the Karlovac sub-depression, close to the village Sjeničak (GPS WGS 1984: 45.422371, 15.797117). The eastern locality is in the Glina sub-depression, in the Paripovac brook north of the village Mali Gradac (GPS WGS 1984: 46.15858, 16.525669). In both localities, this volcanic ash layer is only ~0.5 m thick. At Sjeničak, it is represented by a biotite-bearing montmorillonite clay layer. Below the ash, more than 6 m of interbedded sands, pelites and limestone are present. They bear an accumulated lacustrine mollusk fauna dominated by Mytilopsis bivalves. The ash layer itself is positioned ~2 m below the transgression horizon, which is marked by marine organogenic limestones and sands of the Badenian Stage. The sands comprise clasts in their lower parts, suggesting deposition within a transgressive system tract. At Paripovac brook the ash is gray medium sand with white calcite veins and biotite flakes up to 1.5 mm in diameter. It is intercalated in a thin sedimentary sequence of 2 m of clays and silts bearing accumulated Mytilopsis shells below the ash and about 1 m of clays and lacustrine limestone above the ash.

5.3.2 Methods

The bulk ash samples were crushed, disintegrated in a dilute calgon solution, washed and sieved over a set of sieves between 63 and 500 μm and the largest appropriate mineral fraction was subjected to standard heavy liquid and magnetic separation techniques for sanidine (non-magnetic, δ2.55–2.60). All samples were subsequently handpicked and leached with a 1:5 HF solution in an ultrasonic bath for 5 min.

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The resulting mineral separates were then sent for irradiation at the Oregon State University TRIGA reactor in the cadmium-shielded CLICIT facility for 10 h. Upon return to the VU Argon Laboratory, the samples were pre-heated under vacuum using a heating stage and heat lamp to remove undesirable atmospheric argon. Thereafter, samples were placed in an Ultra High Vacuum sample chamber and degassed overnight. For each ash sample, 10 multiple-grain splits of the mineral separate were fused using a Synrad CO2 laser in combination with a Raylase scanhead as a beam delivery and beam diffuser system. After purification the resulting gas was analyzed with a Mass Analyzer Products LTD 215-50 noble gas mass spectrometer. Beam intensities were measured in a peak-jumping mode in 0.5 mass intervals over the mass range 40–35.5 on a Balzers 217 secondary electron multiplier. System blanks were measured every three to four steps. Mass discrimination was monitored by frequent analysis of aliquots of air. The irradiation parameter J for each unknown was determined by interpolation using a second-order polynomial fitting between the individually measured standards.

Figure 5.2. The map shows the position of geographic and geologic features from the southern Pannonian Basin and adjoining Dinaride-Alpine fold-thrust belts that are referred to in the text. The white-shaded area represents Miocene to Pleistocene infill of the Pannonian Basin System starting with the Middle Miocene marine flooding. The dark green-shaded areas are Lower to Middle Miocene lacustrine deposits. Compiled after Pavelić et al. (2003), Saftić et al. (2003), geologic maps of former Yugoslavia M 1:100000 and M 1:500000 and ESRI ArcGIS base maps. Abbreviations: SZB – Oligocene to Lower Miocene Slovenian–Zagorje Basin, PB – Pannonian Basin, DIB – Dinaride intra-montane basins.

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All 40Ar/39Ar ages have been calculated with the ArArCalc software (Koppers 2002), applying the decay constants of Steiger and Jäger (1977). The age for the Fish Canyon Tuff sanidine flux monitor used in age calculations is 28.201±0.03 Ma (Kuiper et al. 2008). The age for the Drachenfels sanidine flux monitor is 25.42±0.03 Ma (Kuiper et al., in prep). Correction factors for neutron interference reactions are 2.64±0.017×10−4 for (36Ar/37Ar)Ca, 6.73±0.037×10−4 for (39Ar/37Ar)Ca, 1.211±0.003×10−2 for (38Ar/39Ar)K and 8.6±0.7×10−4 for (40Ar/39Ar)K. Errors are quoted at the 1σ level and include the analytical error and the error in J.

5.3.3 Results

An overview of the results is displayed in table 5.1. All relevant analytical data as well as error determination are presented in the online supplementary material. For each of the Glogovnica, Sjeničak and Paripovac ashes, 10 multiple grain single fusion experiments were performed (online supplementary material). Mass discrimination was monitored frequently during the analytical runs and was not always stable. We ignored all data with unlikely high (>1.0150) mass discrimination corrections. Further, we increased the uncertainty in our discrimination correction to 0.3–0.5% depending on the air measurements run before and after our unknowns.

For the Glogovnica ash, 5 out of 10 experiments were selected to calculate the weighted mean age of 18.11±0.06 Ma (Fig. 5.4, online supplementary material), which is concordant with the peak in probability density at 18.14 Ma and with the 18.07±0.08 Ma isochron age (Table 5.1). The uncertainties in J and mass discrimination are 0.3%. Experiments 48A and B were discarded based on their unlikely high mass discrimination correction, and 48I and M were excluded due to their comparatively low K/Ca content, indicating the presence of a lower K mineral. Experiment 48H was discarded because the mass spectrometer encountered peak centering problems during this experiment. The error in the isochron intercept for the Glogovnica experiments is large due to the high percentage of radiogenic argon and resultant clustering of data points.

A weighted mean age of 16.00±0.06 Ma was calculated based on 8 out of the 10 experiments performed for the Sjeničak ash (Fig. 5.4, online supplementary material). The weighted mean age is concordant with the 16.00±0.09 Ma isochron age and very close to the 15.92 Ma peak in probability density (Table 5.1). The uncertainties in J and mass discrimination were 0.3% and 0.5%, respectively. Experiment 50G and 50K were discarded because they are significantly younger than the other 8 experiments. Upon removal, the isochron intercept improved from 305±17 to 295±25.

The weighted mean age of 16.00±0.06 Ma for the Paripovac ash was calculated based on 6 out of the 9 measured experiments (Fig. 5.4, online supplementary material). The weighted mean age is

Figure 5.3. Studied sections showing position of dated samples.

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concordant with the 16.00±0.07 Ma isochron age and the 16.06 Ma peak in probability density (Table 5.1). The mass discrimination factor was more stable during the measurement of the Paripovac experiments and none had to be discarded on these grounds. The uncertainties in J and mass discrimination were estimated to be 0.3%. Experiments 59A and F were excluded when calculating the weighted mean age because they are significantly younger than the other experiments. Upon removal, the isochron intercept improved from 315±31 to 296±15. Although the age of experiment 59L was concordant with the included experiments, it was discarded because problems with peak centering were encountered during measurement.

For all three ash samples the MSWD of the calculated weighted mean ages was smaller than the T-student distribution at the 95% confidence level (online supplementary material). The weighted mean ages were concordant with the isochron ages and very close to the peak in probability density distribution, even though the age populations in the latter were not completely homogeneous. We therefore interpret them to reflect the tuffs’ crystallization ages. The ages for the Paripovac and Sjeničak ashes are in excellent agreement and suggest that we are dealing with the same volcanic event, even though the difference in their K/Ca ratio indicates that we separated and dated a material with a different chemical composition.

Figure 5.4. Weighted mean age, probability density distribution and age and uncertainty of the individual experiments for the Glogovnica, Sjeničak and Paripovac ashes.

Sample Location Weighted Mean Age (Ma)

n N Peak in Probability Distribution (Ma)

Mineral Isochron age

Isochron Intercept

Glogovnica N46 08 20.3 E016 33 01.4 18.11±0.06 5 10 18.14 Sanidine 18.07±0.08 368±45

Sjeničak N45 28 49.7 E015 50 17.4 16.00±0.06 8 10 15.92 Plagioclase/Sanidine 16.00±0.09 295±12

Paripovac N45 16 16.4 E016 14 32.1 16.00±0.06 6 9 16.06 K-Feldspar 16.00±0.07 297±8

Table 5.1. Summary of the 40Ar/39Ar results. MSWD is Mean Square Weighted Deviates, N is the total number of repetitions in the single fusion experiments. n is the number of experiments used to calculate the weighted mean and isochron age.

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5.4 Discussion

5.4.1 Initiation of extensional tectonics in the South Pannonian Basin

The volcanic ash from the Glogovnica tuff at Mt. Kalnik is stratigraphically located in the basal part of the fluvial-lacustrine deposits of the Drava depression. These fluvial conglomerates and sands mark the end of marine sedimentation in the Oligocene-Lower Miocene Slovenia-Zagorje Basin (Pavelić et al. 2001; Fig. 5.2) and mark the start of extensional tectonics in the PBS. The new Ar/Ar dating of the Glogovnica tuff indicates that this initial rifting phase of the SPB took place at 18.11±0.06 Ma (Fig. 5.4 and 5.5). This corresponds stratigraphically roughly to the Eggenburgian/Ottnangian transition in the Paratethys Time Scale (Piller et al. 2007) and to the middle Burdigalian of the standard Geological Time Scale (Lourens et al. 2004). The Eggenburgian/Ottnangian transition corresponds to a major global sea-level low-stand event marking the 3rd order sequence boundary Bur-3 (Piller et al. 2007). This suggests that the marine retreat from the Slovenian-Zagorje Basin and the Transtethyan Corridor is probably a combination of tectonic activity and glacio-eustatic sea level lowering.

The sedimentary successions of the continental basinal infill of the various SPB depressions generally show a similar architecture, with alluvial sediments in the lower part and lacustrine sediments in the upper part (Pavelić 2001). Such an architecture is present everywhere along the basinal margins, e.g. in the region of Mt. Žumberak (Vrsaljko et al. 2005), Mt. Medvednica (Pavelić et al. 2001), Mt. Papuk (Pavelić et al. 1998), Mt. Požeška (Hajek-Tadesse et al. 2009) and Mt. Kalnik (Pavelić et al. 2001). Consequently, Pavelić et al. (1998) suggested a uniform opening of these basins due to tectonic rifting. According to that model, the early rifting was accompanied in the SPB by the extension of vast floodplain environments. The initial leveling of the relief was characterized by the extended accumulation of conglomerates and other fluviatile material infilling the depressions. During a later phase, larger and smaller lakes developed here. This lake development was probably an effect of more humid climate conditions, which promoted lake settlement throughout the region.

In addition, the new age for the extensional tectonics of the SPB correlates exactly with the age of the initial extension and deposition in the intra-mountainous basins of the Dinaride Lake System. The vast area of that System, stretching southwards across the Dinarides into the Outer Dinaride Foreland Basin on the Adriatic microplate, demonstrates the extended spatial range of Pannonian Basin extensional tectonics (Leeuw et al. 2010). The initiation of Lake Sinj in southern Croatia was recently determined by Ar/Ar dating in combination with magneto-biostratigraphic correlations to have occurred at ~18 Ma (de Leeuw et al. 2010). This synchronicity indicates that large-scale geodynamic processes in the Dinaride fold-thrust belt and the Pannonian region were responsible for the initial extension phase in the SPB.

Our new age constraint for the marine continental transition fits very well with the previous lithostratigraphic subdivision of the SPB (Pavelić et al. 2001). This subdivision attributes the lower part of the continental series to the lower Ottnangian. It is also in good agreement with the K/Ar age of 19.21±0.64 Ma for the glauconite sands of the underlying marine Macelj formation (Avanić et al. 2005), which correlates to the Eggenburgian. It also agrees with the correlation of younger lacustrine deposits at Mt. Kalnik to the late Ottnangian; that correlation is based on microfloral assemblages and indicates the last cold spell before the start of the global warming event in the Karpatian (Pavelić et al. 2001). The subsequent thermophilous floral assemblage is known from the similar environmental setting of Mt. Moslavačka Gora at the northern margin of the Sava Depression (Krizmanić 1995). Finally, the calculated age is in accordance with Tibljaš et al. (2002), who suggested that the Glogovnica ash correlates, based on its chemical composition, with Egerian and Eggenburgian andesites and dacites. Our dating supports the authors’ suggestion that a major change in geochemical composition of regionally very common volcanic and volcanoclastic rocks must have taken place during the Ottnangian.

In the North Hungarian Basin, positioned in NE prolongation of the Slovenia-Zagorje Basin, the corresponding continental fluvio-lacustrine series superposes marine deposits with Pecten hornensis of early Ottnangian age (Csepreghyné Meznerics 1967; Piller et al. 2007). The Lower Rhyolite Tuff, intercalated in the lower part of that continental series, was dated by the 40Ar/39Ar technique at 17.02±0.14

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Ma and 16.99±0.16 Ma (Palfy et al. 2007). This suggests a distinctly younger, early Karpatian age for the volcanism.

5.4.2 Paleogeographic changes and faunal bioprovinces

The marine Egerian and Eggenburgian deposits of the Slovenia-Zagorje Basin indicate that this region formed prior to SPB installation at the southeastern margin of the Paratethys Basin (Fig. 5.2). During those times, the partial study area belonged to the Transtethyan corridor that iteratively connected the Paratethys with the Mediterranean-Tethys Sea in northeastern Italy via the North-Hungarian Basin (Rögl and Steininger 1983; Rögl 1998). Our results indicate that marine environments of the Slovenia-Zagorje Basin preceding the SPB extension disappeared at 18.1 Ma during the earliest Ottnangian. This involved a combination of tectonic activity and glacio-eustatic sea level lowering.

The Ottnangian and Karpatian sediments of the Paratethys region are characterized by the occurrence of brackish marine Rzehakia assemblages. They are considered to mark a major endemic event that can be traced all across the Paratethys Sea from Bavaria into the Caucasus (Mandic and Ćorić 2007). During

Figure 5.5. Geochronologic correlation table compiled after Lourens et al. (2004), Strauss et al. (2006), Piller at al. (2008), Hohenegger et al. (2009) and Ćorić et al. (2009) showing approximate depositional durations for different regions within the PBS and stratigraphic position for investigated samples.

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this event, the entire Paratethys Sea is considered to be a separate ecoregion termed Danubian Province (Harzhauser and Piller 2007 and references therein). The absence of Rzehakia shell accumulations in the entire SPB including Styrian, Slovenian, SW Hungarian (up to Varpalota) and North Croatian basins indicates that their connection to the Paratethys was completely lost at that time. Moreover, it shows that no Paratethys ingressions occurred in the SPB during the late Ottnangian and early Karpatian: such an ingression would have certainly brought this peculiar brackish fauna into the lake system.

The Karpatian transgression (Rögl 1998) from the Mediterranean via the Venetian and Styrian basins into the North Hungarian Basin apparently did not cross the Donat line into the North Croatian basins (Fig. 5.2 and 5.6). This transgression must have taken place at a time when Rzehakia was already extinct in the Paretethys because that taxon did not migrate southwards following the re-establishment of marine conditions. An absence of Karpatian deposits south of the Donat line was also suggested by Rasser et al. (2008).

The biostratigraphic data from the SPB (Mytilopsis/dreissenid bivalves) indicate that its freshwater environments are closely related to the Dinaride Lake System that originated at the same time (Mandic et al. 2007; de Leeuw et al. 2010). The peri-Sava sub-depressions particularly show high faunal similarity with the mollusk fauna of the Dinaride intra-mountainous basins (Kochansky-Devide and Slišković 1978). Based on the high-grade endemic character (Harzhauser and Mandic 2009), these faunas are regarded to constitute an independent paleobiogeographic unit that we now term the Illyrian Bioprovince. The extension of the Illyrian Bioprovince did not always exactly correlate with the extension of the Dinaride Lake System. Especially during the Karpatian-Badenian, dreissenid-rich accumulations of the Illyrian Bioprovince extended further north to Mt. Meszesk (Hamor 1970) and Mt. Bakony (Kókay 2006) in S Hungary, to Fohnsdorf (Hölzel and Wagreich 2004) and in the Vienna Basin (Schultz 2003) in E Austria, and further south to the Morava depression (Knežević 1996) belonging to the Serbian Lake (Krstić et al. 2003) (Fig. 5.6).

The lacustrine deposits from the Karlovac and Glina sub-depressions are also characterized by abundant dreissenid bivalve accumulations of the Illyrian Bioprovince type, whereas these accumulations are absent in the older deposits at Mt. Kalnik. Drill holes in the Sava depression indicate that the dreissenid assemblages are restricted to the upper part of the lacustrine interval (Ožegović 1944). The volcanic ashes of Sjeničak and Paripovac are located at the upper part of the terrestrial series and consequently provide the youngest age for the lake deposits and the Illyrian Bioprovince in the SPB. The new Ar/Ar dating of 16.00±0.06 Ma for the dreissenid assemblages for the first time prove that lacustrine sedimentation continued in the SPB at least until the earliest Badenian. This is significantly younger than the previously inferred late Ottnangian age (Kochansky-Devide and Slišković 1978; Bulić and Jurišić-Polšak 2009; Hajek-Tadesse et al. 2009).

The typical sedimentary successions of the terrestrial series of the SPB show alluvial sediments in the lower part and lacustrine sediments in the upper part (Pavelić 2001). Pavelić et al. (1998) inferred a synchronous installation of a huge open lake environment, connecting all depressions of the SPB. This model was based on sections from the Slavonian Mountains where marine transgression indeed flooded one deep, open lake, changing the water chemistry and introducing marine immigrants without changing the depositional facies (Pavelić et al. 1998; Hajek-Tadesse et al. 2009). In contrast, Saftić et al. (2003) claimed, based on results from the Sava and Drava depressions, that the lakes were not connected but formed rather isolated occurrences across the southern Pannonian Basin. The calculated ages of the two tephra layers suggest that the continental fluvio-lacustrine deposition is not coeval across the North Croatian basins (Fig. 5.5). Such sedimentation occurred already in the lower Ottnangian at Mt. Kalnik and during the early Badenian in the southern peri-Sava subdepressions. These significant age differences partly fit tectonostratigraphic models for the early rifting phase of the SPB (Prelogović 1975; Pavelić 2001). These models imply an earlier development of the Drava versus the Sava depression. Accordingly, subsidence and lake development could have first started in the Drava depression, progressing later in southward direction over the Bjelovar and Požega sub-depressions to finally reach the Sava depression and its southern sub-depressions.

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5.4.3 The Badenian transgression in the Southern Pannonian Basin

The marine Badenian transgression and opening to the world oceans represented a crucial phase in the Pannonian Basin formation, especially in the Interior Dinaride Foreland where terrestrial environments persisted since the Eocene time (Rögl 1998, Popov et al. 2004). The Sjeničak and Paripovac ashes are both located slightly below the marine transgression (Fig. 5.3), and the equivalent 40Ar/39Ar ages of 16.0 Ma thus indicate the maximum age of the Badenian transgression in the SPB. This transgression is, however, biostratigraphically dated to an age younger than 14.8 Ma based on nannofossil assemblages corresponding to NN5 at Mt. Medvednica and at Mt. Papuk by Ćorić et al. (2009).

Accepting a maximum age of 14.8 Ma for the Badenian transgression at Karlovac implies a significant hiatus/ interruption of sedimentation between the lacustrine deposits and the marine flooding. This discordance is also recorded at the northern margin of the sub-basin, where Badenian marine deposits directly transgress the basement rocks (Pletikapić 1960). A similar conclusion must be made for the Glina sub-depression where, according to regional stratigraphic data, the studied ash layer is overlain by coal deposits (Jurković 1993; field observation). Apparently, a swamp-related marginal lake environment installed itself, marking the final phase in regional lacustrine deposition. Similarly, very thick coal deposits can be traced in their southeastern continuation (Fig. 5.2), e.g. in the Lješljane Basin north of Prijedor in Bosnia and Herzegovina or in the Banja Luka Basin (Milojević 1976). The Karlovac and Glina sub-depressions could thus represent a zone independent from the Sava depression and related to the North Bosnian basins of the Dinaride Lake System (Fig. 5.2). This suggests that, in contrast to Lake Požega in the Slavonian Mountains (Pavelić et al. 1999; Ćorić et al. 2009, Hajek-Tadesse et al. 2009), the lakes of the Karlovac and Glina sub-depressions were not directly flooded by the Middle Miocene marine water, but vanished before the transgression.

Alternatively, it is possible that the marine transgression was not coeval for the entire SPB. This would diminish the significance of the latter result. In particular the biostratigraphic data available from Bosnian basins suggest that the marine transgression first reached the southern sub-depressions of the Sava Depression in the late Early Badenian (Petrović and Atanacković 1969, 1976; Fig. 5.4). Future investigations of the Drava Depression should focus on demonstrating the age of its initial marine flooding in respect to the previously reported Karpatian datum (Pavelić et al. 2001). Within the Hungarian part of that depression, the marine transgression is attributed to the Badenian (Saftić et al. 2003). Data from Slovenia fix the position of the southernmost Karpatian deposits at the Donat fault (Rasser et al. 2008), striking along the northwestern boundary of the Drava depression (Fig. 5.2). At Mt. Vilanny north of the

Figure 5.6. Paleogeographic map showing extensions of the Dinaride Lake System and the Paratethys during the Early/Middle Miocene transition (compiled after Rögl and Steininger 1983; Hamor 1985; Krstić et al. 2003, and own data). Dashed lines mark the maximal possible extension of the Illyrian Bioprovince. The geographic area is same as in figure 5.1.

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Drava depression, the marine flooding is also settled into the Badenian (Saftić et al. 2003). There, the lake deposits underlying the transgression bear common dreissenid shell accumulations (Hamor 1970), suggesting its relation to the younger part of the SPB basal fluvio-lacustrine infill.

5.5 Conclusions

The volcanic ash from Mt. Kalnik is dated at 18.1 Ma and corresponds to the middle Burdigalian and the Eggenburgian-Ottnangian transition. Its stratigraphic position at the base of the initial fluvial-lacustrine deposits of the Drava depression corresponds with the start of extensional tectonics in the Pannonian Basin System. Our new age correlates exactly with the age of the initial deposition in the intra-mountainous basins of the Dinaride Lake System. This provides evidence for similar large-scale geodynamic processes in the Miocene of the Dinaride fold-thrust belt and the Pannonian Basin.

The volcanic ashes from the Karlovac and Glina sub-depressions are dated at 16.0 Ma, corresponding to the earliest Langhian and earliest Badenian, respectively. This implies that the duration of the SPB extensional phase, characterized by fluvial and lacustrine deposition, lasted more than 2 Myr. Taking into account the revised datum of initial marine flooding in the Southern Pannonian Basin (Ćorić et al. 2009), that duration may even be extended by an additional 1 Myr.

Such a long duration of the continental cycle – in combination with available sedimentation data, sequence stratigraphy, and the position and character of marine transgression – makes continuous basin deepening models improbable. We here suggest a phase of several independent, smaller and larger basins comparable with the Dinaride intra-mountainous basins, and argue that the entire study area prior to Paratethys flooding and finalization of the SPB as tectonic unit paleogeographically and tectonically corresponds to the Dinaride Lake System and Dinaride Basins, respectively (Fig. 5.5 and 5.6).

We consider the term Paratethys exclusively related to a marine and marginal marine ecozone restricted at that time to the adjoining area north of the Donat line (Fig. 5.2 and 5.6). The endemic mollusk fauna of the basal SPB infill is similar to that of the Dinaride intra-montane basins and is attributed to the Illyrian Bioprovince, an ecoregion which is the continental counterpart to the marine Danubian Province (Harzhauser and Piller 2007) of the Paratethys domain.

Dreissenid shell accumulations with a faunal composition related to the fossil fauna of the Illyrian Bioprovince are abundantly present in sections from the Karlovac and Glina sub-depressions, and are now dated to be of earliest Badenian age. These accumulations are restricted to the upper parts of the basal lacustrine deposits of the Southern Pannonian Basin. The presence of related dreissenid shell beds as far as the Vienna Basin (W Austria), Mt. Bakony (central Hungary), Mt. Mescek (south Hungary) or the Morava depression (central Serbia) suggests the strikingly wide extension of the Illyrian Bioprovince during the Early and Middle Miocene.

Acknowledgments

Thanks go to Jakov Radovčić (Croatian Natural History Museum, Zagreb) for help with the field work. We are highly obliged to Davor Pavelić (Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb) and Mathias Harzhauser (Natural History Museum Vienna) for comments and suggestions regarding an earlier version of the manuscript. The study contributes the Austrian Science Fund (FWF) Project P18519-B17: “Mollusk Evolution of the Neogene Dinaride Lake System” and was supported by Dutch Centre for Integrated Solid Earth (ISES) as well as Netherlands Organisation for Scientific Research (NWO) funding.

117


An active volcano on the side of a lake in the Andes

Chapter 6Paleomagnetic and geochronologic constraints on the geodynamic evolution of the Central Dinarides

Arjan de Leeuw, Oleg Mandic, Wout Krijgsman, Klaudia Kuiper, and Hazim Hrvatović

The geodynamic evolution of the Dinaride Mountains of southeastern Europe is relatively poorly understood, especially in comparison with the neighboring Alps and Carpathians. Here, we construct a new chronostratigraphy for the post-orogenic intra-montane basins of the Central Dinarides based on paleomagnetic and 40Ar/39Ar age data. A first phase of basin formation occurred in the late Oligocene when strike-slip faulting related to the extrusion of the Alcapa block penetrated into the orogen. A second phase of basin formation took place between 18 and 13 Ma, concurrent with profound extension in the neighboring Pannonian Basin. Our paleomagnetic results further indicate that the Dinarides did not experience any significant tectonic rotation since the late Oligocene. This implies that the Dinarides were decoupled from the adjacent Adria and the Tisza-Dacia Mega-Units that both underwent major rotation during the Miocene. The Dinaride orogen must consequently have accommodated significant shortening, which is corroborated by our AMS data that indicate post-Middle Miocene shortening in the frontal zone, wrenching in the central part of the orogen, and inversion in the hinterland. These findings comply with structural geological and GPS data in the literature. A review of paleomagnetic data from the Adria plate, which plays a major role in the evolution of the Dinarides as well as the Alps, constrains its rotation since the Early Cretaceous to 48 ± 10° counterclockwise (CCW) and indicates 20° of this CCW rotation took place since the Miocene. The amount of rotation within the Adria-Dinarides collision zone increases with age and proximity of the sampled sediments to undeformed Adria. These results significantly improve our insight in the post-orogenic evolution of the Dinarides and resolve the existing apparent controversy between structural geological and paleomagnetic rotation estimates for the Dinarides as well as Adria.

This chapter is based on: de Leeuw, A., Mandic O., Krijgsman W., Kuiper K.F., and Hrvatović H., Paleomagnetic and geochronologic constraints on the geodynamic evolution of the Central Dinarides. Submitted to Tectonophysics.

120


6.1 Introduction

The Dinarides mountain belt is located on the north-western part of the Balkan Peninsula, continues southwards into the Albanides, Hellenides and Taurides, and forms an integral part of the Alpine-Himalayan orogenic system. In comparison with the neighboring Alps and Carpathians, the Dinarides remain geologically under-explored, mainly due to the politically complicated situation in the 90s. Although its Mesozoic and Paleogene pre- and syn-orogenic history recently received increased attention (Korbar, 2009; Ustaszewski, 2008a, 2008b; Schmid et al., 2008) and efforts were made to better understand its Miocene stratigraphy (Jimenez-Moreno et al., 2008, 2009; Mandic et al., 2008, 2011; de Leeuw et al., 2010), knowledge on its Neogene geodynamic evolution remains limited. After the main orogenic phase in the Eocene, a suite of intra-montane basins formed on top of the orogenic structure (Tari, 2002; Hrvatović, 2006; Pamić et al., 1998) and were occupied by a system of lakes. The exact timing and mechanism of basin formation as well as their paleogeographic history are still poorly understood. This clearly hampers our insight into the post-orogenic evolution of the mountain belt.

The most recent palinspastic reconstruction of south-eastern Europe (Ustaszewski et al., 2008), based exclusively on structural geological data, attributes a 20° CCW post 20 Ma rotation to the Adriatic plate, whereas the Dinarides are thought not to have rotated at all. This stands in sheer contrast with the simultaneous 30° CCW of Adria and the Dinarides inferred from post middle Miocene paleomagnetic data by Márton et al. (2002). This apparent discrepancy between structural geological and paleomagnetic data has a large impact on the rates of shortening predicted for the south-western part of our study area (Ustaszewski et al., 2008).

Recently, we have used integrated 40Ar/39Ar and magnetostratigraphic dating techniques to construct chronologic frameworks for the accumulated lacustrine sediments in various intra-montane basins of the Dinarides (chapters 1, 2, 3, 4, and 5; Jimenez-Moreno et al., 2009; De Leeuw et al., 2010; Mandic et al., 2011). Here, we combine all individual results to present a complete overview of the timing of the main phases of intra-montane basin formation. In addition, we use the paleomagnetic data of these basins to determine the vertical axis rotations of the Dinarides, and measure the AMS to reveal different tectonic stress directions. This provides improved insight in the post Early-Middle Miocene geodynamic evolution of the blocks constituting the Dinarides. A review of the collection of Cretaceous to Paleogene paleomagnetic data that is available in the literature builds a framework for the new Neogene results and shows the pre- and synorogenic rotation of crustal fragments involved in the collision of Adria and Tisza-Dacia. The apparent conflict between these paleomagnetic literature data and the most recent structural geologic palinspastic reconstruction (Ustaszewski, 2008) is ultimately resolved.

6.2 The intra-montane basins of the Central Dinarides

The Dinarides are located on the convergent plate boundary separating the Adriatic and Tisza-Dacia micro-plates in the central part of the Mediterranean region (Fig. 6.1). The post-orogenic evolution of the Dinaride Mountains is characterized by the formation of a large number of intra-montane basins (Pamić et al, 1998). Oligocene strike-slip faulting in response to movement on the Peri-Adriatic fault initiated transtensional depressions (Hrvatović, 2006). In the resulting intra-montane depressions fluvial and lacustrine sediments accumulated. Strike slip faulting was accompanied by volcanism (Tari, 1998).

In the middle Miocene, the north-eastern margin of the Dinarides was affected by profound extension in response to the rifting that initialized the Pannonian Basin (Tari, 2002). It largely subsided below the base-level of the adjacent Paratethys; a Mediterranean sized epi-continental sea that covered large parts of South-Eastern Europe at that time. In the more central and western parts of the Dinarides, this extensional phase reactivated Oligocene transpressive structures and consequently triggered the development of a series of lacustrine intra-montane basins.

According to Hrvatović (2006), the intra-montane basins continued to evolve as pull-apart structures. Ilić and Neubauer (2005), on the other hand, relate this phase of subsidence to pure extension partitioned in two phases; an early Miocene NE-SW directed phase and a middle Miocene NW-SE directed phase.

121

Chapter 6: Paleomagnetic and geochronologic constraints on the geodynamic evolution of the Central Dinarides

notalaB .L

Budva-Cukali Zone

Pre-Karst & Bosnian Flysch Unit

Piemont-Liguria, Vahic, Inacovce-Kriscevo,Szolnok, SavaWestern Vardar Ophiolitic Unit(incl. Meliata-Maliac & Mirdita Ophiolites)Eastern Vardar Ophiolitic Unit (incl. South Apuseni & Transylvanian Ophiolites)

Intra-montane basins of the Dinarides

dashed white lines: outlines of tectonic blocks*

Undeformed part of the Adriatic plateDalmatian Zone

Southern Alps

High Karst Unit

East Bosnian-Durmitorthrust sheetDrina-Ivanjica thrust sheet (incl. Korab, Pelagonides)

Jadar-Kopaonik thrust sheet(incl. Bükk)

Non Adria-derived part of Italy

ALCAPA Mega Unit

Tisza Mega-Unit

Adria-derived thrust sheets S Alps and Dinarides

Dacia Mega-Unit

Ophiolites, oceanic accretionary prisms

Western Imbricated Margin of Adria

Outer perimeter Adriatic Plate includingits imbricated thrust sheets

white lines: outlines of the Pannonian Basin

Tizsa-DaciaPlate

Adriatic Plate

Eastern Alps

Sarajevo

Banja Luka

Split

18˚ 20˚22

14˚ 16˚12˚

18˚ 20˚22˚

14˚ 16˚12˚

46˚

47˚

45˚

44˚

43˚

42˚

46˚

47˚

45˚

44˚

43˚

42˚

Pag

Livno

Bugojno

BanoviciUgljevik

Gacko

Kakanj

SjenicakParipovac

Glogovnica

Alps

Carpathians

Dinarides

Busovača fault

Vrbas fault

Figure 6.1. Plate tectonic and geologic setting of the Dinarides and surrounding area, adapted from Schmid et al. (2008). The Neogene intra-montane basins as well as our sampling sites, main faults, major plate boundaries and the direction of movement of the Adriatic Plate are indicated.

122


Korbar (2009) resorts to yet another model of basin formation and invokes a wedge top position in order to explain the close coexistence of lacustrine and marine environments along the trust front. Formation of such a large suite of intra-montane basins represents a very marked phase in the post-orogenic evolution of the Dinarides. The between 200 and 2500 m thick sedimentary sequences that accumulated in the interior of the mountain chain could provide a detailed record of this event. However, due to the strictly endemic nature of the lacustrine fauna, age inferences remained tentative. It was consequently hard to asses whether sedimentation took place syn- or diachronously, despite numerous lithostratigraphic correlations (Milojević, 1963; Muftić and Luburić, 1963; Pantić, 1961). Correlation to the geodynamic evolution of Adria and the Pannonian domain was thus also problematic.

6.3 A chronostratigraphic framework for the Dinaride basins

Recently, high resolution magnetostratigraphic and 40Ar/39Ar studies were initiated in several of the intra-montane basins (chapters 1, 2, 3, 4, and 5; Jimenez-Moreno et al., 2009; De Leeuw et al., 2010; Mandic et al., 2011). We will here integrate these and other data to arrive at a chronostratigraphic scheme for sedimentation in the intra-montane basins. An overview of the acquired ages is given in table 6.1. Details of the age and error calculations for the investigated volcanic ashes can be found in supplementary table 6.1.

6.3.1 Pag Island (Croatia)

The Island of Pag (Fig. 6.2), that comprises a 110 m thick Miocene succession exposed along the Crnika Beach (Fig. 6.3), represents the northwestern-most constituent of the DLS (Bulić and Jurišić-Polšak, 2009). Magnetostratigraphic data for the Crnika section revealed a long (113 m) reversed polarity interval, followed by a 7 m thick interval of normal polarity at the top (Jimenez-Moreno et al., 2009). Rock magnetic experiments indicate that the magnetic signal is carried by detrital, multi-domain magnetite in the upper part of the section and by greigite in its lower part that is richer in organic material. Combined with biostratigraphic constraints based on mollusks and pollen, the magnetostratigraphic pattern of the Crnika section was correlated to chrons C5Cr and C5Cn.3n of the GPTS, between 17.1 and 16.7 Ma (Jimenez-Moreno et al., 2009) (Fig. 6.3).

6.3.2 Sinj Basin (Croatia)

In the Sinj basin (Fig. 6.2), a 500 m thick limestone dominated succession is well exposed along the Sutina stream near Lučane in the western part of the basin (Jimenez-Moreno et al., 2008; Mandic et al., 2009). Several volcanic ash layers intercalate with the lacustrine sediments and enabled absolute age dating (De Leeuw et al., 2010) (table 6.1). A clear and reliable magnetostratigraphic pattern with 9 reversals was established (Fig. 6.3), firmly anchored to the timescale by 40Ar/39Ar dating of the intercalated volcanic ash layers (De Leeuw et al., 2010). The base of the section is constrained by the 40Ar/39Ar age for the Lučane 3 tuff layer of 17.91±0.18 Ma (Fig. 6.3, table 6.1). The age of the very top of the Lučane section is constrained by correlation of the uppermost normal interval to chron C5Bn.2n, arriving at an age of 15.0 Ma.

6.3.3 Livno-Tomislavgrad Basin

Two successive lacustrine cycles are found in the Livno-Tomislavgrad Basin (Fig. 6.2, 6.3). Around 1700 m of predominantly marl and limestone constitute the first phase, and an additional 500 m of sediments constitute the second. The basal part of the sequence is exposed in the Tušnica section. The Tušnica volcanic ash is located within a 10 m thick coal seam bearing proboscidean remains. The overlying sediments (~1300 m thick) are exposed along the Ostrožac stream. Breccias, derived from the basin margins, first occur in the upper third and coarsen and thicken upwards in the section towards a mega breccia at the top. A second volcanic ash crops out along the shores of Lake Mandek. The reversed polarity interval of the Tušnica section is correlated to chron C5Cr, as constraint by the 40Ar/39Ar age of

123


17.00±0.17 Ma for the Tušnica ash (Fig. 6.3; table 6.1). The Ostrožac section is correlated to the interval between C5Br and C5ABn, in agreement with the 40Ar/39Ar age of 14.68±0.16 Ma for the Mandek ash. These correlations suggest an age of approximately 17 Ma for the base, and 12.6 Ma for the top of the Livno succession.

6.3.4 Gacko Basin

The infi ll of the Gacko basin (Fig. 6.2) is ~360 m thick in the basin centre (Mirković 1980) and was sampled along a condensed 75 m section exposed in the Gračanica open pit coal mine. A 1.5 m thick prominent greenish volcanic ash layer is located in the top part of the section (Mandic et al., 2011). 40Ar/39Ar total fusion experiments for two samples of this ash provided a combined weighted mean age

Alps

Carpathians

Dinarides

Hr BiH

Hr BiH

Dinaride Lake System, Lower Miocene Extension after Krstic 2003

Extent of Dinaride Lake System Affected by the marine transgression from the Paratethys

Neogene intramontane basins of the Dinarides

political boundary

Croatia

Bosnia and Herzegovina

Adriatic Sea

SAVA

SINJ BASIN

LIVNO BASIN

SARAJEVO BASIN

TUZLA BASIN BANOVIĆI BASIN

GACKO BASIN

BUGOJNO BASIN

PAG ISLAND

Kakanj

SAVA BASIN

DRAVABASIN

Soutern Pannonian BasinVukovar

Zagreb

Doboj

Teslić

Bihać

DrvarJajce

Bugojno

Sarajevo

Konjic

Gacko

MostarSplitMiljevina

Ljubljana

40Ar/39Ar ash date

Paleomagnetic Sampling Site

Gračanica

Ostrožac Tušnica

Lučane

Crnika

Ugljevik

Grivice

Gračanica

Sjeničak

Paripovac

Glogovnica

Figure 6.2. Map with an overview of the sampling sites. The former extent of the Miocene Dinaride Lake System, the area affected by the Paratethys transgression, as well as the location of the intra-montane basins is indicated.Modifi ed after Pavelić (2002).

124


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ges

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125


Tim

esca

le, r

egio

nal

and

glob

al e

vent

s

040

Early Miocene MIddle Miocene

Climatic Optimum Deterioration Ice-house

Burdigalian Langhian Ottnangian Karpatian Badenian

NN4 NN5 NN3

Serravallian

NN6

Egg. Sarm.

early syn-rift: extensional strike-slip first rift phase: extension inversion

C5D

r2r

C5D

r1r

C5D

n

C5C

r

C5C

n3n

C5C

n2n

C5C

n1n

C5B

r

C5B

n1n

C5B

n1r

C5B

n2n

C5A

Dr

C5A

Dn

C5A

Cr

C5A

Cn

C5E

n

C5A

Br

C5A

Ar

C5A

Bn

C5A

An

17.8

17.6

17.4

17.2

16.8

16.6

16.4

16.2

15.8

15.6

15.4

15.2

14.8

14.6

14.4

14.2

13.8

18.4

18.2

18.0

17.0

16.0

15.0

14.0

13.0

13.6

13.4

13.2

12.8

12.6

12.4

17.8

17.6

17.4

17.2

16.8

16.6

16.4

16.2

15.8

15.6

15.4

15.2

14.8

14.6

14.4

14.2

13.8

18.4

18.2

18.0

17.0

16.0

15.0

14.0

13.0

13.6

13.4

13.2

12.8

12.6

12.4

C5A

r3r

C5A

r1r

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anic

ash

with

40

Ar/3

9 Ar a

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16.0

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16.0

0±0.

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1±0.

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B

asin

late

Sar

m.

unco

nf.

Crn

ika

Gračanica

Sou

ther

nP

anno

nian

Bas

in

lacustrine (DLS)marine (Paratethys)

Glo

govn

ica

Sje

niča

k

Par

ipov

ac

Low

er

part

of

NN

5

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posi

te

16.2

4±0.

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17.9

1±0.

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15.4

3±0.

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čane

Luča

ne

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ne

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15.3

1±0.

16

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koB

asin

0204060

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ko a

sh

0

200

400

600

800

1000

1200

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0±0.

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asin

(firs

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Man

dek

ash

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ica

ash

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int. carbonate sandbreccias with carbonate clasts

Man

dek

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jB

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300

400

200

500

126


of 15.36 ± 0.16 Ma. However, due to a slight amount of excess argon, the 15.31 ± 0.16 Ma isochron age best reflects its crystallization age (table 6.1). The reversed polarity interval of the upper part of the Gacko section correlates to C5Br, constraint by the 15.31±0.16 Ma age of the Gacko tuff. The basal part of the succession is estimated at 15.85 Ma, assuming that the seven transgressive-regressive sequences correspond to ~100 kyr eccentricity cycles (Mandic et al., 2011). Extrapolation of sedimentation rates suggests that Lake Gacko disappeared around 15.0 Ma.

6.3.5 Bugojno and Sarajevo Basins

The general quality of the paleomagnetic demagnetization diagrams (online supplementary material) was too low to arrive at a reliable magnetostratigraphy for the Bugojno and Sarajevo basins (Fig. 6.2). Both basins, however, bear elephantoid proboscidean remains (Gomphotherium and Prodeinotherium bavaricum) (Malez & Slišković, 1976; Milojević, 1964). The oldest occurrence of these proboscideans in Europe was dated at 17.5 Ma by Palfy et al. (2007) based on radio-isotopic ages for a tuff that overlies fossil footprint bearing sandstones and clays with Gomphoterium remains. The oldest dated occurrence of proboscideans in the Dinarides is at the Tušnica coal mine in the Livno-

Tomislavgrad basin, here dated at 17.0 Ma. The occurrence of proboscideans thus indicates that the sections exposed in the Gračanica coal mine in the Bugojno basin and above the Kakanj coal seam of the Sarajevo basin are younger than 17.5 Ma. Additional age constraints in Bugojno come from a number of small mammal teeth, which pertain to Democricetodon gracilis and Democricetodon mutilus, associations that are correlated to the upper part of MN4 and MN5 (Wessels et al. 2008), i.e between 17 and 13.8 Ma (Agustí et al., 2001).

6.3.6 Banovići basin

The infill of the late Oligocene-early Miocene Banovići Basin (Fig. 6.2) is approximately 500 m thick. The fauna of Turija mammal site, located just below the main coal layer of the basin, best compares with localities from the European MP30/MN1 mammal zones and Anatolian zone B (chapter 4). The magnetostratigraphic pattern of the 167 m long Grivice section reveals a long reversed interval interrupted by a short interval of normal polarity (Fig. 6.4), and correlates best to chrons C6Cr to C6Cn.2r of the GPTS. The MP30/MN1 Turija fauna would thus correlate to the base of C6Cr, at an age of approximately 24 Ma.

6.3.7 South Pannonian Basin

In the area of the Southern Pannonian Basin (Fig. 6.2) back arc extension triggered the deposition of an up to 500 m thick series of continental, alluvial and lacustrine sediments (Pavelić et al., 2003) with fauna highly similar to that of the Dinaride Lakes (chapter 5). The series is generally overlain by transgressive marine deposits that indicate a wide-spread ingression of the Paratethys Sea into the SPB. Two volcanic ash levels are located at the base and at the top of the continental series, respectively (table 6.1). The weighted mean age for the lower, Glogovnica ash, is 18.11 ± 0.06 Ma. Weighted mean ages for the Sjeničak and Paripovac ashes are 16.00 ± 0.09 Ma and 16.00 ± 0.07 Ma respectively. These ages

Cha

ttian

Olig

ocen

eM

ioce

neA

quita

nian

Turija MP30-MN1/2 Anatolian Zone B

BanovićiBasin

C7n.2n

C6Cr

C6Cn.3n

C6Cn.2r

C6Cn.2n

C6Cn.1r

C6Cn.1n

C7n.1r

C7n.1n

C6Br

24.0

23.0

24.4

23.8

23.6

23.4

23.2

24.2

22.8

22.6

22.4

Grivice

MaATNTS*

Figure 6.4. Correlation of the magnetostratigraphy of the Banovici basin to the geological timescale (Gradstein et al. 2004).

127


indicate that the SPB continental phase lasted at least 2 Myr (chapter 5) and that it is largely coeval with the deposition of lacustrine sediments with similar fauna in the more interior parts of the Dinarides.

6.3.8 Ugljevik basin

The coal mine of the Ugljevik basin (Fig. 6.2), situated at the northern rim of the Dinarides, exposes around 100 m of late Oligocene lacustrine deposits superposed by 70 m of marine Paratethys sediments (Vrabac et al. 1995). The small mammal assemblage recovered from the lacustrine sediments (Wessels et al., 2008) resembles the Late Oligocene ones from Thrace and Anatolia (Ünay, 1989; Ünay et al., 2003). Its age is estimated to be late Oligocene but slightly older then Banovići (Wessels et al., 2008).

Our new chronostratigraphic data indicate an early to middle Miocene phase of basin formation and lacustrine deposition in the Central Dinarides (Fig. 6.3). At this time, the Dinaride Lake System stretched out from the Southern Pannonian Basin across the Dinarides as far out as the Pag Island in the north-west and the Gacko basin in the south-west. Pre-dating this Miocene phase, there is a late Oligocene phase of which deposits are present in the Ugljevik and Banovići basins. Chronostratigraphic constraints on this earlier phase remain rudimentary.

6.4 Late Oligocene to middle Miocene paleomagnetic rotation data

The paleomagnetic data also allow determination of the vertical axis rotation these basins experienced since deposition of their lacustrine infill. For the Pag, Sinj, Livno, Tomislavgrad, Gacko and Banovići basins demagnetization diagrams were of high quality and the established paleomagnetic directions are thus also suitable for constraining their rotation history.

For the Crnika section on Pag (Jimenez-Moreno et al., 2009), 121 directions are available. The vast majority is of reversed polarity and only 16 directions constitute the short normal polarity interval at the top of the section. The reversal test fails, because of a number of low inclination intermediate directions at the base of the short normal chron. The remaining normal directions are statistically too few in number. In order to overcome this problem, we have decided to rely on the 102 directions from the long and extensively sampled reversed chron. On these, the VanDamme cutoff was applied and discarded eight outliers. The remaining 94 directions have an average declination of 182 ± 3.3°, and an average inclination of -56.5 ± 2.5° (Fig. 6.8). Bedding planes hardly vary along the Crnika section, which results in a non-significant Tauxe and Watson (1994) fold test.

The 221 paleomagnetic directions established for the magnetostratigraphy of the Lučane section in the Sinj basin (de Leeuw et al. 2010) were subjected to a fold test (Tauxe and Watson, 1994) in order to test their primary origin. Maximum clustering occurs close to 100% unfolding (supplementary Fig. 6.2). This proves the directions are pre-folding and thus most likely primary. Since the dataset consists of both reversed and normal directions, a reversal test was applied. The reversed and normal directions do not share a common true mean direction. This is likely attributable to the incomplete removal of a present day overprint (Fig. 6.5) which displaces both the normal and reverse average in a westward direction. This effect is largely compensated for when the reversed directions are inverted to normal polarity and added to the set of normal directions. In order to discard outliers, the vanDamme cutoff (VanDamme, 1994) is applied. The resulting average for the remaining 180 directions, has a declination of 355 ± 2.7° and an inclination of 56.4 ± 2.1.

Thermal demagnetization diagrams for the Ostrožac samples of the Livno-Tomislavgrad basin are of a higher quality than AF demagnetization diagrams (chapter 2) and we regard the thermal directions as more reliable (Fig. 6.3). We therefore determine the rotation of the Ostrožac section exclusively based on the thermal results. These comprise both normal and reverse directions and for both sets outliers were discarded with the VanDamme cutoff. The reversal test (McFadden & McElhinny, 1990) succeeds with classification C and the two sets of opposite polarity were subsequently merged. This results in an average direction with a 13.8 ± 6.5° declination, and a 50.6 ± 6.1° inclination (Fig. 6.5). For the Tušnica section only AF demagnetization was carried out. These samples have a much higher NRM intensity than those from the Ostrožac section and the established directions are regarded as suitable for a rotation

128


study. After removal of outliers with the VanDamme cutoff, the remaining reversed 12 samples provide an average direction with a 190.2 ± 15.9° declination and a -49.6 ± 15.4° inclination (Fig. 6.5). The Tušnica and Ostrožac sections share a common true mean direction and may thus be combined to arrive at an average 14.0 ± 5.8° declination and a mean 50.3 ± 5.5° inclination for the Livno-Tomislavgrad basin (Fig. 6.5). The combined set of Tušnica and Ostrožac directions was subjected to a fold test (Tauxe and Watson, 1994). Clustering is highest near 100% untilting (supplementary Fig. 6.2), which suggests these directions have a pre-tilt and therefore most likely primary origin.

The upper part of the Gračanica section in the Gacko basin is characterized by reversed directions, interpreted to be of primary origin (Mandic et al., 2011). On the total set of 18 directions the Vandamme cutoff was applied. The remaining 16 directions average at a declination of 180.7 ± 6.2° and an inclination of -51.0 ± 5.8° (Fig. 6.5).

The average direction for the Grivice section of the Banovići basin has a declination of 2.8 ± 2.8° and an inclination of 53.5 ± 2.4° (Fig. 6.5) based on 70 directions of both normal and reversed polarity (chapter 4). The reversal test is positive, but a fold test is not possible since all samples were taken along a section with a relatively constant bedding plane.

In general, the quality of the demagnetization diagrams for samples from the Gračanica open pit coal mine in the Bugojno basin was too low to arrive at a reliable magnetostratigraphy, attributable to the high amount of organic content in the sediments. Despite the poor quality results for most of the section, several distinct levels provided a good paleomagnetic signal (supplementary Fig. 6.2) and were considered suitable for rotational analysis. A total of 14 directions was used to establish an average paleomagnetic direction with a declination of 359.1 ± 5.3° and an inclination of 50.0 ± 5.1° for the Bugojno basin (Fig. 6.5).

The calculated directions and resulting averages and other statistical parameters for these 5 basins are displayed in figure 6.5, table 6.3, and supplementary table 6.3. The expected magnetic field direction at the time of deposition was calculated for each location based on the 20 Ma pole for Europe (Torsvik, 2008). Comparison of the expected and measured directions leads us to conclude that the intra-montane basins of the Central Dinarides hardly rotated since their lacustrine sediments accumulated. The coherence of these results suggests that the Dinaride Block as a whole experienced no tectonic rotation since the Late Oligocene.

6.5 A compilation of Mesozoic and Cenozoic paleomagnetic data: the differential rotation of crustal fragments in the Dinarides

We have made a compilation of available literature data (Márton 1983; Márton et al. 1987, 1990, 2002, 2003, 2008; Kissel et al. 2003; this study) ranging in age from early Cretaceous to Miocene to place our rotation results for the Central Dinarides in a comprehensive framework (table 6.2). To facilitate comparison between the compiled paleomagnetic data and the palinspastic reconstruction of Ustaszewski et al. (2008) we adhere to their tectonic subdivision and group the data accordingly. In the study area, four different micro-terranes are distinguished (Fig. 6.6). The first of these terranes, progressing from the more internal to the more external parts of Adria, is its undeformed segment, exposed on a large part of the Istria peninsula. The second terrane, called SW imbricated Adria and Dalmatian Zone, consists of the southern part of the Croatian coast and islands attributed to the Dalmatian zone. The third terrane, called NW imbricated Adria and High Karst, consists of the northern Croatian islands and a large part of the imbricate structures of the High-Karst west of the Split-Karlovac fault. The fourth and last terrane, called the Dinarid Nappes & SW High Karst, consists of the Budva-Cukali zone, the High-Karst, Dalmatian and Pre-Karst units, the East-Bosnian Durmitor and Drina-Ivanjica nappes and obducted ophiolites,

Figure 6.5. Equal area diagram with ChRM directions for the sampled sections and sites in the different intra-montane basins. The red line indicates the average declination and the grey area the corresponding uncertainty (dDx). For a detailed explanation, we refer to the main text.

129


Expected declination with uncertainty*

Average declination with uncertainty

= present day field direction

Lućane section normal &reversedcombined

tilt adjusted tilt adjusted

Lućane section

geographic

Sinj Basin (18-15 Ma)

VGP

dec = 355.0 ± 2.7inc = 56.4 ± 2.1

A95 = 2.2k = 36.7n = 180

Ostrožac and Tušnica combined tilt adjusted

Livno Basin(17-14 Ma)

geographic

tilt adjustedgeographic

OstrožacSection

OstrožacSection

TušnicaSection

TušnicaSection

VGP

tilt adjusted &transformed

to normal

dec = 14.0 ± 5.8inc = 50.3 ± 5.5

A95 = 5.0k = 16.1

n = 54

geographic tilt adjustedPag Basin(~16.7-17 Ma)

Crnika section

Crnika section

VGP

tilt adjusted &Crnika section transformed

to normal

dec = 2.4 ± 3.3inc = 56.5 ± 2.5

A95 = 2.6k = 51.1

n = 94

Bugojno Basin (Late E. Miocene)

geographic tilt adjusted

VGP

Gračanicasection

Gračanicasection

tilt adjustedGračanicasection

dec = 359.1 ± 5.3inc = 50.0 ± 5.1

A95 = 4.6k = 91.9

n = 14

Banovici Basin(24-23.2 Ma)

geographic tilt adjusted tilt adjusted &transformed

to normal

VGP

Grivicesection

Grivicesection

Grivicesection

dec = 2.8 ± 2.8inc = 53.5 ± 2.4

A95 = 2.3k = 79.8

n = 70

geographicGacko Basin(15.8-15.2 Ma)

tilt adjustedGračanicasection

Gračanicasection

VGP

Gračanicasection

tilt adjusted &transformed

to normal

dec = 0.7 ± 6.2inc = 51.0 ± 5.8

A95 = 5.3k = 58.4

n = 16

130


situated between the Split-Karlovac fault and Skadar - Peć (Shkodra/Scutari - Pejë/Peja) line. Apart from the Crnika section on Pag, located on the NW imbricated Adria and High Karst micro-terrane (nr 3), all other sites are situated on the terrane labeled Dinarid Nappes & SW High Karst (nr 4).

All paleomagnetic data of the four investigated terranes come from sedimentary rocks (table 6.2). After categorization according to location, the data were grouped according to age. For each site the net rotation was calculated based on a comparison of the observed declination with the directions of the magnetic field expected based on the location of the paleomagnetic pole for Eurasia with a corresponding age (Torsvik, 2008). For each age and location group, the mean declination was calculated using Fisher statistics, after the Vandamme cut-off was applied to exclude outliers (table 6.2).

The amounts and timing of rotation for each of the four blocks that constitute the Dinarides that results from our compilation are summarized in table 6.3 and figure 6.6.

Undeformed Adria rotates 48 ± 10° counterclockwise (CCW) since the early Cretaceous of which 34 ± 7° occur since the Middle Cretaceous. Its post Eocene rotation amounts to 28 ± 13° and its post 20 Ma rotation is not constrained by any local paleomagnetic data. The small amount of pre middle Cretaceous sites on the High Karst NW part of the imbricated carbonate platform, forces us to group all of them together and derive a 32 ± 7° CCW rotation for this block. The Eocene rocks are rotated with a similar amount, although the error is very high. The post early Miocene rotation of the NW High Karst block is purely determined by data from the lacustrine Pag locality and amounts to only 3° CCW. The Dalmatian SW part of the imbricated carbonate platform has rotated 21 ± 11° CCW since the Cretaceous, of which 15 ± 7° CCW occurred since the Eocene. The post 20 Ma rotation of this block is poorly constrained. The average rotation of sites in lacustrine sediments and positioned on the SW High Karst imbricated carbonate platform, pre-karst, Bosnian flysch, and Dinaric nappes amounts to only 3 ± 6° CCW and thus indicates that this block did not experience any significant rotation since the late Oligocene.

The paleomagnetic data from the Sutorina Valley in Montenegro (Kissel, 1995), require an additional explanation. The sampled flysch sections in this valley are in principle located on the Dalmatian part of the carbonate platform according to the geological map (1:100.000). Paleomagnetic data indicate that these sites hardly rotated since the Eocene. This does, on the other hand, not match with the 15° CCW rotation of the other sites located on this block. There are two possible explanations. First, the sampled “flysch” sections could be of Miocene instead of Eocene age. Some authors in fact consider parts of the Eocene flysch deposits exposed along the Croatian Adriatic coast as Miocene, based on nannoplankton studies (Mikes et al., 2008 and references therein). This solution would imply the Dalmatian zone docked against the Central Dinarides just prior to the Miocene and did not rotate since. An alternative explanation would be that the Sutorina Valley flysch is actually located in the Budva-Cukali Zone, which then consequently did not rotate since the Eocene.

We realize that we use pre-20 Ma data for undeformed Adria and the Dalmatian zone with post 20 Ma data for the Central Dinarides block. Absence of Miocene rocks on the former two blocks excludes a

Table 6.2. Paleomagnetic rotation data from the literature suitable for analysis, sorted according to zone and time, with the corresponding rotations per site, and per block/time interval, calculated versus stable Europe. Site: the site name mentioned in the literature. Reference: the corresponding publication from which the data were taken. Lithology: the sample lithology. Age: the age of the sampled rocks. The approximate site location is listed to allow calculation of the expected declination and inclination based on the reference pole of corresponding age. Declination and inclination after tilt correction are listed in combination with α95, k and n. N used refers to the amount of samples from which the directions were used for calculation of the mean direction. N original refers to the total amount of measured directions for the site. For some sites the k-value was not listed in the literature but calculated from α95 and n. For most sites, A95 is not known and therefore δDx and δIx cannot be calculated. Although we realize α95 is not a good estimate of the error in declination, since in this case fisher statistics are applied to site directions instead of poles, we adhere to it in order to guarantee a uniform approach. For our own sites, we have listed A95 in order to allow calculation of δDx and δIx. Site mean directions are compared to the expected directions for the site based on the corresponding reference pole according to Torsvik (2007) in order to calculate the rotation and flatening. For the reference poles used, age, latitude, longtitude, A95 and p value are listed.

131


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12.2

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.0M

rlera

-1M

arto

n 20

08Li

mes

tone

Turo

nian

–Con

iaci

an93

.685

.845

.00

14.0

0n

49-2

29

735

590

80.3

169.

12.

653

.955

.5±

2.5

5.0

±3.

2-2

7.0

±11

.36.

5±

7.5

Mrle

ra-2

Mar

ton

2008

Lim

esto

neTu

roni

an–C

onia

cian

93.6

85.8

45.0

014

.00

n60

-21

1328

610

9080

.316

9.1

2.6

53.9

55.5

±2.

55.

0±

3.2

-26.

0±

21.5

-4.5

±10

.6M

rlera

-3M

arto

n 20

08Li

mes

tone

Turo

nian

–Con

iaci

an93

.685

.845

.00

14.0

0n

58-1

313

198

1090

80.3

169.

12.

653

.955

.5±

2.5

5.0

±3.

2-1

8.0

±20

.3-2

.5±

10.6

Brš

ica

quar

ry H

R 6

44–6

57, 7

52–7

57M

arto

n 20

08Li

mes

tone

Turo

nian

–Con

iaci

an93

.685

.845

.00

14.0

0n

57-1

711

288

2090

80.3

169.

12.

653

.955

.5±

2.5

5.0

±3.

2-2

2.0

±16

.6-1

.5±

9.0

Sv.

Nik

ola

quar

ry H

R 6

58–6

68, 7

58–7

65M

arto

n 20

08Li

mes

tone

Turo

nian

–Con

iaci

an93

.685

.845

.00

14.0

0n

52-4

97

2916

1990

80.3

169.

12.

653

.955

.5±

2.5

5.0

±3.

2-5

4.0

±9.

53.

5±

6.0

Mrle

ra-4

HR

518

–527

Mar

ton

2008

Lim

esto

neTu

roni

an93

.688

.645

.00

14.0

0n

47-2

34

135

910

9080

.316

9.1

2.6

53.9

55.5

±2.

55.

0±

3.2

-28.

0±

5.4

8.5

±3.

8K

ažel

a H

R 4

94–5

05M

arto

n 20

08Li

mes

tone

Turo

nian

93.6

88.6

45.0

014

.00

n46

-12

499

1212

9080

.316

9.1

2.6

53.9

55.5

±2.

55.

0±

3.2

-17.

0±

5.3

9.5

±3.

8Va

ltura

qua

rry

HR

607

–614

Mar

ton

2008

Lim

esto

neTu

roni

an93

.688

.645

.00

14.0

0n

53-2

18

667

890

80.3

169.

12.

653

.955

.5±

2.5

5.0

±3.

2-2

6.0

±11

.02.

5±

6.7

Pre

man

tura

Mar

ton

2008

Lim

esto

neTu

roni

an93

.688

.645

.00

14.0

0n

47-4

915

177

790

80.3

169.

12.

653

.955

.5±

2.5

5.0

±3.

2-5

4.0

±18

.08.

5±

12.2

Ste

rna

Mar

ton

1983

Cen

oman

ian

99.6

93.6

45.0

014

.00

n60

.2-1

9.1

764

810

100

8116

6.2

4.1

53.1

56.3

±3.

95.

2±

5.1

-24.

3±

12.1

-3.9

±6.

4U

mag

I-III

Mar

ton

1983

Cen

oman

ian

99.6

93.6

45.0

014

.00

n44

.3-7

6.7

20.3

88

1710

081

166.

24.

153

.156

.3±

3.9

5.2

±5.

1-8

1.9

±23

.612

.0±

16.5

Kam

enja

k, N

jive

HR

506

–517

Mar

ton

2008

Lim

esto

neC

enom

ania

n99

.693

.645

.00

14.0

0n

671

1118

1125

100

8116

6.2

4.1

53.1

56.3

±3.

95.

2±

5.1

-4.2

±23

.7-1

0.7

±9.

3K

amen

jak,

Njiv

e H

R 6

15–6

27M

arto

n 20

08Li

mes

tone

Cen

oman

ian

99.6

93.6

45.0

014

.00

r85

-33

270

1025

100

8116

6.2

4.1

53.1

56.3

±3.

95.

2±

5.1

-8.2

±29

.8-2

8.7

±4.

0K

amen

jak,

Gra

kalo

vac

HR

467

–481

Mar

ton

2008

Lim

esto

neC

enom

ania

n99

.693

.645

.00

14.0

0n

47-2

15

6016

1610

081

166.

24.

153

.156

.3±

3.9

5.2

±5.

1-2

6.2

±7.

29.

3±

5.1

Kam

enja

k, G

raka

lova

c H

R 4

88–4

93M

arto

n 20

08Li

mes

tone

Cen

oman

ian

99.6

93.6

45.0

014

.00

r-6

4-1

504

270

66

100

8116

6.2

4.1

53.1

56.3

±3.

95.

2±

5.1

-155

.2±

8.4

120.

3±

4.5

Um

agM

arto

n 20

08Li

mes

tone

Cen

oman

ian

99.6

93.6

45.0

014

.00

n35

-36

1328

68

100

8116

6.2

4.1

53.1

56.3

±3.

95.

2±

5.1

-41.

2±

13.4

21.3

±10

.9Lo

vrec

ica

HR

766

–773

Mar

ton

2008

Lim

esto

neC

enom

ania

n99

.693

.645

.00

14.0

0n

46-2

66

470

38

100

8116

6.2

4.1

53.1

56.3

±3.

95.

2±

5.1

-31.

2±

8.0

10.3

±5.

7N

ovi G

rad

IIIM

arto

n 19

83A

lbia

n11

2.0

99.6

45.0

014

.00

n27

.4-8

5.4

28.5

85

411

080

.719

1.4

3.6

54.3

55.2

±3.

60.

5±

4.4

-85.

9±

26.2

27.8

±23

.0N

ovi G

rad

IM

arto

n 19

83A

lbia

n11

2.0

99.6

45.0

014

.00

n56

.4-6

520

.310

75

110

80.7

191.

43.

654

.355

.2±

3.6

0.5

±4.

4-6

5.5

±31

.3-1

.2±

16.5

Ant

enal

qua

rry

HR

415

–423

, 790

–797

Mar

ton

2008

Lim

esto

neA

lbia

n11

2.0

99.6

45.0

014

.00

n39

-34

551

1616

110

80.7

191.

43.

654

.355

.2±

3.6

0.5

±4.

4-3

4.5

±6.

316

.2±

4.9

Lant

erna

HR

424

–438

Mar

ton

2008

Lim

esto

ne#N

/A!

112.

099

.645

.00

14.0

0n

42-3

35

5915

1511

080

.719

1.4

3.6

54.3

55.2

±3.

60.

5±

4.4

-33.

5±

6.5

13.2

±4.

9La

nter

na H

R 4

39–4

43M

arto

n 20

08Li

mes

tone

Alb

ian

112.

099

.645

.00

14.0

0r

6710

967

55

110

80.7

191.

43.

654

.355

.2±

3.6

0.5

±4.

49.

5±

19.2

-11.

8±

7.7

Nov

a Va

s-2

HR

528

–547

Mar

ton

2008

Lim

esto

neA

lbia

n11

2.0

99.6

45.0

014

.00

n40

-37

936

920

110

80.7

191.

43.

654

.355

.2±

3.6

0.5

±4.

4-3

7.5

±10

.115

.2±

7.7

Kan

fana

r, qu

arry

HR

572

–577

, 818

–824

Mar

ton

2008

Lim

esto

neA

lbia

n11

2.0

99.6

45.0

014

.00

n50

-24

427

07

1311

080

.719

1.4

3.6

54.3

55.2

±3.

60.

5±

4.4

-24.

5±

6.1

5.2

±4.

3B

uje

Mar

ton

2008

Lim

esto

neA

lbia

n11

2.0

99.6

45.0

014

.00

n39

-21

1444

48

110

80.7

191.

43.

654

.355

.2±

3.6

0.5

±4.

4-2

1.5

±14

.916

.2±

11.6

Vila

nija

Mar

ton

2008

Lim

esto

neA

lbia

n11

2.0

99.6

45.0

014

.00

n42

-33

1123

99

110

80.7

191.

43.

654

.355

.2±

3.6

0.5

±4.

4-3

3.5

±12

.413

.2±

9.2

Aver

age,

with

Van

dam

me

cuto

ff11

289

45.0

014

.00

50.2

-28.

34.

938

2428

5.9

100

8116

6.2

4.1

53.1

56.3

±3.

95.

2±

5.1

-33.

5±

7.4

6.1

±5.

0

UN

DEF

OR

MED

AD

RIA

, 112

-151

Ma

Stra

nci +

15

Cer

ven

Mar

ton

1983

Bar

rem

ian-

Apt

ian

130.

011

245

.00

14.0

0n

29.4

-42.

219

.916

517

120

78.4

196.

52.

656

.652

.8±

2.7

0.6

±3.

1-4

2.8

±18

.623

.4±

16.1

Lim

ski K

anal

Mar

ton

1983

Bar

rem

ian-

Apt

ian

130.

011

245

.00

14.0

0n

35.4

-76.

123

.312

58

120

78.4

196.

52.

656

.652

.8±

2.7

0.6

±3.

1-7

6.7

±23

.417

.4±

18.8

Šoš

ici H

R 6

81–6

88, 7

82–7

89M

arto

n 20

08Li

mes

tone

Vala

ngin

ian–

Apt

ian

140.

211

245

.00

14.0

0n

43-3

58

4210

1613

075

.219

3.6

2.9

59.8

49.3

±3.

30.

1±

3.4

-35.

1±

9.2

6.3

±6.

9K

anfa

nar,

quar

ry H

R 5

58–5

71M

arto

n 20

08Li

mes

tone

Vala

ngin

ian–

Apt

ian

140.

211

245

.00

14.0

0n

49-2

819

175

1413

075

.219

3.6

2.9

59.8

49.3

±3.

30.

1±

3.4

-28.

1±

24.0

0.3

±15

.4Ta

r, P

erci

Mar

ton

2008

Lim

esto

neVa

lang

inia

n–A

ptia

n14

0.2

112

45.0

014

.00

n42

-44

68

813

075

.219

3.6

2.9

59.8

49.3

±3.

30.

1±

3.4

-44.

1±

2.7

7.3

±2.

6N

ova

Vas-

1M

arto

n 20

08Li

mes

tone

Vala

ngin

ian–

Apt

ian

140.

211

245

.00

14.0

0n

42-6

58

66

130

75.2

193.

62.

959

.849

.3±

3.3

0.1

±3.

4-6

5.1

±2.

77.

3±

2.6

Rov

inj

Mar

ton

1983

Vala

ngin

ian-

Hau

teriv

ian

140.

213

045

.00

14.0

0n

48.6

-38.

224

.25

105

140

70.4

176.

76.

163

.944

.4±

7.7

6.4

±6.

8-4

4.6

±31

.1-4

.2±

20.3

Kirm

enja

k qu

arry

HR

581

–590

, 798

–809

Mar

ton

2008

Lim

esto

neTi

thon

ian

(Upp

er J

uras

sic)

150.

814

5.5

45.0

014

.00

n45

-50

849

822

150

67.7

148.

25.

962

.146

.6±

7.1

17.9

±6.

7-6

7.9

±10

.51.

6±

8.6

Mon

dala

co q

uarr

y H

R 6

69–6

80, 7

74–7

81M

arto

n 20

08Li

mes

tone

Tith

onia

n (U

pper

Jur

assi

c)15

0.8

145.

545

.00

14.0

0n

52-5

07

539

2015

067

.714

8.2

5.9

62.1

46.6

±7.

117

.9±

6.7

-67.

9±

10.6

-5.4

±8.

0Av

erag

e, w

ith V

anda

mm

e cu

toff

151

112

45.0

014

.00

43.8

-48.

18.

438

99

9.4

130

75.2

193.

62.

960

49.3

±3.

30.

1±

3.4

-48.

2±

9.7

5.5

±7.

2

132


Site

Ref

eren

ceLi

thol

ogy

Age

App

r. Si

te lo

catio

nD

irect

ion

afte

r tilt

cor

rect

ion

Ref

eren

ce P

ole

Expe

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IEx

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ed D

Rot

atio

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ning

Sta

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ngP

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n A

95A

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I xdI

Dx

dDR

∆RF

∆F(M

a)(M

a)(°

N)

(°E

)(°

)(°

)(°

)us

edor

igin

al(M

a)(°

N)

(°E

)(°

)(°

)(°

)(°

)(°

)(°

)(°

)(°

)(°

)(°

)

NW

PA

RT

OF

THE

IMB

RIC

ATED

C

AR

BO

NAT

E PL

ATFO

RM

: HIG

H K

AR

ST,

16-1

7 M

a

Pag

(Crn

ica

sect

ion)

this

stu

dyla

cust

r. m

arls

1716

44.4

515

.03

n+r

56.5

2.4

2.1

51.1

9495

2.6

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137.

43

48.4

60.6

±2.

65.

7±

4.0

-3.3

±4.

44.

1±

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PA

RT

OF

THE

IMB

RIC

ATED

C

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NAT

E PL

ATFO

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: HIG

H K

AR

ST,

37-5

6 M

aLu

kini

177

1–17

81M

arto

n 20

03M

arl

Bar

toni

an40

.437

.245

.50

13.9

0r

44-7

615

1011

1240

82.3

150.

52.

850

.358

.9±

2.5

6.9

±3.

6-8

2.9

±17

.114

.9±

12.2

Lopa

r, S

an M

arin

oM

arto

n 19

90Fl

ysch

mid

-late

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ene

48.6

33.9

44.7

014

.70

r55

-43

28.3

85

1240

82.3

150.

52.

851

.058

.3±

2.6

6.9

±3.

6-4

9.9

±44

.73.

3±

22.7

Kas

telin

aM

arto

n 19

90Li

mes

tone

mid

-late

Eoc

ene

48.6

33.9

44.7

514

.66

n35

-39

15.8

167

1640

82.3

150.

52.

851

.058

.3±

2.6

6.9

±3.

6-4

5.9

±15

.823

.3±

12.8

Om

isal

jM

arto

n 19

90Fl

ysch

mid

Eoc

ene

48.6

37.2

45.2

014

.50

n2

5915

.516

78

4082

.315

0.5

2.8

50.6

58.7

±2.

56.

9±

3.6

52.1

±12

.756

.7±

12.6

Njiv

ice

Mar

ton

1990

Lim

esto

nem

id E

ocen

e48

.637

.245

.18

14.5

0n

4639

4.1

140

1010

4082

.315

0.5

2.8

50.6

58.7

±2.

56.

9±

3.6

32.1

±5.

512

.7±

3.9

Dol

ovo

Mar

ton

1990

Lim

esto

nem

id E

ocen

e48

.637

.245

.18

14.5

0r

20-1

317

543

640

82.3

150.

52.

850

.658

.7±

2.5

6.9

±3.

6-1

9.9

±14

.838

.7±

13.8

Pre

snic

a 25

–31

Mar

ton

2003

Mar

lLu

tetia

n48

.640

.445

.50

13.9

0r

51-4

910

357

750

79.1

154.

22.

653

.256

.2±

2.5

8.7

±3.

2-5

7.7

±13

.15.

2±

8.2

Kub

ed 1

–8M

arto

n 20

03M

arl

Lute

tian

48.6

40.4

45.5

013

.90

n50

113

208

850

79.1

154.

22.

653

.256

.2±

2.5

8.7

±3.

2-7

.7±

16.6

6.2

±10

.6N

amel

ess

9–14

Mar

ton

2003

Mar

lLu

tetia

n48

.640

.445

.50

13.9

0n

47-1

09

715

650

79.1

154.

22.

653

.256

.2±

2.5

8.7

±3.

2-1

8.7

±10

.99.

2±

7.5

Sla

nisc

eM

arto

n 19

90Li

mes

tone

early

-mid

Eoc

ene

55.8

37.2

44.7

014

.70

r49

-14

6.3

947

850

79.1

154.

22.

653

.955

.5±

2.6

8.7

±3.

2-2

2.7

±8.

16.

5±

5.4

Kam

por

Mar

ton

1990

Lim

esto

neea

rly-m

id E

ocen

e55

.837

.244

.75

14.6

6r

18-1

084.

973

1317

5079

.115

4.2

2.6

53.9

55.6

±2.

58.

7±

3.2

-116

.7±

4.9

37.6

±4.

4S

tara

_Bas

ka 1

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1987

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1990

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Mar

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1990

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1990

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1990

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1990

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1987

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1990

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1990

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1987

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134


Figu

re 6

.6. (

belo

w)

Cret

aceo

us t

o pr

esen

t da

y bl

ock

rota

tions

com

pile

d fr

om t

he a

vaila

ble

liter

atur

e on

top

of

the

20 M

a re

cons

truc

tion

of U

stas

zew

ski e

t al

. (2

008)

of

the

diffe

rent

tect

onic

uni

ts th

at th

e D

inar

ides

com

pris

es. A

rrow

s in

dica

te th

e av

erag

e ro

tatio

n of

rock

s of

the

indi

cate

d ag

e. T

he p

artia

l arc

inte

rsec

ting

the

arro

ws

indi

cate

s th

e co

rres

pond

ing

unce

rtai

nty.

The

rot

atio

n po

le o

f Ad

ria w

as d

raw

n ac

cord

ing

to U

stas

zew

ski e

t al

. (2

008)

.

Blo

ckTe

cton

ic

Rec

onst

ruct

ion

Pale

omag

netic

Dat

a

<20

Ma

14-2

4 M

a34

-56

Ma

66-7

1 M

a89

-112

Ma

112-

151

Ma

UN

DEF

OR

MED

AD

RIA

-20

-28

± 13

-34

± 7

-48

± 10

NW

PA

RT

OF

THE

IMB

RIC

ATED

CA

RB

ON

ATE

PLAT

FOR

M: H

IGH

KA

RST

-5-3

± 4

-34

± 28

-32

± 7

SW P

AR

T O

F TH

E IM

BR

ICAT

ED C

AR

BO

NAT

E PL

ATFO

RM

: DA

LMAT

IAN

ZO

NE

-20

- 0-1

5 ±

7-2

1 ±

11

SW P

AR

T O

F TH

E IM

BR

ICAT

ED C

AR

BO

NAT

E PL

ATFO

RM

: D

ALM

ATIA

N O

R B

UD

VA-C

UK

ALI

ZO

NE

not i

nclu

ded

?-1

± 5

SW IM

BR

ICAT

ED C

AR

BO

NAT

E PL

ATFO

RM

(HIG

H K

AR

ST),

PRE-

KA

RST

, BO

SNIA

N F

LYSC

H, A

ND

DIN

AR

IC N

APP

ES0

-3 ±

6

Tabl

e 6

.2. S

umm

ary

of t

he c

ompi

led

rota

tion

for

each

of t

he b

lock

s th

e D

inar

ides

com

pris

e pe

r tim

e pe

riod.

For

the

pur

pose

of c

ompa

rison

, the

am

ount

of r

otat

ion

sinc

e 20

Ma

as p

redi

cted

by

the

tect

onic

rec

onst

ruct

ion

by U

stas

zew

ski (

2008

) is

list

ed.

135


rota

tion

pole

for A

dria

45˚N

, 6˚4

0’E

0 7 . c

y a d - t n e s e r p t s a o c

y a d - t n e s e r p t s a o c

Adriatic Plate

Adriatic Plate

Adriatic Plate

Cre

tace

ous

to p

rese

nt d

aybl

ock

rota

tions

on

top

of20

Ma

reco

nstr

uctio

n

EEEEEEEEEEEEEEuuuuuuuuuuuuuuuuuuuuuuuurrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrroooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooororrorrorrorrorrorrorooooooooppppppppppppeeeeeeeeeeeeeeeeeeeeeeeeeee

aaannnnnMMMMMMMMMMMMMMMMMMMMMMMMM

rrrrrrrrrrrrrrrrrrrrrrrrrrrrrggggggggggggggggggggggggggggggggggggggggggggggin

20 Ma

20 Ma

35 Ma

90 Ma

110 Ma

35 Ma60 Ma65 Ma

35 Ma20 Ma

20 Ma

20 Ma

110 Ma

20 Ma

Rot

atio

n in

ferr

ed fr

om

tect

onic

reco

nstru

ctio

n

Rot

atio

n in

dica

ted

by p

aleo

mag

netic

data

unce

rtain

ty

Bloc

k

SW im

brica

ted A

dria,

Dalm

atian

Zon

e

shortening

Ca 210 km

Dina

rid N

appe

s &

SW H

igh

Kars

t

NW im

brica

ted A

dria,

Hi

gh K

arst

Unde

form

ed A

dria

adap

ted

from

Usz

tasz

ewsk

i et a

l. 20

08

136


better paleomagnetic estimate of their post 20 Ma rotation. For the Central Dinarides, currently no data older than Late Oligocene are available.

The new paleomagnetic data from the Late Oligocene to Middle Miocene Dinaride Lakes clearly fit in the framework provided by the compiled Mesozoic and Cenozoic paleomagnetic data. Both magnitude and timing of the rotation is furthermore in good agreement with the rotations estimated by Ustaszewski et al. (2008).

The apparent mismatch between paleomagnetic and structural geological data concerning the post-Eocene rotation of Adria originally arose since a post-Pontian, i.e. younger than 6.0-4.7 Ma (Krijgsman et al., 2010), 30° rotation of sites in the Croatian part of the Pannonian basin was postulated to result from the push of CCW rotating Adria and thus assumed to have affected the whole Dinaride Block (Márton et al., 2003). However, paleomagnetic directions from Late Oligocene to Middle Miocene lacustrine sites located on the SW High Karst imbricated carbonate platform, pre-karst, Bosnian flysch, and External Dinaride nappes confirm that this block did not rotate since the late Oligocene, in agreement with the data of Kissel (1995). The counterclockwise rotation of Adria can therefore not have driven the very young rotation of sites in the southwestern Pannonian basin. Their rotation is most probably attributable to the last tectonic inversion event (Latest Miocene to Pleistocene) in that part of the Pannonian Basin, which in places is characterized by NE-SW oriented strike slip faulting and temporally corresponds with the timing of the rotations (Márton et al., 2002). It is thus most likely that Adria rotated only 20° since 20 Ma, as indicated by structural geological data (Ustaszewski et al., 2008).

The lack of rotation of the Central Dinarides implies decoupling of Dinarides from the Tisza-Dacia Mega-Unit. The latter block experiences a major clockwise rotation during the Miocene (Hinsbergen et al., 2008). The differential movement between the two blocks could be accommodated along the sinistral strike-slip faults observed in the Southern Pannonian Basin (Pavelić, 2001; Tari, 2002). Ustaszewski (2008) assumed that the Dinarides have remained attached to the Tisza-Dacia Mega-Unit but nevertheless had to make some geometrical adjustments along the Sava Zone thus taking a certain amount of decoupling into account.

6.6 Neogene to recent deformation: AMS as an indicator of post-depositional strain

The anisotropy of the low field magnetic susceptibility (AMS) of sedimentary rocks provides a rapid and precise description of the average preferred mineral orientation, or fabric (Mattei, 1997). This fabric may in turn reflect the regional stress field (Tarling and Hrouda, 1993) and its recognition can shed light on the tectonic evolution of the area under consideration. In this study, we measured the AMS of 400 samples from 9 different sections according to the same methodology as applied by Vasiliev et al. (2009). The AMS tensors were calculated according to Jelinek (1977). Error ellipses of the susceptibility axes are calculated according to (Jelinek, 1978).

Figure 6.7 provides an overview of the acquired AMS data for the Dinaride basins. The sediments of the Sinj, Gacko and Bugojno basins displayed only weak anisotropy, whilst for the other basins a clear AMS pattern could be established. Here, the minimum axis is generally oriented perpendicular to the bedding plane, which is characteristic for sedimentary fabrics. The maximum and intermediate axes are generally orthogonal and interpreted to reflect post-depositional strain.

For the lacustrine sediments that accumulated on top of the Dinaride Carbonate Platform, i.e. the Pag and Livno-Tomislavgrad basin, the maximum axis of anisotropy (red arrows in Fig. 6.7) aligns with the average structural trend of the mountain range. This suggests that these basins were subject to compression orthogonal to the Dinaridic strike after deposition of the lake sediments, in sheer contrast with results from the more internal parts of the orogen, i.e. the Sarajevo, and Banovići basin. Here, the maximum susceptibility axis is oriented perpendicular to the structural trend. This is interpreted to indicate post-depositional extension perpendicular to the mountain range. In Ugljevik, the maximum susceptibility axis is oriented nearly E-W and parallel to the average local strike in agreement with subsurface data (Horvath, 1995). In our view, this reflects a phase of N-S compression rather then E-W extension. Our results thus suggest that while NE-SW directed compression continued in the external

137


n o t a l a B . L

Sara

jevo

Split

18˚

20˚

22

14˚

16˚

12˚

18˚

20˚

22˚

14˚

16˚

12˚

46˚ 47˚

45˚

44˚

43˚

42˚

46˚

47˚ 45

˚ 44˚ 43˚ 42˚

Pag

Sin

j Li

vno B

ugoj

no

Ban

ović

i

Ugl

jevi

k

Gac

ko

Kak

anj

Sara

jevo

Sa

raje

vo

Sara

jevo

Sa

raje

vo

Sara

jevo

Ban

ović

i (G

rivic

e)

Pag

(Crn

ika)

(Tuš

nica

)

Sara

jevo

(Kak

anj)

Sinj

(Luč

ane)

Ugl

jevi

kB

ugoj

no(G

rača

nica

)

(Ost

roža

c)G

acko

(Gra

čani

ca)

Livn

o-To

mis

lavg

rad

Figu

re 6

.7. O

verv

iew

of

AMS

dire

ctio

ns f

or t

he s

uite

of

sam

pled

sec

tions

. In

the

equ

al a

rea

diag

ram

s, c

ircle

s in

dica

te t

he m

inim

um a

xes,

squ

ares

th

e in

term

edia

te,

and

tria

ngle

s th

e m

axim

um a

xes

of e

long

atio

n. A

niso

trop

y in

the

sam

ples

fro

m t

he S

inj,

Gac

ko a

nd B

ugoj

no b

asin

s w

as v

ery

low

, w

hich

res

ults

in a

larg

e sp

read

in d

irect

ions

. Fo

r th

e ot

her

loca

tions

the

min

imum

axi

s is

ort

hogo

nal t

o th

e be

ddin

g pl

ane,

as

expe

cted

for

a

sedi

men

tary

fabr

ic. T

he re

d ar

row

s in

dica

te th

e di

rect

ion

of p

oten

tial e

xten

sion

whi

le th

e bl

ue a

rrow

s in

dica

te th

e di

rect

ion

of p

oten

tial c

ompr

essi

on.

Und

erly

ing

map

with

tec

toni

c un

its a

dapt

ed fro

m U

stas

zew

ski e

t al

(20

08),

for

lege

nd s

ee fi

gure

6.2

.

138


Dinarides in post Middle Miocene times, the internal Dinarides underwent from NE-SW to N-S directed extension. The north-eastern boundary of the orogen was affected by a post Langhian phase of N-S compression.

Our interpretation agrees well with the general post-orogenic tectonic framework for the Dinarides. In the Late Miocene and Pliocene, the collisional systems surrounding the Pannonian basin became locked and the region was subject to a compressional stress field (Huismans, 2002). The continuing NW-ward motion of the Adriatic indenter led to N-S shortening across the Dinarides, associated with surface uplift and erosion and induced dextral wrenching along orogen parallel strike-slip faults (Ilić and Neubauer, 2005). Along the southern margin of the Pannonian basin, subsurface data demonstrate general inversion (Horvath, 1995). It is this phase of N-S compression that is also reflected in the AMS data for the Ugljevik basin. In the external and southern internal Dinarides, the direction of shortening was more NE-SW directed (Ilić and Neubauer, 2005; Marović et al., 1999; Fodor et al., 1999; Oldow et al., 2002), which is corroborated by the AMS results for the Pag and Livno-Tomislavgrad basins.

It is striking that in the late Oligocene sediments of the Banovići basin and the Early Miocene sediments of the Sarajevo basins the maximum axis of susceptibility has a NE-SW direction. This signifies either orogen parallel compression or, alternatively, extension perpendicular to the orogen. The latter scenario seems in this case more viable since the second phase of intra-montane basin formation, in which extension penetrated deep into the Dinarides, largely postdates the sampled sediments. It still remains remarkable that the late Miocene to recent inversion is not reflected in the AMS of these sediments.

6.7 Consequences for the post-orogenic evolution of the Dinarides

Our results provide new insight into the post early Oligocene evolution of the Dinarides. The newly constructed chronology elucidates the timing of intra-montane basin formation. A first cycle of lacustrine sediments accumulated in basins induced by strike-slip faults penetrating deeply into the orogen (Hrvatović, 2006) in the latest Oligocene (Fig. 6.4). Optimum climatic conditions (Zachos et al., 2001) stimulated the formation of these lakes (chapter 4). Sedimentation apparently stalled in the Aquitanian and Early Burdigalian. A second and extensive phase of lacustrine deposition took place from 18 to ~13 Ma (Fig. 6.3). The Dinaride Lake System spread out over large parts of the orogen, when the Miocene Climatic Optimum (Zachos et al., 2001) induced favorable climatic conditions. The coincidence of this phase of intra-montane basin formation with the main phase of extension in the Pannonian Basin System suggests a causal link. Rifting induced extension apparently penetrated into the Dinarides and affected even its westernmost external reaches. This is corroborated by the data of Ilić and Neubauer (2005) who studied paleostress indicators near Prijepolje in the Central part of the Eastern Dinarides. They document a phase of NE-SW extension that results in opening of the intra-montane basins and occurs in conjunction with the main phase of extension in the Pannonian Basin. A phase of orogen parallel NW-SE directed extension follows and stimulates further growth of the basins.

Concurrent with the accumulation of DLS sediments in the intra-montane realm, lacustrine and alluvial sediments accumulate in the Sava and Drava depressions (Fig. 6.3). In the lower part of calcareous nannoplankton zone NN5 these basins subside below the base-level of Paratethys and are flooded with marine water (Ćorić et al., 2009). Whereas some authors invoke a strike-slip mechanism to account for the subsidence of the Sava and Drava depressions (Tari, 2002; Hrvatović, 2006), others characterize them as half-grabens (Pavelić, 2001; Fodor et al., 1999).

Around 15 Ma, coal formation indicates shallowing of the Gacko basin, shortly after which sedimentation comes to a halt. Simultaneously, coal formation in the Sinj basin intensifies and breccias, originating from the margins, enter the lake. Carbonate sand layers enter the Livno-Tomislavgrad basin around the same time, and soon after, the first limestone breccias appear in the Mandek section. Apparently, compressional tectonic activity intensifies since the intercalating breccias coarsen and thicken. Deposition continues until at least 13 Ma. While sedimentation in the intra-montane basins comes to a halt, the central part of the study area is inverted (Tari, 2002). In the Sava and Drava depressions, deposition comes to a halt

139


as well, and a Late Sarmatian erosional unconformity develops (Saftić et al., 2003).The Late Miocene to Pliocene evolution of the mountain chain is characterized by renewed shortening

and dextral wrenching (Pribicević et al., 2002; Tari, 2002; Picha, 2001; Ilić and Neubauer, 2005). During this period most major faults accommodated a significant amount of strike-slip motion and it significantly influenced the present day structural fabric of the study area. Interestingly, several authors (Hrvatović, 2006; Tari, 2002) also invoke a strike-slip mechanism for the formation of the Miocene intra-montane basins. One of the main arguments is that the basins are often associated with the major faults that at present bear a strike-slip signature, and generally have an en-echelon shape. The detailed stress analysis by Ilić and Neubauer (2005), on the other hand, reveals NE-SW extension during the Early to Middle Miocene. The responsible extensional faults are in most cases reactivated and overprinted during late-stage wrenching. Although we cannot exclude a strike-slip component during the Miocene, it is thus conceivable that the intra-montane basins formed due to orogen-perpendicular extension and received their en-echelon shape later.

The current seismic activity and ongoing convergence between Adria and Europe (Oldow et al. 2002; Bennett et al, 2008; Grenerczy et al. 2005) indicate that deformation in the Dinarides has not stopped yet. A change in GPS velocities (Bennett, 2008; Grenerczy et al., 2005) between the central Dinarides, the Dalmatian coast and Adria, indicates differential motion between these plates. This supports a model in which the larger part of the Adria push is accommodated by deformation within the Dinarides and only a small amount transferred further east. Miocene, Pliocene and even Quaternary sediments are affected by faults (Dragičević et al., 1999; Prebičević et al., 2002) and the frontal thrusts are at present located in the Adriatic, just offshore the Croatian Islands (Tari, 2002; Schmid et al. 2008; Korbar, 2009).

6.8 Conclusions

An initial phase of basin formation struck the Dinarides in the latest Oligocene as evident from e.g. the Banovići Basin. A second and more profound phase started around 18 Ma and continued until at least 13 Ma. It thus coincided with a phase of severe extension in the Pannonian Basin System, which suggests a causal link. At that time, the Dinaride Lake System extended from the Pag Island in the far west to the Gacko Basin in the south and the Southern Pannonian Basin in the East. The reigning optimum climatic conditions stimulated formation of the lakes. Our new paleomagnetic results and a reinterpretation of available literature data indicate that the Dinarids did not rotate since the deposition of the DLS sediments. This implies that the rotation of Adria was not transferred to the Croatian margin of the Pannonian Basin and that the shortening resulting from this rotation must mainly have been accommodated within the Dinarides, in correspondence with present day GPS results. AMS results corroborate post Middle Miocene shortening along the frontal zone of the Dinarides. In the more central parts of the Dinarides, the AMS pattern most likely reflects extension orthogonal to the average Dinaride strike. The absence of rotation in the Dinarides implies the orogen remained largely decoupled from the Tisza-Dacia block since 24 Ma.

Acknowledgments

We are highly indebted to Stjepan Ćorić (Geological Survey Vienna), Sejfudin Vrabac (RGN Tuzla), Enes Šišić (RMU Banovići), Zlatko Ječmenica (RMU Ugljevik), Hamdija Puljarga (RU Gračanica), Jovan Olujić (Geozavod - Zvornik), Boško Vuković (Rudnik i TE Gacko), Davor Pavelić and Alan Vranjković (RGN Zagreb), Jeronim Bulić (HPM Zagreb) and Mathias Harzhauser (NHM Vienna) - this study would not be possible without their support and logistic help. We thank Roel van Elsas for assistance with mineral separation, Jan Wijbrans, Guillaume Dupont-Nivet, Douwe van Hinsbergen for discussion, and Cor Langereis for critically reviewing the manuscript. The study contributes to the Austrian FWF Project P18519-B17: ‘Mollusk Evolution of the Neogene Dinaride Lake System” and was supported by the Netherlands Research Centre for Integrated Solid Earth Sciences (ISES) and by the Netherlands Organization for Scientific Research (NWO/ALW).

Sampling a vertical cliff face in Transylvania

Chapter 7Age of the Badenian Salinity Crisis; impact of Miocene climate variability on the Circum-Mediterranean region

Arjan de Leeuw, Krzysztof Bukowski, Wout Krijgsman, and Klaudia Kuiper

Massive evaporites were deposited in the Central European Paratethys Sea during the Badenian Salinity Crisis (BSC). The scarcity of absolute age constraints so far hampered a thorough understanding of these salt deposits. Here, we present a robust chronology for this catastrophic event by 40Ar/39Ar dating of volcanic tuffs below and within the Badenian salts in southern Poland. The onset of BSC evaporite deposition is dated at 13.81 ± 0.08 Ma and the entire event is estimated to have lasted 200 to 600 k.y.. Correlation to oxygen isotope records shows that the BSC evaporites are just preceded by glacial event Mi-3b, suggesting a causal relationship. The corresponding sea level fall most likely restricted the open marine connection to the Mediterranean, thereby trapping the salt in the deep Paratethys basins.

This chapter is based on: de Leeuw A., Bukowski K., Krijgsman W., and Kuiper K.F. (2010) Age of the Badenian salinity crisis; impact of Miocene climate variability on the circum-Mediterranean region. Geology 38: 715-718.

142


7.1 Introduction

The circum-Mediterranean Miocene is marked by several occurrences of extraordinary paleoenvironmental catastrophes during which large water-masses were cut off from the open ocean, resulting in the formation of hypersaline waters, complete destruction of biological ecosystems, and deposition of giant (100–1000m) evaporite units in the deep as well as marginal basins. These so-called salinity crises occurred in the Middle Miocene (~15 Ma) of the Red Sea (e.g., Rouchy et al., 1995), the Late Miocene (~6 Ma) of the Mediterranean (e.g., Hsü et al., 1973) and the Badenian (~14 Ma) of the Paratethys (e.g., Peryt, 2006) – the larger precursor of the Black Sea – and are commonly considered to have been caused by a complex interplay of tectonic and glacio-eustatic processes that resulted in progressive closure of the marine gateways, and hence in the obstruction of the hydrological exchange between different basins.

An accurate chronostratigraphic framework is a key prerequisite for the correct deciphering of evaporite basin evolution, as has been demonstrated for the Messinian Salinity Crisis (MSC) of the Mediterranean (Krijgsman et al., 1999; Roveri et al., 2008). In this paper, we focus on constructing a reliable chronology for the Badenian Salinity Crisis (BSC) by isotopic (40Ar/39Ar) dating of volcanic tuff layers located directly below and within the Badenian halite units of the northern Carpathian Foredeep in southern Poland (Fig. 7.1). The new data help to distinguish between tectonic and paleoclimatic causes and to elaborate on the progression of evaporite formation in these highly restricted environments.

7.2 The Badenian Salinity Crisis (BSC) of the Paratethys

The Badenian stage marks the last period of significant connectivity between the Mediterranean and Paratethys. The only postulated seaway was situated in the narrow area between the Alps and the Dinarids (Fig. 7.1), which progressively decreased during the lower Badenian by a combination of tectonic and glacio-eustatic processes (Rögl, 1998). This resulted in the formation of massive salt deposits in the Central European Paratethys basins.

The Badenian evaporites of southern Poland are 30–100 m thick and consist of Ca-sulphates (anhydrite and gypsum) or halite with intercalations of claystones and minor Ca-sulphates. Sulphates occur in a broad belt along the northern and the central parts of the Carpathian Foredeep and its foreland. Halite is limited to a small area along the northern rim of the Carpathians (Fig. 7.1). The halite is commonly underlain by deep marine siliciclastics and carbonates (Skawina Beds) and overlain by deep marine to brackish siliciclastic deposits (Chodenice and Gliwice Beds) (Porębski et al., 2003; Peryt, 2006). Facies analyses suggest that the deep Badenian evaporites originated from density-stratified brines (Garlicki 1979; Bąbel, 1999). Halite precipitation commenced in the deepest parts of the basin where the heaviest brines occurred, and slightly pre-dated the onset of Ca-sulphates at the basin margins (Peryt, 2006).

Based on the absence of the nannofossil Sphenolithus heteromorphus, the base of the Badenian evaporites was correlated to zone NN6 of the geological time scale (Peryt, 1997), corresponding to the time interval of approximately 13.65–13.0 Ma (Lourens et al., 2004). This was supported by K-Ar dating of the tuff horizon (WT-3) occurring below the salt in southern Poland (Fig. 7.1) that gave an age of 13.6 ± 0.2 Ma (Dudek et al., 2004), although the 40Ar/39Ar age for the same tuff provided an age of 12.0 ± 0.3 Ma (Bukowski et al., 2000).

7.3 Radio-isotope dating

We decided to re-date the key volcanic ashes of the Polish successions by the 40Ar/39Ar method to better understand the processes underlying the BSC. Two tuff layers (WT-1 and WT-3) were sampled in the historical – exploited during more than seven centuries – salt mines in Wieliczka and Bochnia (Bukowski, 1999; Dudek et al., 2004). WT-1 is located a few meters below the first halite deposits, while WT-3 is found in the middle part of the halite unit. The 40Ar/39Ar ages of the hornblende (WT-1) and biotite (WT-3) separates obtained from these layers were determined applying (standard) incremental heating techniques (see online supplementary material). Ages were calculated using the recalibrated

143

Chapter 7: Age of the Badenian Salinity Crisis; impact of Miocene climate variability on the Circum-Mediterranean region

Figu

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144


Fish Canyon Tuff sanidine neutron flux monitor age of 28.20 ± 0.03 Ma (Kuiper et al., 2008), thereby allowing direct comparison to astronomically dated geological records. Results are displayed as age spectra diagrams (Fig. 7.2). Duplicate measurements provided plateau ages of 13.78 ± 0.04 and 13.83 ± 0.04 Ma for WT-1. Plateau ages for the two WT-3 duplicates amount to 13.58 ± 0.01 and 13.61 ± 0.02 Ma. None of the inverse isochron intercepts deviates from the value of atmospheric argon. We arrive at a weighted mean age of 13.81 ± 0.03 Ma for WT-1 and 13.60 ± 0.01 Ma for WT-3. Errors increase to 0.08 and 0.07 Ma respectively if the uncertainties in standard age and decay constants are included (see online supplementary material). All errors are quoted at the 1 sigma level.

7.4 The onset of the BSC

Reliable chronologic constraints provide better means to discriminate between climatic and tectonic processes by direct correlation to the global oxygen isotope records and astronomical target curves. Our 40Ar/39Ar results indicate that evaporite deposition in the Carpathian Foredeep started shortly after 13.81 ± 0.08 Ma. This indicates that the BSC took place simultaneously with or directly after Mi3b (Fig. 7.3), the major step in Middle Miocene global cooling dated at 13.82 ± 0.03 Ma in the Mediterranean (Abels et al., 2005), and clearly expressed in many isotope records worldwide (e.g. Holbourne et al., 2005). A significant decline in temperature has also been documented from below the evaporites by the displacement of warm-water planktonic foraminiferal assemblages and the expansion of the cool-water populations (Gonera et al., 2000; Bicchi et al., 2003). Harzhauser and Piller (2007) furthermore indicate that the Mid-Badenian Extinction Event, a sharp decline in the number of gastropod and foraminifera taxa that characterizes evaporite free Paratethys successions coeval with the BSC, coincides with a decline in the share of thermophilous mollusc taxa. This indicates a drop in sea-surface temperature from 16-18° C to 14-15° C. A causal relationship between the Mi3b cooling event and the BSC thus seems likely. At first sight, it is counter intuitive that evaporites form under colder climatic circumstances since this would principally lead to less evaporation. The BSC, however, was preceded by such intense climatic optimum (Böhme, 2003) that the hydrological budget (precipitation – evaporation) remained negative, directly after the cooling event.

In the Early and Middle Badenian, and thus during the BSC, the circulation in the Central Paratethys was anti-estuarine (Baldi, 2006). The Mi3b cooling step triggered a significant drop (~40-50 m) in global sea level (John et al., 2004; Westerhold et al., 2005) that deteriorated the water exchange through the gateway to the Mediterranean. Model results show that under negative hydrological budgets salinity will rise if the connecting strait is sufficiently shallow (Meijer and Krijgsman, 2005). While changes in precipitation patterns at low and mid latitudes accompanying the global drop in temperature might have stimulated evaporite formation in Central Europe, the Mi3b sea level lowering most likely played a crucial role in the onset of the BSC. It restricted the deep outflow of dense saline waters from the Carpathian Foredeep, trapping the salt within the Paratethys basins and consequently set off the BSC.

The Mi3b event furthermore coincided with minimum eccentricity values associated with the 400-kyr cycle and minimum obliquity amplitudes associated with the 1.2-Myr cycle (Abels et al. 2005). It implies that this specific orbital configuration indirectly triggered the BSC, thereby providing an extreme example of the influence of long period astronomical cyclicity on the paleoenvironment.

7.5 Progression and termination of the BSC

The difference in age between volcanic ash WT-1 and WT-3 is 210 ± 32 k.y. Since all samples were co-irradiated side by side and measured consecutively in the same setup, the uncertainty in the age difference (or duration) is much lower than the uncertainty in the absolute age of the individual volcanic ashes, since we do not need to take into account uncertainties in standard ages and decay constants. Assuming the upper half of the evaporites was deposited in a similar time span as the lower half, the total package would be deposited in ~400 k.y. Taking uncertainties in the stratigraphic position of WT3 and potential changes in sedimentation rate into account, we estimate the total duration of evaporite

145


200

150

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Skawina Beds (Lower Badenian)

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Tabl

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Sum

mar

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/39Ar

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Squa

re W

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wei

ghte

d m

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age.

146


deposition was 200 to 600 k.y. Although we realize this is only a rough first order approximation, it is the best estimate on the basis of isotopic age data currently available. The BSC was previously suggested to comprise only 20–35 k.y., based on observations of cyclic cm-scale laminae within the Ca-sulfate and halite deposits, under the assumption that these represent annual accumulations (Peryt, 2006). Our results indicate that the observed cyclicity is of ~10–30 yr, implying a driving mechanism different from seasonality. The 11 yr sunspot cycle is a potential external forcing factor. Internal forcing could result from autocyclic behavior in regional climate systems, nowadays evident in e.g., the El Nino-Southern Oscillation and North Atlantic Oscillation. The laminae could furthermore represent storm events that often have a recurrence interval of several years (e.g., Peryt, 2006).

Even though only a rough age estimate for the end of the BSC can be derived, the newly acquired 40Ar/39Ar ages do not support a glacio-eustatic cause, since after 13.82 Ma global climate gradually deteriorated and sea-level did not rise significantly. The termination of the BSC is thus most likely related to a tectonically-driven transgression resulting in the dilution of brines by inflowing marine water (e.g., Krzywiec, 2001; Baldi, 2006). The circulation pattern in the Central Paratethys changed to estuarine, which might have enhanced the cooling of Central Europe (Baldi, 2006). Many authors envisage a new connection to the Indian Ocean, due to the presence of Indo-Pacific elements in the Late Badenian fauna (e.g. Rogl, 1998). This hydrological change must have taken place in a relatively short time span, because the basin water rapidly gained normal salinity.

7.6 Comparison to the Messinian Salinity Crisis of the Mediterranean

The Badenian Salinity Crisis was not a unique event in the circum-European region. The Mediterranean Sea has experienced an even more dramatic crisis when it became progressively isolated from the Atlantic Ocean during the Messinian (e.g., Hsü et al., 1973; Rouchy and Caruso, 2006). This Messinian salinity crisis (MSC), astronomically dated between 5.96 and 5.33 Ma (Krijgsman et al., 1999), generated more than 1000 m of salt in the deep Mediterranean basins. The 630 k.y. duration of the MSC is of the same order of magnitude as the duration of the BSC, although MSC halite deposition lasted at maximum ~80 k.y. (Krijgsman and Meijer, 2008). The MSC halites also have a cyclic laminated character, similar to the BSC evaporites (e.g., Roveri et al., 2008). This indicates that sub-Milankovitch forcing is an important component in Miocene climate evolution and especially evident during evaporite deposition. The timing of the MSC does not coincide with a major glacio-eustatic sea level fall (Hodell et al., 2001). Tectonic restriction of the sea strait to the Atlantic Ocean and consequent blocking of the inflow of normal marine water is thus its most likely cause. It follows that the MSC and BSC are two end-member types in terms of the mechanism of gateway restriction; tectonic sill uplift and eustatic sea level lowering.

While tectonic sill uplift provided the right conditions, the exact timing of the MSC may well have been controlled by the 400-kyr component of the Earth’s eccentricity cycle (Krijgsman et al., 1999). For the BSC, the trigger was a drop in sea level provoked by coincident minima in the long period astronomical cycles. This suggests that these orbital cycles exert a large influence on these semi-isolated basins and are a key factor in the initiation of salinity crises in the (circum-)Mediterranean area.

7.7 Conclusions

Our newly acquired 40Ar/39Ar ages indicate that the evaporites of the BSC were deposited shortly after 13.81 ± 0.08 Ma, suggesting they were triggered by the worldwide cooling event Mi3b. Sea level lowering induced by this main step in the Middle Miocene climate transition most likely restricted the already narrow straits connecting the Paratethys basin to the world’s oceans. Due to preferential blockage of the deep saline outflow, salt could accumulate, eventually leading to brine formation and start of the BSC. Evaporite formation lasted 200 to 600 k.y. before incipient tectonism permitted normal marine water into the basins, which diluted the salt-precipitating brines. Since both the BSC and the MSC were likely indirectly triggered by a specific orbital configuration, they represent extreme examples of the influence long-period astronomical cycles can have on the paleoenvironment.

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0.8 1.2 1.6 2.5 2 1.5 1

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Figure 7.3. Detailed correlation figure showing the chronostratigraphic position of the Wieliczka halite deposits. The BSC coincides with the start of the Miocene Icehouse period. The foraminifera assemblages in the drill-core Gliwice 19 indicate cooling just before deposition of evaporites (data from Bicchi et al., 2003). Stable oxygen and carbon isotope data from ODP site 1146 in the Caribbean (Holbourn et al., 2007) show the Middle Miocene global climate transition with its major step, Mi3b, at 13.82 Ma. Ages of the magnetic chrons originate from Lourens et al. (2004) with adjustments according to Hüsing et al. (2010). Position of the Langhian-Serravallian boundary according to Abels et al. (2005).

Acknowledgments

We thank Roel van Elsas for assistance with mineral separation and Jan Wijbrans for discussion. This research was supported by the Netherlands Research Centre for Integrated Solid Earth Sciences (ISES) and by the Netherlands Organization for Scientific Research (NWO/ALW). We would like to express our gratitude for the comments from three anonymous reviewers that significantly improved the manuscript.

View over the Mureş Valley, Transylvania

View over the Mureş Valley, Transylvania

Chapter 8Paleomagnetic & chronostratigraphic constraints on the evolution of the middle Miocene Transylvanian Basin: implications for Central Paratethys Stratigraphy & emplacement of the Tisza-Dacia plate

Arjan de Leeuw, Sorin Filipescu, Liviu Maţenco, Wout Krijgsman, Klaudia Kuiper, and Marius Stoica

Chronostratigraphic control on the Paratethys stages remains rudimentary compared to the cyclostratigraphically constrained Mediterranean stages. We here derive better age constraints on the Badenian, Sarmatian and Pannonian Central Paratethys regional stages through integrated biostratigraphic, magnetostratigraphic, and 40Ar/39Ar research in the Transylvanian Basin. Six new 40Ar/39Ar ages were determined for tuffs intercalating with the generally deep marine basin infill. The now in total 9 radio-isotopically dated horizons in the basin were traced along seismic lines into a synthetic seismic stratigraphic column in the basin center and serve as first order tie-points to the astronomically tuned Neogene timescale. Paleomagnetically investigated sections were likewise traced and their polarity in general corroborates the 40Ar/39Ar results. The new, high resolution timeframe for the sedimentary infill of the Transylvanian Basin constrains the onset of accumulation of the Dej Tuff Complex, indicative of a period of intensive volcanism, between the first occurrence (FO) of Orbulina suturalis at 14.56 Ma and 14.38±0.06 Ma. During the subsequent Badenian Salinity Crisis (BSC) up to 300 m of salt accumulate in the basin center. After the end of the BSC normal marine conditions return, but the fauna of the upper Badenian Velapertina Biozone suggests the absence of a direct connection with the Mediterranean. The strong faunal turnover that marks the Badenian-Sarmatian Boundary is dated at 12.80±0.05 Ma. A second phase of intense volcanism occurs at 12.4 Ma and leads to deposition of the middle Sarmatian tuff complex (Ghiriş, Hădăreni, Turda and Câmpia Turzii tuffs). Rates of sediment accumulation strongly diminish in the basin center at the onset of the Pannonian stage coincident with an approximately 20° CW tectonic rotation of the Tisza-Dacia plate. Concurrent enhanced uplift in the Eastern and Southern Carpathians leads to the isolation of the Central Paratethys and triggers the transition from marine to freshwater conditions. An additional Pannonian to post-Pannonian 6° of CW rotation relates to the creation of antiform geometries in the Eastern Carpathians which are notably larger in the north than in the south. An 8.4 Ma age is determined for the uppermost Pannonian sediments preserved in the central part of the basin. Two sections belonging to middle Pannonian Zone D, and the lower part of Zone E (Subzone E1) cover the 10.6-9.9 Ma time-interval.

This chapter is based on: de Leeuw A., Filipescu S., Maţenco L., Krijgsman W., Kuiper K.F., and Stoica M., Paleomagnetic and chronostratigraphic constraints on the evolution of the middle Miocene Transylvanian Basin and implications for Central Paratethys Stratigraphy and emplacement of the Tisza-Dacia plate. Submitted to Global Planetary Change.

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8.1 Introduction

During the Cenozoic, collision of the northward moving African continent with Eurasia led to the Alpine Orogeny. Along the course of this process, the vast water mass of the Tethys Ocean disintegrated and gave birth to the Mediterranean and Paratethys Seas (e.g. Seneš, 1973; Rögl, 1999) (Fig. 8.1). These huge land-locked seas were subsequently affected by the rising mountain chains that almost fully encircle them. The complex tectonic evolution of the Africa-Eurasia convergence zone, with its multitude of colliding micro-continents (Csontos 1992; Schmid et al. 2008; Ustaszewski et al. 2008), induced a similarly intricate regional paleogeographic history with frequently changing seaways and land-bridges (Popov et al. 2004, 2006; Rögl 1998, 1999; Harzhauser et al. 2002, 2007b).

Gradual growth of the Alpine-Carpathian-Dinaridic orogenic system induced progressive restriction of the Western, Central and Eastern Paratethys (Fig. 8.1) Repeated isolation led to large changes in water chemistry (Matyas et al., 1996; Harzhauser et al. 2007a; Vasiliev et al., 2010a; Peryt, 2006) and inflicted periods of severe endemism (e.g. Steininger et al., 1988; Magyar et al., 1999; Harzhauser et al. 2002, 2003; Kováč et al. 2007). This geodynamically controlled paleogeographic and biogeographic differentiation caused major difficulties in stratigraphic correlation between the different parts of the Paratehys and the Mediterranean (Piller et al. 2005) and thus led to the establishment of regional chronostratigraphic scales for the Central Paratethys summarized in the series “Chronostratigraphie und Neostratotypen” (Cicha et al. 1967; Steininger and Seneš 1971; Báldi and Seneš 1975; Papp et al. 1973, 1974, 1978, 1985; Stevanovic et al. 1990). A high resolution chronologic framework was established over the past few decades for the Mediterranean stages, on which the global timescale currently relies (Lourens et al. 2004). In comparison, chronologic control on most of the Paratethys regional stages remains rudimentary, although significant progress has been achieved in the last decade (e.g. Krijgsman et al., 2010 and references therein). In particular, precise dating of the Middle-Late Miocene sediments of the Central Paratethys, (Geary et al., 2000; Magyar et al., 2007; Vasiliev et al., 2010b) remains an outstanding problem.

In order to overcome part of this problem we conducted an integrated biostratigraphic, magnetostratigraphic and geochronologic study in the Transylvanian Basin. This basin was part of the Central Paratethys during the Miocene and it exposes a thick pile of siliciclastic deposits suitable for paleomagnetic investigations. A number of intercalating volcano-sedimentary products (mostly tuffs) provide key stratigraphic horizons potentially datable with the 40Ar/39Ar method. The continuity of sedimentation, reasonable exposures due to tectonic exhumation of the basin towards the end of the Miocene, and a significantly lower amount of tectonic disturbance and related structural complications and unconformities (Sztanó et al., 2005; Horváth et al., 2006; Krézsek et al., 2010) in comparison with other parts of the Pannonian Basin, make the Transylvanian Basin a favorable study location. The limited size of the Transylvanian Basin and its mature stage of exploration (Krézsek and Bally, 2006) provide the opportunity to correlate outcrops over large distances based on subsurface imagery.

The Transylvanian Basin also provides an optimal location to examine the intriguing post-20 Ma rotation associated with the invasion of the Tisza-Dacia plate into the Carpathian embayment, because of its largely undeformed Middle-Late Miocene sedimentary succession. A precise quantification of the translation mechanics of Tisza-Dacia around the Moesian indenter, driven by the roll-back of a slab attached to the European continent (e.g. Balla, 1987; Ustaszewski et al., 2008), is hampered by contrasting amounts of rotation derived by the paleomagnetic, paleogeographic and structural geology studies, which vary from 10° to almost 70° clockwise rotation after the Paleogene (Csontos, 1995; Fügenschuh and Schmid, 2005; van Hinsbergen et al., 2008 and referencess therein).

We aim here to derive better age constraints on the Middle to Late Miocene regional stages (Badenian, Sarmatian, and Pannonian) of the Central Paratethys through integrated magnetostratigraphic, radio-isotopic and biostratigraphic research in Transylvania. Taking advantage of the resulting chronostratigraphic and paleomagnetic data, the amount as well as the partitioning in time of the Miocene rotation of Tisza-

151

Chapter 8: Paleomagnetic and chronostratigraphic constraints on the evolution of the Transylvanian Basin (Romania)

Dacia can be unraveled. These data provide a more detailed insight in the mechanism of emplacement of the Tisza-Dacia block, supported by recent advances in the understanding of the exhumation and kinematics of the Carpathians.


The Middle to Late Miocene Transylvanian Basin is a 200 km long and 250 km wide semi-isolated back-arc basin (Krézsek et al. 2010) situated in the eastern part of the Central Paratethys and bounded by the Eastern Carpathians, Southern Carpathians and Apuseni Mountains (Fig. 8.2). The Middle to Late Miocene subsidence and subsequent exhumation of the Transylvanian Basin are both closely related to, and concurrent with, major episodes of deformation in the Carpathians in conjunction with the subduction of the Eastern European underneath the Tisza-Dacia plate and the collision that followed (Mațenco et al., 2010).

During the Early Miocene deep marine setting occurred only in the northern part of the basin while continental settings prevailed elsewhere. A Middle Miocene regional transgression established relatively deep marine settings troughout the basin with mixed carbonate-siliciclastic platforms near the margins and siliciclastic environments in deeper areas (Filipescu and Girbacea, 1997; Krézsek et al., 2010). At this time, the Central Paratethys was well connected with the Mediterranean (Fig. 8.3) and there was a great similarity in their paleontological record. The onset of calk-alkaline volcanism related to subduction processes in the exterior Carpathians induced acidic volcanism that lead to widespread tuff deposition in the Central Paratethys (Pécskay et al., 1995; Seghedi et al., 2004b). In the Transylvanian Basin an up to 50m thick tuff complex accumulated (Fig. 8.3, Seghedi and Szakács, 1991). This so called Dej Tuff thus constitutes a major lithostratigraphic marker within the in total maximally 100 m thick lower Badenian sedimentary package.

Paratethys

Africa

Eurasia

Mediterranean

Figure 8.1. Middle Miocene paleogeographic map for the Africa-Eurasia collision zone after Rögl (1999). Red star indicates the Transylvanian Basin.

152


Severe restriction of the Central Paratethys at 13.82 Ma (Peryt, 2006; de Leeuw et al., 2010; chapter 7) induced hypersaline conditions. These led to the deposition of around 300 m of salt in the deeper parts of the Transylvanian Basin and gypsum along the western margin (Krézsek et al. 2006).

In the late Badenian, the Central Paratethys became connected with the Eastern Paratethys that occupied the current Black Sea and Caspian Sea regions. Hypersaline conditions disappeared even though the re-united Paratethys remained semi-isolated (Kováč et al, 2007; Popov et al. 2004; Rögl, 1999). At this time, subsidence rates in the Transylvanian Basin increased tremendously and a, locally over 3 km thick, upper Badenian to Pannonian siliciclastic sedimentary infill accumulated (Krézsek and Filipescu, 2005). The lowermost post-salt succession onlaps the evaporites in the northern, western and southern parts of the basin and thickens from a few 100 m in the west to more than 2000 m in the southeast (Krézsek and Bally, 2006). Depositional environments were characterized by clay-dominated deep marine fans with dominantly planktonic assemblages that suggest anoxic bottom conditions (Krézsek and Filipescu, 2005). During the late Badenian and Sarmatian, the basin’s depocenter was located at its current eastern margin and the corresponding part of the infill thus thins out towards the west.

The Sarmatian part of the basin infill comprises fine grained clastics, such as marls and sandstones, subordinate conglomerates and evaporites. These demonstrate a higher variety of sedimentary environments. Sea-level changes in combination with incipient tectonics in the adjacent mountain chains triggered alternation between normal marine, brackish, and lacustrine conditions (Krézsek and Filipescu, 2005).

At the advent of the Pannonian, the Central Paratethys became fully isolated from the marine realm and turned into a lake (Magyar et al., 1999). The Pannonian sediments of the Transylvanian Basin comprise lacustrine fans and lowstand deltas with sand dominated facies in the more proximal-, and marl dominated facies in the central, more distal part of the basin (Krézsek et al., 2010). The regressive trend was interrupted by a phase of regional uplift towards the end of the Pannonian that exhumed the basin to its present-day altitude of between 300 and 600 m (Mațenco et al., 2010).

Volcanic activity was common in the neighbouring Eastern Carpathians and Apuseni Mountains during the Badenian, Sarmatian and Pannonian (Szakács and Seghedi, 1995; Seghedi et al., 2004a). It generated a large number of volcaniclastics in the sedimentary sequence of the Transylvanian Basin.

8.3 Stratigraphic framework and sampling approach

Our research relies on the stratigraphic framework of Krézsek and Filipescu (2005), and Krézsek and Bally (2006). Sections were investigated using an integrated stratigraphic approach taking multiple samples for both biostratigraphic and paleomagnetic analysis at the same stratigraphic level. We focused on sections that include tuffs and tuffites and sampled them for 40Ar/39Ar dating. We use the Astronomically Tuned Neogene Timescale (Lourens et al., 2004) with adaptations by Hüsing et al. (2010) and supplemented with short term fluctuations in the geomagnetic field identified by Krijgsman and Kent (2004) as a global chronostratigraphic reference framework.

In order to retrieve the relative stratigraphic relation, all sections (Fig. 8.2) were traced along seismic reflectors to a composite stratigraphic column (Fig. 8.4) near the basin’s depocenter where the Middle to Upper Miocene succession is as complete as possible (Fig. 8.2). For each of the sections, an uncertainty in stratigraphic position was estimated based on the proximity to available seismic lines, clarity of line-to-line reflector correlation and the presence of potential sedimentary architectures affecting precision, such as clinoforms with lower reflectivity. The synthetic seismic section in figure 8.4 is vertically scaled in seconds two-way travel time. A depth conversion of all elements projected into this line was performed using average interval velocities derived from neighboring well logs (sonic or density logs, see De Broucker et al., 1998; Krézsek et al., 2010 for further details).

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Figure 8.2. Geological map (A) and east-west cross section of the Transylvanian Basin (B).

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8.4 40Ar/39Ar dating of key stratigraphic horizons

Some of the tuffs present in the Transylvanian Basin (e.g. the Borșa-Apahida, Hădăreni, Ghiriş, Sărmășel and Bazna tuffs) extend over a wide area. Their strong acoustic impedance contrast facilitates correlation across seismic lines (Ionescu, 1994) and they represent excellent regional stratigraphic markers. Several of these tuff levels were sampled for 40Ar/39Ar dating in order to acquire absolute ages for the corresponding stratigraphic horizons.

8.4.1 Methods

The volcanic ashes were processed at the Department of Isotope Geochemistry (VU University Amsterdam). Bulk samples were crushed, disintegrated in a calgon solution, washed and sieved over a set of sieves between 63 and 250 μm. The residue was subjected to standard heavy liquid as well as magnetic mineral separation techniques for k-feldspar. Except for the sample from Apahida, all samples contained ample k-feldspar for 40Ar/39Ar dating. The k-feldspar separates were leached with a 1:5 HF solution in an ultrasonic bath for 5 min. After careful handpicking, they were loaded in a 10 mm ID quartz vial together with Fish Canyon Tuff (FC-2) and Drachenfels (Dra-1, f250-500 and Dra-2, f>500) sanidine that served as flux monitors. The vial was irradiated in the Oregon State University TRIGA reactor in the cadmium shielded CLICIT facility for 10 hours.

Upon return in the laboratory, mineral separates were split into at least 9 duplicate fractions and loaded in a Cu-tray. The loaded Cu-tray was pre-heated to ~200ºC under vacuum using a heating stage and a heat lamp to remove undesirable atmospheric argon. The tray was then placed in the sample house and the system (extraction line + sample house) was degassed overnight at ~150ºC. Incremental heating was performed with a Synrad CO2 laser in combination with a Raylase scanhead as a beam delivery and beam diffuser system. After purification the resulting gas was analyzed with a Mass Analyzer Products LTD 215-50 noble gas mass spectrometer. Beam intensities were measured in a peak-jumping mode in 0.5 mass intervals over the mass range 40–35.5 on 2 a Balzers 217 secondary electron multiplier. System blanks were measured every three to four steps. Mass discrimination was monitored by frequent analysis (~every 10 hours) of aliquots of air. The irradiation parameter J for each unknown was determined by interpolation using a second-order polynomial fitting between the individually measured standards (see online supplementary material of chapter 6).

All age calculations use the decay constants of Steiger and Jäger (1977). Steps with less than 1% radiogenic argon were immediately discarded. The age for the Fish Canyon Tuff sanidine flux monitor used in age calculations is 28.201 ± 0.03 My (Kuiper et al. 2008). The age for the Drachenfels sanidine flux monitor is 25.42 ± 0.03 My (Kuiper et al. in prep). Correction factors for neutron interference reactions are 2.64±0.017×10−4 for (36Ar/37Ar)Ca, 6.73±0.037×10−4 for (39Ar/37Ar)Ca, 1.211 ± 0.003 × 10−2 for (38Ar/39Ar)K and 8.6 ± 0.7×10−4 for (40Ar/39Ar)K. Errors are quoted at the 1σ level and include the analytical error and the error in J. All relevant analytical data as well as error determination can be found in the online supplementary material.

8.4.2 Results

The lower Badenian part of the basin infill is dominated by volcaniclastic deposits of the Dej Tuff complex (Fig 8.2). We dated two samples of the Dej Tuff taken from outcrops near Ciceu Giurgeşti and Dej. For these samples a larger number of repetitive experiments had to be performed, since both contained a large reworked component. This resulted in abnormally high ages for part of the experiments that therefore had to be rejected. For each of the samples at least 7 experiments, provided consistent, homogenous and stratigraphically realistic ages (Fig. 8.5). These were selected to calculate their weighted mean ages. The resulting statistically equivalent weighted mean ages of the samples of the Dej Tuff taken at Ciceu Giurgeşti and Dej are 14.38±0.06 Ma and 14.37±0.06 Ma (Table 8.1).

Several tuff levels occur in close stratigraphic distance to each other in the middle Sarmatian part of the basin infill. The lowermost of these is the Hădăreni Tuff, which is recognized as a seismic reflector

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zsek

an

d Fi

lipes

cu (

2005

) an

d Fi

lipes

cu a

nd S

ilye

(200

8).

156


throughout the larger part of the basin (Fig. 8.4). It crops out in a classic locality at the base of the Brâul Alb (‘White Belt’) hill near Hădăreni where it was sampled. At the top of the same hill, stratigraphically about 100 m higher, there is another tuff level. This level, known as the andesitic Ghiriş Tuff (Mârza and Mészáros, 1991), also crops out at a site just west of Ghirişu Român where it was sampled. Between the Apuseni Mountains and Hădăreni, where the E-W flowing Arieş River has cut outcrops in the south-directed hillsides, the diapirs of the western salt diapir alignment (Krézsek and Bally, 2006) cause tilting and multiple repetition of upper Badenian to middle Sarmatian strata. Two additional middle Sarmatian tuff levels were sampled in outcrops near Turda and Câmpia Turzii respectively (Fig. 8.2). The 40Ar/39Ar experiments for the Turda, Hădăreni, Câmpia Turzii and Ghiriş tuffs provided homogenous age populations (Fig. 8.5). The peaks in the probability density distributions are concordant with the calculated weighted mean ages and we interpret them to reflect the respective tuff’s crystallization age. The Turda, Hădăreni, Câmpia Turzii and Ghiriş tuffs are thus respectively 12.37±0.04 Ma, 12.37±0.05 Ma, 12.35±0.04 Ma, and 12.38±0.04 Ma old (Table 8.1).

The calculated weighted mean ages, peaks in probability density distribution, isochron ages and several other parameters for each of the investigated ashes are listed in (Table 8.1). The full analytical data are available in the online supplementary material. The MSWD was smaller than the statistical T ratio at the 2 sigma level for all reported weighted mean ages. Isochron ages are concordant with the weighted mean ages and the trapped 40Ar/36Ar component is in all cases atmospheric. The calculated weighted mean ages are also concordant with the peaks in probability density distribution and we interpret them to reflect the respective tuff’s crystallization age.

8.5 Resulting time frame and sedimentation rates

The performed 40Ar/39Ar measurements thus provided ages for six stratigraphic horizons. The number of horizons with a radio-isotopically determined age can be augmented with the base and top of the evaporite sequence for which a 13.8 Ma and 13.36 Ma 40Ar/39Ar age was recently established (chapter 7; De Leeuw et al. 2010). Vasiliev et al. (2010) furthermore determined an 11.62±0.04 Ma age for Oarba Tuff that is located in the uppermost Sarmatian part of the basin infill (Fig. 8.4). These radio-isotopic ages provide a first order timeframe for the Middle to Late Miocene infill of the Transylvanian Basin that enables the calculation of average sedimentation rates.

At the onset of the Badenian the Paratethys was well connected with the Mediterranean and endemism does not hamper biostratigraphic correlation to the global timescale. The base of the Badenian coincides with the Early-Middle Miocene boundary (Krézsek and Filipescu, 2005) which is dated at 15.97 Ma (Lourens et al. 2004). The 13.8 Ma onset of evaporite deposition provides a minimum age for the top of the lower Badenian. Whereas the early Badenian thus lasted for over 2 Ma, not more than 100 m of sediments accumulated (Krézsek and Filipescu, 2005). Rates of deposition were consequently rather low.

After the Badenian salinity crisis (BSC), rates of deposition increase tremendously. The upper Badenian to upper Sarmatian part of the basin infill is around 2.6 km thick in the basin centre (Fig. 8.6). The 13.36 Ma termination of the BSC provides a maximum age for its base and the 11.62 Ma old Oarba Tuff provides an approximate age for its top. Based on these two absolute ages an average sedimentation rate of 1.5 m/kyr can be calculated for this infill.

Our synthetic seismic section (Fig. 8.4) demonstrates that the Hădăreni, Ghiriş, Turda and Câmpia Turzii tuffs are all distributed in an interval with a thickness lower than 0.1 sTWT, which is around 130 m given the average interval velocity of Sarmatian sediments. We collectively call these tuffs the ‘middle Sarmatian tuff complex’ (MSTC) since they demonstrably co-occur in a relatively limited stratigraphic interval and according to our 40Ar/39Ar data follow upon each other in a short time-interval.

Our composite stratigraphic column for the late Badenian and Sarmatian (Fig. 8.6) shows that there are about 1.44 km of sediments between the top of the salt (13.36 Ma) and the base of the MSTC (12.36 Ma). The respective upper Badenian and lower Sarmatian sediments were thus deposited at a rate of approximately 1.44 m/kyr. The upper Sarmatian sediments overlying the MSTC have a stratigraphic

157


thickness near 1.16 km. Using the 11.62 Ma age for the Oarba Tuff as tie-point this package was deposited at a comparable rate of around 1.56 m/kyr. This indicates that from the end of the Badenian Salinity Crisis to the fi nal part of the Sarmatian deposition in the Transylvanian Basin was very rapid and occurred at a fairly constant rate. Lower sedimentation rates are obviously to be expected near the western basin margin and in the vicinity of rising diapirs that started their upward movement already in the late Badenian (Krézsek and Filipescu, 2005).

It should be noted that the calculated Badenian, Sarmatian and Pannonian sedimentation rates do not permit direct quantitative conclusions on basin evolution without a proper subsidence history

0 . 0

1 . 0

2 . 0

3 . 0

0 . 0

1 . 0

2 . 0

3 . 0

s T W

T s T W

T 7 6 7 8 8 0 8 2 8 4 8 6 8 8 9 0 9 2 k m 9 6

Vh2-Bs

Vh1

Pn

Bn

Vh2-Bs

Vh1

Pn

Bn

S alt

Sighişoara

Târnăveni

MediaşŞoimuşViforoasa

Oarba DOarba A

Oarba Beta

Turda

Câmpia Turzii

Gherla

Apahida

Fâgâraş

Dej Tuff, Dej Dej Tuff, Ciceu Giurgeşti

Hâdâreni TuffCâmpia Turzii Tuff

Ghiriş Tuff

Oarba Tuff

Figure 8.4. Synthetic seismic section near the depocenter of the Transylvanian Basin. For location see fi gure 8.2. Surface outcrops have been traced along seismic refl ectors from their respective position in the basin into this seismic section in order to determine their position in the stratigraphic succession. The section is vertically scaled in seconds two-way travel time. Vertical brackets indicate the uncertainty in stratigraphic position. For a detailed explanation we refer to the main text.

158


analysis taking into account factors such as paleo-bathymetry, compaction and sea-level variations. However, these sedimentation rates provide a first order qualitative indication of the sedimentary influx associated with processes such as subsidence or increases in input from the source areas.

8.6 Paleomagnetism

Sections were sampled for paleomagnetic investigation since their magnetostratigraphic polarity pattern can provide additional constraints for the correlation to the global timescale and the directional data can be used to unravel the amount as well as the partitioning in time of the Miocene rotation of the Transylvanian Basin (see Fig. 8.11 and Table 8.2 for locations and results).

We will discuss the magnetic polarity pattern of the investigated sections, taking the composite stratigraphic column (Fig. 8.6, 8.8) and the stratigraphic position of the sampled sections inferred from it as a starting point. In order to place the magnetostratigraphic pattern of the investigated sections into the composite stratigraphic column at the right scale, a scaling factor has to be determined. According to the geological map and our own observations the stratigraphic distance between the Badenian-Sarmatian boundary and the Câmpia Turzii Tuff is around 400 m in the area where the respective sections were sampled. In the basin center, the corresponding interval is around 500 m and thus 1.25 times as thick. The upper Badenian to middle Sarmatian sections were scaled accordingly in the composite stratigraphic column. The stratigraphic distance between the Oarba Beta and A outcrop is, based on field data, approximately 530 m. In the composite stratigraphic column, the same stratigraphic interval is nearly 800 m, or 1.5 times as thick. The magnetostratigraphic pattern of Oarba Beta was thus scaled accordingly, which indicates that its earlier correlation to the ATNTS by Vasiliev et al. (2010) does not agree with the correlation to our composite stratigraphic column. Its limited stratigraphic extent in combination with its position in the stratigraphy precludes the occurrence of two reversals (Fig. 8.6 and 8.8).

Approximate scaling factors for the Pannonian sections can also be derived from the sequence of outcrops at Oarba de Mureș. The stratigraphic distance between Oarba A and D is, based on field data, around 120 m. In the composite stratigraphic column, the same interval covers about 215 meters and is thus 1.8 times as thick. All Pannonian sections are scaled accordingly.

8.6.1 Methods

All samples were collected with a hand-held electric drill. The orientation of the standard paleomagnetic cores and corresponding bedding planes were measured by means of a magnetic compass, and corrected for the local magnetic declination. In the laboratory, thermal as well as alternating field demagnetization techniques were applied to isolate the characteristic remanent magnetization (ChRM). The natural remanent magnetization (NRM) of the samples was measured after each demagnetization step on a 2G Enterprises DC Squid cryogenic magnetometer (noise level 3•10-12 Am2). Heating occurred in a laboratory-built, magnetically shielded furnace employing 10-30°C temperature increments. AF demagnetization was accomplished by a laboratory-built automated measuring device applying 5-20 mT increments up to 100 mT by means of an AF coil interfaced with the magnetometer. The presence of iron sulfides in the studied samples was anticipated. In order to overcome the problem of gyroremanence during alternating field demagnetization the specifically designed per component demagnetization scheme of Dankers and Zijderveld (1981) was applied. In addition, small (2-5 mT) field steps were taken in the 20-40 mT range. The ChRM was identified through assessment of decay-curves and vector end-point diagrams (Zijderveld 1967). ChRM directions were calculated by principal component analysis (Kirschvink 1980) and are based on at least four consecutive temperature or field steps.

8.6.2 Results

Several characteristic demagnetization diagrams, of mostly marls and clays, are depicted in figure 8.7. Their NRM intensity ranges between 1 and 1000 *10-4 A/m. A viscous overprint is generally removed at 100°C or 15 mT respectively. During progressive stepwise thermal demagnetization two and sometimes

159


sing

le e

xper

imen

t w

ith a

naly

tical

un

certa

inty

prob

abili

ty d

ensi

ty

dist

ribut

ion

10

12

14

16

0

0.00

2

0.00

4

0.00

6

0.00

8

0.01

11

13

15

Oar

ba-C

Dej

Hăd

ăren

i

Turd

a

Ghi

rişu

Rom

ân

Câm

pia

Turz

ii

Cic

eu G

iurg

eşti

Age

(Ma)

Probability

Figu

re 8

.5.

(lef

t) P

roba

bilit

y de

nsity

dia

gram

s fo

r th

e m

ultip

le t

otal

fu

sion

exp

erim

ents

for

the

inve

stig

ated

tuf

fs.

Sam

ple

Loca

tion

Wei

ghte

d M

ean

Age

(Ma)

n/N

MSW

DPP

D (M

a)%

40A

r(r)

Min

eral

Cys

tal

size

(μm

)In

vers

e

Isoc

hron

age

Inve

rse

Isoc

hron

In

terc

ept

Isoc

hron

M

SWD

Ref

eren

ce

Dej

N47

.159

060,

E02

3.88

2390

14.3

7±0.

067/

131.

3314

.34

95k-

feld

spar

200-

250

14.2

9±0.

1332

8±48

1.46

this

stu

dyC

iceu

Giu

rgeş

tiN

47.2

4148

0,E

024.

0328

9014

.38±

0.06

8/27

0.91

14.4

190

k-fe

ldsp

ar18

0-20

014

.31±

0.10

309±

170.

94th

is s

tudy

Câm

pia

Turz

iiN

46.5

6576

0,E

023.

8901

8712

.35±

0.04

14/1

50.

5212

.35

99k-

feld

spar

250-

500

12.3

6±0.

0429

2±48

0.57

this

stu

dyH

ădăr

eni

N46

.471

167,

E02

3.98

5811

12.3

7±0.

059/

130.

1312

.37

88k-

feld

spar

250-

500

12.3

7±0.

0529

4±4

0.13

this

stu

dyG

hiriş

u R

omân

N46

.799

330,

E02

3.95

3930

12.3

8±0.

048/

90.

7512

.39

98k-

feld

spar

200-

250

12.3

8±0.

0530

0±53

0.87

this

stu

dyTu

rda

N46

.566

940,

E02

3.82

2020

12.3

7±0.

0410

/10

1.4

12.3

998

k-fe

ldsp

ar20

0-25

012

.36±

0.05

315±

361.

15th

is s

tudy

Oar

baN

46.4

5705

2,E

024.

2886

2211

.62±

0.04

11/1

31.

7186

Bio

tite

300-

500

Vasi

liev

et a

l. 20

10

Tabl

e 8

.1. (

belo

w)

40Ar

/39Ar

age

s fo

r sev

eral

tuffs

of t

he T

rans

ylva

nian

Ba

sin.

MSW

D i

s M

ean

Squa

re W

eigh

ted

Dev

iate

s, P

PD i

s th

e pe

ak i

n pr

obab

ility

den

sity

, N

is

the

tota

l nu

mbe

r of

rep

etiti

ons

in t

he s

ingl

e fu

sion

exp

erim

ents

and

n is

the

am

ount

incl

uded

whe

n ca

lcul

atin

g th

e w

eigh

ted

mea

n an

d in

vers

e is

ochr

on a

ge.

%40

Ar(r

) is

the

rad

ioge

nic

amou

nt o

f 40Ar

. Err

ors

are

give

n at

95

per

cent

con

fiden

ce le

vel.

MSW

D,

40Ar

* (%

), a

nd I

nver

se i

soch

ron

inte

rcep

t w

ere

dete

rmin

ed b

ased

on

the

exp

erim

ents

sel

ecte

d fo

r ca

lcul

atio

n of

the

wei

ghte

d m

ean

or

isoc

hron

age

. Age

s in

bol

d ar

e co

nsid

ered

to m

ost a

ccur

atel

y re

pres

ent

the

resp

ectiv

e tu

ffs c

ryst

alliz

atio

n ag

e.

160


three components can be established. A first, low temperature component appears between 100-200°C. The corresponding directions are generally of normal polarity and close to the present day field direction. A notable exception is the Câmpia Turzii section, where the 100-200°C component bears both normal and reversed polarities. A second component appears between 200-300°C. For the Badenian and Sarmatian sections, the corresponding directions are clearly different from the present-day field direction and indicative of a significant counterclockwise rotation. Approximately 90% of the NRM had been removed at 300°C in samples from the Făgăraș, Gherla and Câmpia Turzii sections. For these localities we interpret the 200-300°C component as the ChRM. For samples of the Șoimuş and Viforoasa sections, a higher temperature (300-360°C) component could be established. At 360°C generally 99% of their NRM had been removed. We thus interpret the 300-360°C as the ChRM for the Viforoasa and Șoimuş sections. In alternating field demagnetization diagrams of marly samples generally only a single component appears. The corresponding directions are established in the 20-40mT interval. Above 40mT the onset of gyroremanence often distorts the demagnetization diagrams.

The samples taken from the Pâglişa section consisted of volcanic tuff or tuffite, and displayed a different demagnetization behaviour. Their NRM intensity was generally around 1000 *10-4 A/m. Two components were identified during thermal demagnetization. A first component appears between 100 and 260°C. The corresponding directions are close to the present day field direction and we interpret this component as a present-day field overprint. A second component is isolated between 280 and 480°C. Although also of exclusively normal polarity, the corresponding directions differ significantly from the present-day field direction. We interpret the 280-480°C component as the ChRM of the Pâglişa section.

The ChRM temperature interval and gyroremanent behaviour upon AF demagnetization indicate that the magnetic carrier in most samples from the Transylvanian Basin is an iron sulfide, most likely greigite. Iron sulfides are very commonly the main magnetic carrier in Middle to Late Miocene of the Paratethys (Vasiliev et al. 2010). Correlation of the iron sulfide based magnetic polarity patterns of boreholes in the Pannonian Basin to the GPTS (Sacchi et al., 1997; Juhász et al., 1999; Sacchi and Horváth, 2002; Sacchi and Muller, 2004; Magyar et al., 1999, 2007) is often intricate. The reliability of greigite as a magnetic carrier has moreover frequently been questioned, because greigite is thermodynamically metastable and the timing of NRM acquisition by greigite is not well constrained due to its diagenetic formation (Vasiliev et al. 2008). The straightforward correlation of the magnetostratigraphy of greigite bearing sections in the Carpathian Foredeep (Vasiliev et al., 2007), Mediterranean (Hüsing et al., 2009) and the Transylvanian Basin (Vasiliev et al., 2010), however, shows that greigite may preserve its NRM for geological times and that reliable magnetostratigraphic results can be obtained if only a dedicated greigite specific demagnetization approach is taken.

8.6.3 Correlation to the timescale

The polarity of the sampled sections in general corresponds to the polarity predicted for the respective stratigraphic interval by the time-scale correlation based on 40Ar/39Ar data and thus provides additional support (Fig. 8.6). The Făgăraș outcrop, which has a reversed polarity, correlates to chron C5Ar.3r. The marls underlying the tuff exposed at Apahida were deposited during chron C5Ar.2n, and the reversed polarity interval exposed at Gherla correlates to the base of chron C5Ar.1r. At the top of the Turda section the magnetic polarity changes from normal to reverse. This reversal correlates to the C5An.2n to C5An.1r

Figure 8.6. Biostratigraphy, magnetostratigraphy and 40Ar/39Ar ages for the investigated late Badenian to Sarmatian sections and tuffs, and their consequential correlation to the ATNTS (Lourens et al., 2004; Hüsing et al., 2010; Krijgsman and Kent, 2004). Stratigraphic column and position of the sections therein based on the synthetic section in figure 8.4. Scaling and first order correlation of the stratigraphic succession to the ATNTS is based on the radio-isotopic ages for the top of the evaporites and the Oarba Tuff. Due the scale imposed by the high sedimentation rate detailed lithological changes in the depicted individual sections would be indiscernible. Detailed bio-, litho-, and magnetostratigraphic information about each of the outcrops is provided in the online supplementary material.

161


Upp

er B

aden

ian

Mid

dle

Bad

.S

arm

atia

n

Sectionname

Tracedposition

Magnetostratigraphy

A. dividens Zone

Tenuitellinata Zone

Velapertina Zone

A. dividens Zone

Badenian - Sarmatian boundary

A. dividens Zone

2.0

1.0

1 .5

2.0

1.0

1 .5

3.0

2.5

4.0

3.5

3.0

Depth Paratethys stages

unce

rtain

ty

Oarba tuff (11.62±0.06 Ma)

Ghiriş Tuff (12.38±0.04 Ma)

Câmpia Turzii 2 Hâdâreni Tuff (12.37±0.04 Ma)

Fagaras*

Oarba Beta

Turda*

Câmpia Turzii*

Gherla

Apahida

Turda Tuff

Câmpia Turzii Tuff

sTwT km

13.0

12.0

13.5

12.5

11.5

ATNTSTime (Ma) Polarity

(12.35±0.04 Ma)

50

110

5

20

150

10

(12.37±0.04 Ma)

40

top salt (13.36 Ma)

Badenian Salinity Crisis evaporites C5ABn

C5AAr

C5AAn

C5Ar.3r

C5Ar.2n

C5Ar.2r

C5Ar.1n

C5Ar.1r

C5An.2n

C5An.1r

C5An.1n

C5r.3r

C5r.2n

C5r.2r

162


transition taking the acquired 12.37±0.04 Ma radio-isotopic age for the Turda Tuff into account.The magnetostratigraphic pattern of the Câmpia Turzii, however, requires additional explanation.

The 150 m thick Câmpia Turzii section covers ~0.2 Ma. Its polarity is dominantly reversed, but a large number of single samples have a normal polarity. The demagnetization diagrams on which the magnetostratigraphic pattern is based are of high quality for both normal and reversed samples. They do, on the other hand, all display multiple and sometimes even up to four components. These can be of normal or reversed polarity independent of their demagnetization temperature or field interval. We consider it most likely that the primary magnetic component of the Câmpia Turzii section is reversed, but demagnetization behaviour is so complex that reliable magnetostratigraphic information is hard to extract.

For the uppermost Sarmatian to Pannonian part of the basin infill we do not have 40Ar/39Ar dates that can guide first order estimates on sedimentation rates and correlation to the ATNTS. We thus have to rely on the magnetostratigraphic information acquired (Fig. 8.9). For the correlation of the magnetostratigraphic pattern of the Oarba sections we follow Vasiliev et al. (2010), except for Oarba Beta. The polarity of the newly sampled Pannonian sections and sites is generally normal. We thus correlate the Târnăveni, Sighișoara, Viforoasa and Șoimuş sections to the long normal Chron C5n.2n. The short reversed intervals of the Viforoasa section might represent small-term fluctuations in the geomagnetic field (Krijgsman and Kent, 2004). The uppermost part of Șoimuş, which bears a reversed polarity, in our view correlates to C5n1r. This correlation implies that in the Pannonian the sedimentation rate is much lower than during the upper Badenian to Sarmatian time-interval and amounts to 0.36 m/kyr (Fig. 8.10). This is in agreement with seismic interpretations that demonstrate a gradual change from subsidence to uplift during the Pannonian stage (Mațenco et al., 2010).

Along the course of our study it has become clear that the quality of demagnetization results for and the preservation of NRM in the sediments of the Transylvanian Basin is highly variable. Sections for which demagnetization results were of low quality were immediately excluded from further analysis. The polarity of most sections with high quality demagnetization diagrams corresponds to the one expected based on correlation to the GPTS (Lourens et al., 2004) according to our 40Ar/39Ar based

PIG04

N

up/W

FA05

N

up/W AP35

N

up/WCT2 120

N

up/W

CT16

N

up/W

CT21

N

up/W

VIF12 N

up/W

TU33

N

up/W

TH TH TH TH

TH TH AF TH

marl marl marl marl

marl marl marl tuffite

15 mT

40 mT

0 ˚C

200 ˚C

325 ˚C

300 ˚C

200 ˚C

20 ˚C

100 ˚C

200 ˚C

315 ˚C

80 ˚C

180˚C

285˚C

315˚C

0 mT

25 mT

100 mT

150 ˚C

260 ˚C 470 ˚C

100 mT

100 ˚C 260 ˚C

280 ˚C

360 ˚C

Figure 8.7. Bedding plane corrected characteristic demagnetization diagrams for paleomagnetic samples from the Transylvanian Basin. Diagrams for thermally demagnetized samples are marked ‘th’ while those for alternating field demagnetized samples are marked ‘af’. Sections are indicated with abbreviations: CT2, Câmpia Turzii 2; FA, Fâgâras; TU, Turda; AP, Apahida; PIG, Pâglisa; CT, Câmpia Turzii; VIF, Viforoasa.

163


chronostratigraphic framework. This consistency provides independent support for the reliability of the acquired paleomagnetic results. The intricate polarity pattern of the Câmpia Turzii section, however, points out that magnetostratigraphic interpretation in iron-sulfide dominated sediments remains delicate and great care has to be taken.

8.7 Biostratigraphy

8.7.1 Methods

All sections were investigated in detail for micropaleontological analyses primarily focusing on foraminifera and ostracods. Samples were processed by standard micropaleontological methods, sieved on 63μm mesh and hand-picked under the microscope. A detailed description of the micropaleontological assemblage characteristic for each section or part thereof is provided in the online supplementary material.

Each section will be attributed to one or more particular biozones. The biostratigraphic zonation of the Badenian and Sarmatian is based on Popescu (1999), Filipescu and Silye (2008), and Beldean et al. (2010). The biostratigraphic scheme for the Pannonian is based on ostracods and follows Jiřiček (1975, 1985), who referred to the standard Pannonian Biozones of Papp (1951, 1985).

During the Early Badenian the Paratethys and Mediterranean were well connected (Rogl, 1999; Popov, 2004) and biostratigraphic events can be correlated and should coincide. Endemism hampers direct correlation of the middle Badenian, Sarmatian and Pannonian sections. Moreover, relatively deep water environments characterized the larger part of the Transylvanian Basin at that time. Most of the specifically designed biostratigraphic zonations for the Middle Miocene of the Central Paratethys were constructed in shallow water environments and rely on benthic taxa. They are therefore not always suitable for the deep-water sediments of the Transylvanian Basin (Filipesu and Silye, 2008). This sometimes complicates attribution of the sampled sections to a specific biozone. Fitting the ostracod assemblages from the Transylvanian Basin in the chronostratigraphic framework of the Pannonian stage is rather difficult because of pertinent differences regarding the Pannonian ostracod biozonation between authors (Brestenska, 1961; Pokoný, 1944; Krstić, 1973, 1985; Jiřiček, 1975, 1985, Rundić, 1997, 2006). Assignment of the Late Badenian to Pannonian sections to a specific biozone is for all these reasons often tentative.

8.7.2 Results

Three successive biozones can be identified in the Ciceu Giurgeşti section. The fauna of the lowermost part of the section is dominated by biserial planktonic foraminifera of the Lower Miocene Streptochilus pristinum Biozone (Beldean et al., 2010). The second part of the outcrop, which starts with a conglomerate bed with clasts up to 15 cm in diameter, contains foraminifera of the Lower Badenian Praeorbulina glomerosa Biozone. Samples from the remaining part of the section belong to the Orbulina suturalis Biozone.

Nannoplankton data by Mezáros and Șuraru (1991) indicate that the Dej Tuff in the Pâglişa area pertains to the NN5 calcareous nannoplankton zone. The associated foraminiferal assemblage belongs to the Orbulina suturalis Biozone.

The Făgăraş outcrop exposes sands and marls that belong to the middle to upper part of the Upper Badenian Velapertina Biozone. SEM investigations of samples from the Făgăraş 2 outcrop revealed frequent microperforate globigerinids belonging to the genera Tenuitella and Tenuitellinata assigned to the uppermost Badenian Tenuitellinata Biozone (Filipescu and Silye, 2008).

The faunal assemblage of the marls of the Apahida section is dominated by small size foraminifers which are mostly in their juvenile stage. Specimens of Tenuitellids occur together with Anomalinoides dividens. The former are very common planktonics in the late Badenian and rare in the lowermost Sarmatian, whereas a bloom of latter marks the lowermost Sarmatian. This situation suggests a transitional zone positioned just below the Badenian-Sarmatian boundary. A similar situation occurs at Unguraş where the micropaleontological assemblage included several Tenuitellinata specimens and one specimen

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of Anomalinoides dividens. The Gherla section clearly belongs to the lower transgressive part of the Sarmatian corresponding to the Anomalinoides dividens Biozone. Dilution of the faunal assemblage in combination with reworking from shallower areas and definitions based on acmes instead of FO and LO complicate exact biozone specification for the Hădăreni, Ghiriş, Câmpia Turzii 2 and Turda sections. The diluted microfaunal assemblage from the Hădăreni outcrop contains a few specimens of small rotaliids (Elphidium spp., Nonion spp.) and miliolids (Quinqueloculina spp., Sinuloculina spp., Varidentella spp.). These characterize the lower half of the Sarmatian, but the exact biozone remains unclear. In this part of the basin infill, biozones can, in general, only be inferred based on paleoenvironmental changes caused by relative sea-level changes. The Câmpia Turzii section exposes 150 m of clays, marls, and sandstones that belong to the Anomalinoides dividens Biozone. The Câmpia Turzii 2 section, situated stratigraphically 250 m above the Câmpia Turzii section, exposes some 30 m of marls and subordinate sandstones with on top of it an 8 m thick tuff bed, named the Câmpia Turzii Tuff here. Biostratigraphic samples from this section contain frequent Articulina sarmatica in combination with rare Anomalinoides dividens and most likely belongs to the Articulina Zone (middle-upper Volhynian), in which Anomalinoides is rare but still occurs. The 150 m of marls and sandstones overlying the Turda Tuff might belong to the Elphidium reginum Biozone.

The biostratigraphy of five successive upper Sarmatian and lower Pannonian outcrops at Oarba de Mureș was studied in detail by Sztanó et al. (2005), Vasiliev et al. (2010), and Filipescu et al. (2011). Vasiliev et al. (2010) attribute the Oarba Beta section to the Porosonion aragviensis Biozone. Renewed inspection of the biostratigraphic samples points out that it is, however, difficult to differentiate between the Elphidium reginum, Dogielina sarmatica and Porosononion aragviensis zones. The section was originally assigned to the Porosononion aragviensis Biozone because evolved species of Porosononion are common in the upper part of the section. The whole section, on the other hand, contains species of Elphidium. No specimens of Dogielina sarmatica were encountered. We thus refrain from exact biozone determination for the Oarba Beta section. The larger part of outcrop A belongs to the upper Sarmatian Porosononion aragviensis Biozone. Samples from the top part of outcrop A contain typical Pannonian deep-water ostracods and thus Filipescu et al. (2011) place the Sarmatian–Pannonian boundary 2.3 m below the top of the section. This is in good agreement with the results of Sütő, & Szegő (2008), who found massive numbers of Mecsekia ultima 3.4 m below the top and the index taxon for the base of the Pannonian (Spiniferites bentonii pannonicus) 1.4 m below the top. Outcrops B, C and D are Pannonian in age and contain a rich ostracod association specific for low salinities. Sztanó et al. (2005) identified several mollusks indicative of the Lymnocardium praeponticum Zone of the early Pannonian in the lower part of outcrop B.

The microfauna of the Viforoasa and Şoimuş sections is dominated by Pannonian ostracods. The larger part of the ostracod assemblages can be attributed to Zone 5 (Amplocypris abscissa Zone) and the lower part of Zone 6 (Hemicytheria croatica Zone) of the basal part of the upper Pannonian (Lower Serbian) as defined by Krstić (1973, 1985) and mentioned by Rundić (1997, 2006). They can also be correlated with Zone D and the first part of Zone E (Subzone E1) according to Jiřiček (1975, 1985), who referred to the standard Pannonian biozones of Papp (1951, 1985). The lower part of Viforoasa section seems to be older than the Şoimuş section and might correlate to the upper part of Zone 4 (Propontoniella candeo Zone) (Krstić, 1973, 1985) of the upper Slavonian. The Târnăveni, Mediaş and Sighişoara sections were exclusively sampled for rotation research and only few biostratigraphic samples were taken from a single stratigraphic level in each of the quarries. The faunal assemblage indicated a Pannonian age.

Figure 8.8. Lithology, biostratigraphy, magnetostratigraphy, 40Ar/39Ar ages, and consequential correlation to the ATNTS for the investigated early Badenian sections. 1. and 5. GPTS according to Lourens et al. (2004), Hüsing et al. (2010), and Krijgsman and Kent (2004). 2. Biozones according to Krezsek and Filipescu (2005). Ages of bioevents according to Abdul Aziz et al. (2008) and Lourens et al. (2004b). 3. Central Paratethys regional stages. 4. Confidence interval for the acquired 40Ar/39Ar ages for the Dej Tuff.

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Orb

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8.8 A new chronology for the Middle to Upper Miocene of the Transylvanian Basin

Our new radio-isotopic, magnetostratigraphic and biostratigraphic results allow, in combination with the high resolution seismic correlation of surface outcrops, the establishment of a new chronology for the middle to upper Miocene of the Transylvanian Basin.

8.8.1 Late Karpatian and early Badenian: the Streptochilus pristinum, Praeorbulina glomerosa and Orbulina suturalis biozones and deposition of the Dej Tuff complex

The Ciceu Giurgeşti, Dej and Pâglisa outrcrops (Fig. 8.8) reflect the early Badenian evolution of the last megasequence deposited in the Transylvanian Basin. The lower part of the Ciceu Giurgeşti, Dej, and Pâglișa sections comprise the sediments that accumulated before the onset of volcanic activity related to the Dej Tuff complex. The presence of the upper Karpatian and two Badenian biozones in an interval of only 24 m corroborates extremely low sedimentation rates. The transition from Karpatian to Badenian coincides with the Early to Middle Miocene transition and occurs after 7 m and before 11.5 m where the FO of Praeorbulina glomerosa is registered. In the Mediterranean the FO of Praeorbulina glomerosa was dated to occur at 15.97 Ma (Lourens et al. 2004b). It most likely approximates the deposition of the conglomerate located at 8 m. A second biostratigraphically correlatable horizon (Fig. 8.8) is the FO of Orbulina suturalis, indicative for the start of the Orbulina suturalis Biozone. It was dated to occur at 14.56 Ma in the Mediterranean (Abdul Aziz et al., 2008).

The 14.38±0.06 Ma 40Ar/39Ar age for the Dej Tuff from Ciceu Giurgești provides the third tie-point to the global timescale. Deposition of this tuff slightly postdates the FO of Orbulina suturalis (Fig. 8.8) and provides an independent minimum age constraint on the first occurrence of Orbulina suturalis in the Central Paratethys. Since this dated tuff horizon is one of the basal horizons of the Dej Tuff complex, the acquired age should be indicative of the onset of related volcanism. The statistically indistinguishable 14.37±0.06 Ma age determined for the Dej tuff at Dej provides additional support for this conclusion. This age contrasts strongly with the much older age of between 15.4 and 15 Ma previously attributed to the Dej Tuff based on K-Ar and fission track measurements (Szakács et al. 2000). Since the potassium-argon method is restricted to whole rock measurements, the strong reworked component that our experiments demonstrate to be present in the Dej Tuff, might have induced seemingly higher ages. Since multiple single fusion 40Ar/39Ar experiments are performed on only a small selected fraction of minerals in the rock, detection of, and correction for reworking is possible. We thus conclude that the here calculated 14.37±0.06 Ma and 14.38±0.06 Ma crystallization ages provide more reliable ages for the Dej Tuff.

The part of the tuff complex sampled at Pâglişa also pertains to the Orbulina suturalis Biozone and is exclusively of normal polarity. It was thus deposited during chron C5ADn. Our results imply that accumulation of the over 80 m thick volcaniclastic complex took much shorter than previously thought. Tuffs and tuffites belonging to the Dej Tuff complex disappear from the sedimentary record before the onset of the BSC at 13.82 Ma. Tuff deposition can thus have lasted a maximum of 0.6 Ma.

8.8.2 Late Badenian and Sarmatian Biostratigraphy

Our results provide new age constraints on the Badenian-Sarmatian boundary and the Velapertina s.l., Tenuitellinata and Anomalinoides dividens Biozones. The upper Badenian Făgăraș 2 outcrop falls into the uppermost Badenian Tenuitellinata Biozone (Filipescu and Silye, 2008). The faunal assemblage of the marls of the Apahida Tuff reflects the very top part of the same biozone and suggests it is very close to the base of the Sarmatian. The Gherla section belongs to the lower Sarmatian Anomalinoides Biozone and therefore provides an upper limit for the position of the Badenian-Sarmatian boundary, which is consequently positioned between 2.73 and 2.93 km, and is dated between 12.70 and 12.83 Ma (Fig. 8.6). The observed faunal assemblages suggest that the boundary is stratigraphically closer to the Apahida than to the Gherla section and is thus likely close to 12.8 Ma old (the Apahida Tuff is traditionally placed at the boundary – Vancea, 1960). The top of the Tenuitellinata Biozone (Filipescu and Silye, 2008) and the base of Anomalinoides dividens Biozone, coincide with the Badenian-Sarmatian boundary. The

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base of Velapertina Biozone corresponds to the end of the salinity crisis and is consequently 13.36 Ma. The boundary between the Velapertina and Tenuitellinata Biozones is located in the Făgăraş 2 section.

The Gherla and Câmpia Turzii 1 outcrops belong to the Anomalinoides dividens Biozone (Fig. 8.9). The transition between the Anomalinoides dividens and Articulina sarmatica Biozones is hard to pinpoint since it occurs gradually. Anomalinoides dividens continues to be present in the Articulina sarmatica Biozone although it becomes less abundant. Some fauna characteristic for the Articulina Zone moreover already appears in the top of the Anomalinoides dividens zone. However, at the level of the Câmpia Turzii 2 and Hădăreni outcrops the faunal assemblage becomes more diverse than in the underlying strata. The abundance of Anomalinoides decreases and the assemblage is enriched in specimens of miliolids (Articulina, Varidentella, Sinuloculina) and rotaliids (Elphidium spp.), most likely in response to environmental changes in the basin. This suggests it is adequate to place the zone-boundary at this level. The Anomalinoides dividens Biozone should, in our opinion, therefore end at the stratigraphic level of the Hădăreni Tuff. The Câmpia Turzii Tuff is the first tuff level above the Câmpia Turzii 1 section, which pertains to the Anomalinoides dividens Biozone. The faunal assemblage is, in analogy with the Hădăreni Tuff, enriched at the level of the Câmpia Turzii Tuff. This suggests that the Hădăreni and Câmpia Turzii Tuffs represent a single tuff level that coincides with the top of the Anomalinoides dividens Biozone. The top of the Anomalinoides is consequently around 12.4 Ma old. Differentiation between Biozones in the investigated deep water sections becomes more problematic above this level and we thus refrain from age estimates for the Articulina sarmatica, Dogielina sarmatica and the base of the Porosononion aragviensis Biozones.

8.8.3 The Sarmatian-Pannonian Boundary

In the Oarba de Mureș section, situated near the centre of the Transylvanian Basin, the Sarmatian-Pannonian boundary is located 46 m above the 11.62±0.04 Ma old Oarba Tuff. Based on calibration of the magnetostratigraphic pattern of outcrop A-D to the ATNTS, Vasiliev et al. (2010) have determined an age of 11.3±0.1 Ma for the Sarmatian-Pannonian boundary in the Transylvanian Basin. The basal Pannonian ostracod zone A is missing at Oarba de Mureș (Filipescu et al, 2011). This suggests the marine conditions characteristic for the Sarmatian stage persisted longer in the Transylvanian Basin than in the Pannonian Basin and inhibited colonization by the fresh -water ostracods that characterize the base of the Pannonian there. This implies environmental changes around the Sarmatian-Pannonian boundary were gradual across the Central Paratethys.

8.8.4 The Pannonian

Marked differences exist regarding the Pannonian ostracod biozonation between different authors (Brestenska, 1961; Pokoný, 1944; Krstić, 1973, 1985; Jiřiček, 1975, 1985, Rundić, 1997, 2006). Moreover, ostracod assemblages seem to be environment sensitive. Pannonian ostracod associations from the central part of the Transylvanian Basin, including the Viforoasa and Şoimuş sections, reflect a more deep, basinal and possibly fresher water environment (Krézsek & Filipescu, 2005) than in the other parts of Pannonian Basin where the ostracod biozones were defined. The faunal assemblage of the Viforoasa and Şoimuş sections can, however, be correlated with the Middle Pannonian Zone D and the lower part of Zone E (Subzone E1) according to Jiřiček (1975, 1985). According to our magnetostratigraphic correlation of these outcrops and their position in the synthetic seismic stratigraphic column (Fig. 8.9), they correspond to the time-interval between 10.6 and 9.9 Ma. This is in good agreement with results by Harzhauser et al. (2004) who attribute an age of 10.56 Ma to the base of Zone D. These authors do, on the other hand, attribute an age of 10.36 Ma to the top of Zone E1. Adoption of a 10.36 Ma maximum age for the Şoimuş section would lead to an even larger difference in sedimentation rates for the Sarmatian and Pannonian intervals in the Transylvanian Basin. Moreover, the polarity of the uppermost part of this section warrants correlation to a reversed Chron. We therefore prefer correlation of the reversal in the Şoimuş section to the transition from C5n.2n to C5n.1r and infer an age of 10.0 Ma

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for this level. Based on extrapolation of the sedimentation rate that results from this correlation to the Pannonian deposits overlying the sampled sections results in an around 8.4 Ma age for the uppermost Pannonian deposits in the depocenter of the Transylvanian Basin.

8.9 The rotation of the Transylvanian Basin during the Miocene

The paleomagnetic directions established in our magnetostratigraphic research can be used to calculate the horizontal plane rotation of the Transylvanian Basin during and since the Middle Miocene and thus provide insight in the tectonic evolution of the Tisza-Dacia terrain. The data quality requirements for paleomagnetic rotation analysis are higher than for magnetostratigraphic studies since not only the polarity but also the direction of the ChRM needs to be firmly established. The Gherla, Câmpia Turzii and Oarba Beta sections are discarded from further analyses, because the corresponding demagnetization diagrams are either cryptic or of insufficient quality to determine a paleomagnetic direction reliable enough for rotation analysis. For each of the remaining sections and sites the average declination is subsequently calculated using Fisher statistics on the VGP distribution and applying the Vandamme cutoff (Vandamme, 1994) to discard outliers (Table 8.3, Fig. 8.11).

The net rotation per site was calculated and the observed average declination was with the direction of the expected magnetic field, based on the location of the paleomagnetic pole for Eurasia with a corresponding age (Torsvik, 2008). After inspection of the results, sites were grouped according to age. It shows that Badenian and Sarmatian sites have a significantly larger rotation than Pannonian sites. The absence of major faults in the Middle Miocene infill of the Transylvanian Basin, except for those that resulted from local up-doming due to salt movements, favors the treatment of the basin as a single rigid block. All Badenian and Sarmatian sites were consequently grouped together and the same was done for Pannonian sites. An average rotation per time interval was subsequently calculated (Table 8.3).

Porosonionaragviensis

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Figure 8.9. Biostratigraphy, magnetostratigraphy, and 40Ar/39Ar ages for the investigated late Sarmatian and Pannonian sections, and their consequential correlation to the ATNTS (Lourens et al., 2004; Hüsing et al., 2010; Krijgsman and Kent, 2004). Stratigraphic column and position of the sections therein based on the synthetic section in figure 8.4. Scaling and first order correlation of the stratigraphic succession to the ATNTS is based on the 40Ar/39Ar for the Oarba Tuff (Vasiliev et al. 2010) and correlation of the reversal in the Şoimuş section to the top of C5n.2n.

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The average rotation of the Badenian and Sarmatian sites is 26.1 ± 5.1°. The average rotation of the Pannonian sites is 6.2 ± 3.9°. This indicates the Transylvanian Basin rotated approximately 20° CW (clockwise) from the late Sarmatian to early Pannonian and another 6° CW since the Pannonian.

8.10 Comparison with structural geological observations

A number of structural geological features can be observed that corroborate CW rotation of the basin during the Middle to Upper Miocene and fulfill its mechanical requirements. At the transition from Sarmatian to Pannonian (11.3 Ma) the collision of Tisza-Dacia with the Eastern-European craton evidently accelerates in the Eastern Carpathians and Transylvanian Basin. Exhumation data demonstrate that parts of the Eastern Carpathians underwent enhanced uplift around 12-11 Ma, coeval with the last significant event of nappe stacking (Sanders et al., 1999; Merten et al., 2010; Necea, 2010). Along the eastern margin of the Transylvanian Basin, near the contact with the Eastern Carpathians, successive uplifts on the order of 300m occur in the latest Sarmatian and earliest Pannonian (Mațenco et al., 2010). These were associated with strain partitioning along the boundary between the Tisza-Dacia and ALCAPA continental blocks, where 25-40 km of sinistral strike-slip was recorded along the Dragoș Vodă – Bogdan Vodă (DVBV) fault system (Tischler et al., 2007; Gröger et al., 2008). This sinistral movement has been accommodated by differential shortening at the contact between the Eastern Carpathians and the European lower plate (Tischler et al., 2008) and necessitates a concurrent clockwise rotation of Tisza-Dacia with respect to the European foreland on the order of 10°. In the meantime, final docking

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Figure 8.10. Sedimentation rates for the upper Badenian to Sarmatian, and Pannonian part of the basin infill. It should be noted that these do not allow direct quantitative conclusions on basin evolution without a proper subsidence history analysis taking into account factors such as paleo bathymetry, compaction and sea-level variations. However, these sedimentation rates provide a first order qualitative indication of the sedimentary influx associated with processes such as subsidence or increases in input from the source areas.

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of the South Carpathians against the Moesian plate occurred. Amounts of S-ward vergent shortening are variable along the contact between them and range from zero at its westernmost limit to around 35-50 km near the SE Carpathians (Ștefănescu et al., 1988). This asymmetry in shortening also implies around 15° of clockwise rotation of Tisza-Dacia with respect to stable Europe. The around 10° of difference in rotation estimates between structural geological observations and our paleomagnetic data might be accommodated by further strain partitioning at the contact between Tisza-Dacia and stable Europe (see Fügenschuh and Schmid, 2005) or reflect uncertainties in either of the observations.

The collision that started in the late Sarmatian continued until ~8-9 Ma although its effects in terms of exhumation and kinematics were reduced (Gröger et al., 2008; Merten et al., 2010). Deformation migrated towards the interior of the thrust wedge forming large antiformal geometries. These were notably larger in the northern part of the Eastern Carpathians than in the SE Carpathians (Mațenco et al., 2010). This difference in exhumation can be related to a larger amount of shortening in the Eastern Carpathian areas adjacent to the northern part of the Transylvania Basin and therefore explain the observed 6° Pannonian to post-Pannonian rotations.

Our results are in agreement with the near 30° CW rotation recorded by sediments directly underlying the Sarmatian strata of the Southern Carpathians (Dupont-Nivet et al., 2005). Sediments younger than 6.2 Ma are, according to these authors, on the other hand, not rotated. These findings demonstrate that Tisza-Dacia (including the Transylvanian Basin and surrounding orogens) rotated as a rigid block around the Moesian indenter during middle to late Miocene times, without the requirement of internal differential strain partitioning structures.

The observed CW rotation of the Transylvanian Basin is driven by the eastward roll-back of the European slab during its subduction below the Eastern Carpathians and provides new constraints on the concurrent emplacement of the Tisza-Dacia upper plate into the Carpathian embayment.

8.11 Conclusions

Our new radio-isotopic, magnetostratigraphic and biostratigraphic results provide a new chronology for the Middle to Upper Miocene of the Transylvanian Basin when stratigraphically calibrated to a synthetic seismic section in the basin center through subsurface tracing. This improves insight in the basin’s evolution and moreover provides new time constraints on the Middle Miocene Central Paratethys regional stages. The magnitude and timing of paleomagnetically determined tectonic rotations furthermore give new constraints on the emplacement of the Tisza-Dacia plate into the Carpathian embayment.

In the early Badenian (16 to 13.82 Ma), when marine connections between the Central Paratethys and the Mediterranean still existed, up to 100 m of relatively deep water sediments accumulated under a slight extensional regime (Krézsek et al., 2010). Intense volcanism along the eastern margin of the Transylvanian Basin caused deposition of the Dej Tuff complex, an important lithostratigraphic marker for the early Badenian. The onset of Dej Tuff volcanism is constrained between the first occurrence (FO) of Orbulina suturalis at 14.56 Ma (Abdul Aziz et al. 2008) and 14.38±0.06 Ma, based on new 40Ar/39Ar ages for two of the lowermost tuff levels. Tuff accumulation ceased before the onset of the Badenian Salinity Crisis (BSC) at 13.82 Ma (chapter 7; de Leeuw et al. 2010). The BSC was triggered by a glacio-eustatic restriction of the connection between the Central Paratethys and the Mediterranean and lead to the accumulation of up to 300 m of salt in the central part of the Transylvanian Basin. The establishment of a marine connection to the Eastern Paratethys at the onset of the late Badenian ends salt deposition restores normal marine conditions. The strongly endemic nature of late Badenian faunal assemblages suggest the marine connection to the Mediterranean ceased to exist. Accumulation rates strongly increased in comparison with the early Badenian, and a marked W-E thickening of the upper Badenian in the basin suggests this relates to an increase in tectonic activity in the Eastern Carpathians. The transition from Badenian to Sarmatian is marked by the strongest extinction event in the history of the Central Paratethys (Harzhauser and Piller, 2007). All planktic foraminifera and over 500 gastropod species disappear. We date the Badenian-Sarmatian Boundary in the Transylvanian Basin at 12.80±0.05 Ma. The upper Badenian consequently lasts from 13.36 to 12.80±0.05 Ma. At 12.4 Ma, a second pulse

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of intense volcanic activity occurred, leading to deposition of the Ghiriş, Hădăreni, Turda and Câmpia Turzii tuffs that are collectively called the middle Sarmatian tuff complex. The tuffs accumulate in a short time as indicated by their statistically indistinguishable ages of 12.38±0.04 Ma, 12.37±0.05 Ma, 12.37±0.04 Ma, and 12.35±0.04 Ma. The interval between the onset of the Sarmatian and the middle Sarmatian tuff complex belongs to the Anomalinoides dividens Biozone which accordingly covers the 12.80±0.05 Ma to 12.4 Ma time interval. The basin is struck by a second major environmental change with a heavy impact on its biota at the Sarmatian-Pannonian boundary dated at 11.3 Ma by Vasiliev et al. (2010). The complete disappearance of foraminifera from the faunal record and a subsequent proliferation of ostracods mark a transition to freshwater conditions registered throughout the whole Central Paratethys. This was triggered by tectonic uplift of the Eastern and Southern Carpathians which isolated the Central Paratethys from the Eastern Paratethys (Krézsek et al., 2010). Concomitant enhanced

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Figure 8.9. Biostratigraphy, magnetostratigraphy, and 40Ar/39Ar ages for the investigated late Sarmatian and Pannonian sections, and their consequential correlation to the ATNTS (Lourens et al., 2004; Hüsing et al., 2010; Krijgsman and Kent, 2004). Stratigraphic column and position of the sections therein based on the synthetic section in fi gure 8.4. Scaling and fi rst order correlation of the stratigraphic succession to the ATNTS is based on the 40Ar/39Ar for the Oarba Tuff (Vasiliev et al. 2010) and correlation of the reversal in the Şoimuş section to the top of C5n.2n.

172


uplift in the Carpathians is corroborated by the timing of paleomagnetically determined tectonic rotation of the Tisza-Dacia block. The average rotation of sampled Badenian and Sarmatian sites is 26.1 ± 5.1°. The average rotation of the Pannonian sites is 6.2 ± 3.9°. This indicates the Transylvanian Basin rotated approximately 20° CW from the late Sarmatian to early Pannonian. This rotation was accommodated sinestral strike slip along the Dragoș Vodă – Bogdan Vodă (DVBV) fault system and differential shortening observed in the Eastern and Southern Carpathians. Magnetostratigraphic correlation of the investigated Pannonian sections, which belong to middle Pannonian Zone D, and the lower part of Zone E (Subzone E1) of Jiřiček (1975, 1985), to the ATNTS (Lourens et al. 2004) indicates that these cover the 10.6-9.9 Ma time-interval. This suggests an 8.4 Ma age for the uppermost Pannonian deposits in the depocenter of the Transylvanian Basin. Our new chronostratigraphic results confirm that during the Late Badenian and Sarmatian, coincident with intensive nappe stacking in the neighboring East Carpathians (Mațenco et al. 2007), sediment accumulation rates in the center of the Transylvanian Basin were much higher than during the Pannonian, when a gradual change from subsidence to uplift occurred (Mațenco et al., 2010). In the Eastern Carpathians, deformation at this time migrated towards the interior of the thrust wedge. The resulting antiformal geometries were significantly larger in the NE Carpathians than in the SE Carpathians (Mațenco et al., 2010) which can explain the observed 6° Pannonian to post-Pannonian rotation.

Acknowledgments

We would like to express great appreciation for the commitment of Andrei Briceag, Arnoud Slootman, and Jaap Verbaas who have contributed to the conceptual and practical advancement of this research. This study would not have been possible without the benevolence of multiple brick factories that have provided access to their quarries. We thank Mr. Vasile for his hospitality, Mr. Moldovan for logistic support, Roel van Elsas for assistance with mineral separation, Jan Wijbrans, Guillaume Dupont-Nivet, Douwe van Hinsbergen for discussion, and Cor Langereis for critically reviewing the manuscript. This study was supported by the Netherlands Research Centre for Integrated Solid Earth Sciences (ISES) and by the Netherlands Organization for Scientific Research (NWO/ALW).

173


Sect

ion

Age

Site

loca

tion

Obs

erve

d di

rect

ion

Ref

eren

ce P

ole

pEx

pect

ed I

Expe

cted

DR

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ion

Flat

teni

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tLo

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Ds

α95

kΑ

95n

Pol

Age

Lat

Long

A95

I x±

dID

x±

dDR

±∆R

F ±

∆F

(Ma)

(Ma)

(°N

)(°

E)

(°)

(°)

(°)

(°N

)(°

E)

(°)

(°)

(°)

(°)

(°)

(°)

(°)

(°)

(°)

(°)

Med

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9.6

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146

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654

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114

14.

910

n10

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544

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621

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146

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44.1

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114

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1087

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110

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1087

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9±

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825

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1

Tabl

e 8

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Pale

omag

netic

dat

a an

d ro

tatio

n re

sults

. Se

ctio

n: n

ame

of p

aleo

mag

netic

sam

plin

g lo

calit

y. A

ge:

age

of t

he s

ectio

n ac

cord

ing

to o

ur c

hron

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atig

raph

ic

resu

lts. S

ite lo

catio

n: la

titud

e an

d lo

ngitu

de o

f sam

plin

g lo

calit

y. O

bser

ved

dire

ctio

n: in

clin

atio

n (I

) an

d de

clin

atio

n (D

) of

mea

n pa

leom

agne

tic d

irect

ion

in t

ilt c

orre

cted

co

ordi

nate

s. (α

95)

is t

he 9

5% c

onfid

ence

circ

le c

alcu

late

d ap

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ng fi

sher

sta

tistic

s to

the

obs

erve

d di

rect

ions

, (A 95

) th

e 95

% c

onfid

ence

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le c

alcu

late

d ap

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sher

st

atis

tics

to c

orre

spon

ding

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Ps,

(k)

the

prec

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, (

n) t

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site

s us

ed t

o ca

lcul

ate

mea

n di

rect

ion,

(po

l) th

e po

larit

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the

site

. Re

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pole

: ag

e, la

titud

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d lo

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and

A 95 (

95%

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Tor

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(200

8).

Rota

tion:

ver

tical

axi

s ro

tatio

n w

ith 9

5% c

onfid

ence

lim

it (p

ositi

ve in

dica

tes

cloc

kwis

e ro

tatio

n). F

latt

enin

g: fl

atte

ning

of

incl

inat

ion

with

95%

con

fiden

ce li

mit

(rot

atio

n an

d fla

tten

ing

are

deriv

ed fro

m o

bser

ved

dire

ctio

n m

inus

exp

ecte

d di

rect

ion

at lo

calit

y ca

lcul

ated

fro

m r

efer

ence

pol

e).

174


Epilogue: a scenario for the evolution of southeastern EuropeThe importance of high resolution age constraints for our understanding of the geological record is often underestimated or overlooked. Geological phenomena have for centuries been interpreted without ways to quantify their age or duration. We thus feel habitually confident explaining what we see, even in the absence of good time control. Radio-isotopic dating methods have now given us a way to quantify absolute ages. Magnetostratigraphy and cyclostratigraphy allow us to correlate events over large distances and, even more importantly, across biogeographic boundaries. We are thus, at present, better equipped than ever to establish accurate time constraints.

‘How old is it?’ is the recurrent question in this work. It might seem ridiculously simple. It does neither need an extensive set of hypotheses, nor long elaboration. It is, in all its simplicity, nevertheless one of the most fundamental questions in geology. The research approach we have followed along the course of this project might seem opportunistic, and it probably is. There is, however, a great need for high resolution age constraints, and a large potential to improve our understanding of the geological record on the basis thereof.

Southeastern Europe is an exciting part of the Africa-Europe collision zone. The collision of Adria, Alcapa and Tisza-Dacia with the southern margin of Europe uplifted the Alps, Dinarides and Carpathians. Progressive fragmentation of the Tethys Ocean generated a suite of (semi-)isolated basins. Subduction and back-arc extension related volcanism left their signatures in the Pannonian Basin and the surrounding mountains. The ubiquitous geodynamic activity in the region self-evidently had a large effect on the paleoenvironment. Biogeographic boundaries regularly arose and disappeared, triggering either endemism or the sudden arrival of new species. Modern geological research methods that were extensively tested in the Mediterranean can now be applied in this stunning area that has gradually become accessible to us over the last three decades.

I have, in my research approach, intentionally refrained from hypothesizing about the outcome of the intended chronostratigraphic work. Age speculation is exactly the problem that the conducted experiments were meant to overcome. A synthesis of the acquired results in a scenario for the evolution of southeastern Europe at this point seems adequate. The achieved time constraints allow us to better link separate events in the Dinarides, Central Paratethys and Eastern Carpathians during the Oligocene and Miocene.

The Adriatic Plate that acted as a promontory of the African continent played an important role in the geological history of Central Europe. It collided with the Tisza-Dacia Plate in the Eocene, which lead to widespread flysch deposition in the Dinarides. The subsequent rise of the Dinaride-Anatolian Land led to the birth of the Mediterranean and Paratethys as it dissecting the former Tethys Ocean at the advent of the Oligocene. The collision, catastrophic as it may have been for life in the marine realm, gave small mammals a limited opportunity to migrate between Western Asia and Central Europe, as the 24 Ma old fauna from Banovići testifies. The timing of the formation of the Banovići Basin in the interior of the Dinarides coincides with the extrusion of the Eastern Alps due to collision of the northern margin of Adria with Europe. This suggests the basin may have formed in conjugation with transform faults triggered by the eastwards escape. A global climatic optimum is recognized in the Late Oligocene (Zachos et al. 2005) and there was apparently sufficient precipitation in the Dinarides for lakes to form in the newly generated basins.

In the adjacent intra-Carpathian embayment, European oceanic crust was meanwhile subducted along the eastern margin of Tisza-Dacia. The block translated into the embayment and moved around the Moesian platform. This led to a ca. 50° clockwise rotation of the block in post Oligocene times (Schmid et al. 1998). Our paleomagnetic data indicate that the Dinarides did not experience any significant rotation since 24 Ma. This implies weak coupling of the two plates and suggests differential horizontal movements had to be accommodated in the Southern Pannonian Basin, in line with the most recent palinspastic reconstruction (Ustaszewski et al. 2008).

175

Epilogue

Roll-back of the subducted slab induced back arc extension in the Pannonian Basin, forming on top of the Tisza-Dacia and ALCAPA plates, from 18 Ma onwards. Extension was profound and penetrated into the Dinarides. Extensional basins started to form along the northeastern margin and in the interior of the mountain chain. In the beginning, a thick sequence of fluvio-lacustrine sediments accumulated along the margin. In the Badenian, the marginal basins subsided below the base level of the Paratethys and were flooded. The basins in the interior of the mountain chain had in the meantime been occupied by the lakes of the Dinaride Lake System. Thick lacustrine deposits could accumulate due to a combination of beneficial tectonic and climatic factors. A surprisingly diverse mollusk fauna developed as a result of the long persistence of the lakes. Their water level, mollusks and riparian fauna responded to climatic changes induced by variations in the eccentricity of the Earth’s orbit. In the Pannonian Basin, the realm of the Central Paratethys, a thick syn-rift sequence with Karpatian and Badenian sediments accumulated. Profound extension caused high rates of subsidence. Open marine conditions prevailed. At this time, a connection between the Central Paratethys and the Mediterranean existed across the northernmost part of the Dinarides. This might have been a direct result of the extensional collapse of the orogen. Back-arc extension and subduction triggered extensive volcanism and tuff layers are frequently recognized in the marine and lacustrine deposits of southeastern Europe.

A strong glacio-eustatic event, induced by severe worldwide climatic deterioration, caused a major global sea level drop at 13.8 Ma. Communication between the Central Paratethys and the Mediterranean became restricted. The outflow of saline bottom water from the Paratethys was blocked and salt became trapped in the deeper parts of the basin. This initiated the Badenian Salinity Crisis. Up to 300 m thick salt deposits accumulated. Decimeter-scale cyclicity within the salts reflects climatic fluctuations with a period of ca. 10-30 years, implying a driving mechanism different from seasonality.

Nappe stacking intensified in the Eastern Carpathians, situated along the eastern boundary of the Tisza-Dacia Block, in the Late Badenian (Matenco and Bertotti, 2000). Subsidence rates in the Transylvanian basin increased tremendously. The Central Paratethys reconnected with the Eastern Paratethys via straits through the Eastern and Southern Carpathians. Its connection with the Mediterranean was lost. The Late Badenian fauna of the Transylvanian Basin had an open marine character and suggests influxes from the Indian Ocean. The faunal turnover that marks the Badenian-Sarmatian boundary is dated to occur at 12.8 Ma in the Transylvanian Basin. Restricted marine conditions established in the whole Paratethys realm.

Tisza finally collided with the Eastern European Craton in the Late Sarmatian. This collision promoted even more rapid growth of relief, severing the connection between the Central and Eastern Paratethys at the advent of the Pannonian. All marine fauna disappeared from the Central Paratethys and a freshwater environment named Lake Pannon established itself in the intra-Carpathian area. Due to the collision, Tisza-Dacia and Europe could no longer converge. Due to its continuing counterclockwise rotation, the Adriatic Plate impinged into the Dinarides that could no longer move eastward. Grand scale inversion took place. Rates of deposition slowed down in Transylvania and the basin was gradually inverted. The Pannonian basin was likewise inverted and a regional erosional unconformity developed. This unconformity is clearly recognizable in the North Croation Basin (i.e. Southern Pannonian Basin) situated along the eastern margin of the Dinarides. In the interior of the Dinarides, inversion triggered uplift of the margins of the prolific lacustrine basins and induced the progradation of conglomerates. This terminated the 5 Ma life-time of the Dinaride Lake System.

The foregoing scenario for the evolution of southeastern Europe illustrates that improved age constraints can facilitate interpretation of the geological record and enable correlation of events in seemingly unrelated systems. The evolution of Miocene semi-isolated marine and lacustrine systems in southeastern Europe is evidently intimately linked to the regions geodynamic history. 40Ar/39Ar, magnetistratigraphic and cyclostratigraphic dating techniques have proven to be powerful chronostratigraphic tools in settings where conventional biostratigraphic methods are hampered by endemism. Many basins locked up in the collision zone between Europe and Africa still lack accurate age control and there thus remains a large need for similar studies.

Winter in the botanical gardens where the paleomagnetic laboratory is situated

Paleomagnetists skating on the moat of Fort Hoofddijk

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Nederlandse Samenvatting

Introductie

Gedurende het Cenozoicum (ca. 65 Ma tot nu) leidde convergentie tussen Europa en Afrika tot het op elkaar botsen van verscheidene microcontinenten gelegen langs de zuidrand van Europa. Dit veroorzaakte het omhoogkomen van een aantal gebergten waaronder de Alpen, Karpaten, Dinariden, Helleniden en Pontiden (e.g. Csontos 1992; Schmid et al. 2008; Ustaszewski et al. 2008). De complexe geodynamische evolutie van het gebied veroorzaakte een evenzo ingewikkelde paleogeografische geschiedenis (bijv. Rögl, 1998, 1999; Popov, 2004; Harzhauser et al. 2002, 2007a; Magyar, 1999). Met de tijd fragmenteerde de Tethys Oceaan, hetgeen leidde tot het ontstaan van de Middellandse Zee en de Paratethys (bijv. Rögl, 1999). Voortdurende gebergtevorming zorgde voor progressieve isolatie en verdere fragmentatie van deze bekkens. Hun evolutie staat in nauw verband met de geschiedenis van de aangrenzende gebergten. Dit proefschrift gaat in op de evolutie van de Centrale Paratethys en het aangrenzende Dinariden gebergte.

De Centrale Paratethys was een epicontinentale zee welke een groot gedeelte van Centraal en Zuidoost-Europa besloeg gedurende het Oligoceen (ca. 34-23 Ma) en Mioceen (ca. 23-5 Ma). Aan het begin van het Midden Mioceen (ca. 16 Ma) was de Centrale Paratethys verbonden met de naburige Middellandse Zee en de Oostelijke Paratethys. De bewaard gebleven Midden- en Laat Miocene afzettingen van de Centrale Paratethys onthullen terugkerende perioden van afsluiting van de aangrenzende zeeën. Deze momenten van isolatie oefenden een grote invloed uit op de samenstelling van het zeewater (bijv. Matyas et al., 1996; Latal 2004, 2006; Harzhauser et al. 2007b; Vasiliev et al., 2010; Peryt, 2006) en de fauna (bijv. Steininger en Wessely, 2000; Magyar et al., 1999; Harzhauser et al. 2002, 2003; Kováč et al. 2007) van de Centrale Paratethys en veroorzaakten het uitsterven van een groot aantal soorten (Harzhauser en Piller, 2007).

De Dinariden bevonden zich langs de zuidrand van de voormalige Centrale Paratethys en scheidden deze af van de Middellandse Zee. Gedurende het Late Oligoceen en het Mioceen lag er lange tijd een uitgebreid merensysteem in de intramontane bekkens van het gebergte (Mandic en Harzhauser, 2007; Harzhauser en Mandic, 2008; Mandic et al. 2007). De afzettingen van deze meren voorzien ons van een gedegen getuigenis van de geodynamische en paleogeografische geschiedenis van de Dinariden. De erin bewaard gebleven molluskenfossielen bieden ons bovendien een uitgelezen mogelijkheid de evolutie van fauna in een afgesloten ecosysteem met een lange levensduur te bestuderen. Veelvuldig in de meerafzettingen aangetroffen fossiele zoogdieren voorzien ons van een beter inzicht in de Oligocene en Miocene terrestrische fauna van Zuidoost-Europa.

Probleemstelling en doel van dit proefschrift

De intrinsieke isolatie van de Dinaride Meren en de progressieve isolatie van de Centrale Paratethys leidden tot zwaar endemisme onder de fauna. Het bijna exclusief voorkomen van locale soorten bemoeilijkt eenvoudige biostratigrafische correlatie van de afzettingen van beide systemen naar radio-isotopisch of cyclostratigrafisch gedateerde afzettingen elders. De ouderdom van de afzettingen is dus moeilijk in hoge resolutie te bepalen op basis van conventionele methoden. Dit staat wereldwijde en zelfs regionale correlaties van de gebeurtenissen vastgelegd in het sedimentaire archief in de weg en weerhoudt ons ervan ze eenduidig in de tijd te plaatsen. Dit beperkt dus ons inzicht in de geodynamische en paleogeografische geschiedenis van Zuidoost-Europa.

Het hoofddoel van deze thesis is daarom om nieuwe ouderdomsbepalingen met een hoge resolutie te bewerkstelligen voor zowel het Dinariden Meren Systeem als de Centrale Paratethys. Hiervoor zullen 40Ar/39Ar dateringen evenals cyclostratigrafische en magnetostratigrafische correlatie van de belangrijkste sedimentaire archieven aangewend worden. De verkregen resultaten zullen bovenal in een uitgebreid geologisch kader geplaatst worden en de implicaties met betrekking tot de paleogeografische en geodynamische evolutie van Zuidoost-Europa zullen worden verkend. 40Ar/39Ar-, cyclostratigrafische-

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Samenvatting

Een impressie van de paleogeografie van de Afrika-Europa collisiezone in het Mioceen (volgens Rögl, 1999).

en magnetostratigrafische dateringsmethoden zijn niet direct afhankelijk van de aanwezigheid van geschikte fossielen en dus van grote waarde wanneer er weinig fossielen voorhanden zijn, of endemisme biostratigrafische correlatie bemoeilijkt.

Dateren op basis van 40Ar/39Ar metingen

Wanneer vulkanen uitbarsten spuwen ze vaak grote hoeveelheden vulkanisch materiaal uit. Op een afstand van de vulkaan valt dit terug op het aardoppervlak en kan zich verzamelen in lagen van tuff of tuffiet op de bodem van een marien- of lacustrien bekken. De mineralen die zich vormen ten tijde van de uitbarsting bevatten vaak significante hoeveelheden kalium (K), een van de hoofdmineralen waaruit de aardkorst bestaat. Kalium heeft drie isotopen die in de natuur voorkomen, waarvan 40K radioactief is en met de tijd vervalt naar 40Ar. Het 40Ar gas wat zich vormt in de mineralen van een tuff vanaf het moment dat deze bij de vulkaanuitbarsting kristalliseren, zit gevangen in de kristalstructuur en blijft dus bewaard. De snelheid waarmee 40K isotopen vervallen naar 40Ar is bekend. De ouderdom van een monster van de desbetreffende tuff kan dus berekend worden wanneer de hoeveelheid ouder- (40K) en dochterisotoop (40Ar) gemeten worden. Bij de conventionele K-Ar dateermethode worden de hoeveelheden 40K, een vaste stof, en 40Ar, een gas, gemeten met verschillende meettechnieken in twee verschillende apparaten. De 40Ar/39Ar dateermethode is ontwikkeld om veel van de inherente analytische problemen op te lossen. Om 40Ar/39Ar dateren mogelijk te maken wordt eerst een klein mineraal separaat van een tuff bestraald in een kernreactor. Door het invangen van een neutron wordt de 39K aanwezig in het monster omgezet in 39Ar. De hoeveelheid geproduceerde 39Ar is direct proportioneel met de hoeveelheid 40K aanwezig in het monster, aangezien de 40K/39K ratio in aardse materialen aangenomen wordt constant te zijn. Aangezien zowel 40Ar als 39Ar gassen zijn, kan de ratio van beiden in één enkel experiment gemeten worden op een massaspectrometer. 40Ar/39Ar leeftijden voor tuffen in marine en lacustrine afzettingen voorzien ons van een hoge resolutie ouderdom voor het respectievelijke stratigrafische niveau. Ze zijn dientengevolge van onschatbare waarde als ijkpunten voor cyclostratigrafische of magnetostratigrafische correlaties. Uitgebreide informatie over de basisprincipes van 40Ar/39Ar onderzoek zijn te vinden in Kuiper (2003).

Afrika

Eurasië

Middellandse Zee

Dinariden

ParatethysCentrale Oostelijke

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Magnetostratigrafie

Het aardmagneetveld is gedurende de geologische geschiedenis vaak van polariteit veranderd. Een wisseling van polariteit duurt in het algemeen een paar duizend jaar en kan gezien worden als wereldwijd synchroon en instantaan op geologische tijdschaal. Wanneer fijnkorrelige sedimenten accumuleren en verstenen leggen zij in de regel de heersende richting van het magneetveld vast. Deze vastgelegde primaire magnetisatie wordt later vaak gedeeltelijk overprint door de inwerking van verschillende processen (bijv. vloeistofcirculatie) op de sedimenten. Paleomagnetische metingen kunnen, desondanks, vaak de primair vastgelegde magnetische veldrichting achterhalen. Daarvoor worden gesteentemonsters gedemagnetiseerd door stapsgewijze verhitting in een veldvrije oven, of door blootstelling aan een wisselveld van stapsgewijs toenemende sterkte. De primaire magnetische component wordt dan zichtbaar als een sequentie van vector eindpunten welke idealiter in een rechte lijn de oorsprong naderen. Intensieve studies van sedimentaire archieven wereldwijd en het in kaart brengen van magnetische anomalieën in de oceaan hebben uitgewezen dat ompolingen van het aardmagneetveld onregelmatig plaatsvinden. Elke tijdsperiode in de geologische geschiedenis heeft daarom zijn eigen karakteristieke patroon van ompolingen. Wanneer een karakteristiek ompolingspatroon herkend wordt in een sedimentaire opeenvolging, dan kunnen de desbetreffende afzettingen met andere afzettingen met een soortgelijk magnetisch patroon gecorreleerd worden, ongeacht de tussenliggende afstand of potentiële biogeografische barrières. Dit maakt magnetostratigrafische correlatie tot een krachtig instrument voor chronostratigrafisch onderzoek. De basisprincipes van magnetostratigrafisch onderzoek worden in detail uitgelegd in Langereis et al. (2010).

Cyclostratigrafie

Cyclische variaties in de baan van de Aarde beïnvloeden de distributie en hoeveelheid van invallende zonne-energie. Dit veroorzaakt periodieke veranderingen in het wereldwijde klimaat. Deze worden vaak geregistreerd in sedimentaire opeenvolgingen als cyclische veranderingen in lithologie en fauna. Wanneer deze cycli herkend worden, kunnen ze vergeleken worden met bekende variaties in zonne-instraling, berekend uit astronomische oplossingen voor het gedrag van het zonnestelsel (bijv. Laskar et al., 2004). Cyclostratigrafische analyse kan op deze manier leiden tot een tijdskader van enorm hoge resolutie voor de onderzochte sedimentaire opeenvolgingen. De basisprincipes van cyclostratigrafie worden in detail uitgelegd in Strasser et al. (2007).

Synopsis

In het eerste gedeelte van deze thesis behandelen we het Dinariden Meren Systeem. Het doel is een beter inzicht te krijgen in de ouderdom van de lacustriene afzettingen in de intramontane bekkens van de Dinariden en vervolgens de geodynamische en paleogeografische significantie hiervan te onderzoeken. In hoofdstuk 1 stellen we een gedetailleerd tijdsframe op voor de lacustrine sedimenten van het Sinj Bekken in Kroatië. Paleomagnetische resultaten voor de Lučane sectie voorzien deze van een magnetostratigrafie. 40Ar/39Ar leeftijden voor drie vulkanische assen aanwezig in de Lučane sectie maken het mogelijk het verkregen magnetostratigrafisch patroon aan de geologische tijdschaal te correleren. Op basis van deze correlatie stellen we dat de lacustriene sedimenten in het Sinj Bekken tussen 18.0 en 15.0 miljoen jaar geleden werden afgezet. In hoofdstuk 2 onderzoeken we de lacustriene afzetting in het naburige Livno-Tomislavgrad Bekken. Wederom gebruiken we een combinatie van magnetostratigrafische en 40Ar/39Ar resultaten om de ouderdom van de sedimenten van het Livno Meer te bepalen. Hieruit volgt dat het meer van 17.0 tot 13.0 Ma geleden bestond. De levensduur van het Sinj Meer en het Livno Meer overlappen dus grotendeels. Aangezien kalkarenieten en breccias, afkomstig van de bekkenrand de onderzochte sectie rond 14.8 Ma bereikten, kort waarna het Livno Meer verdween, is deze verdwijning waarschijnlijk te wijten aan een verandering in het tectonische regiem. De bekkenranden werden hierdoor opgeheven en meer blootgesteld aan erosie dan voorheen. In hoofdstuk 3 verkennen we de evolutie van het Gacko Meer in zuidwest Bosnië en Herzegovina. We identificeren cyclische variaties in

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het waterniveau van het meer. Deze worden vervolgens, geleid door magnetostratigrafische en 40Ar/39Ar resultaten, tentatief gekoppeld aan de astronomische curven. Deze correlatie suggereert dat de sedimenten van het Gacko Meer afgezet werden tussen 15.8 en 15.2 Ma geleden. Het cyclische karakter van de onderzochte sedimenten suggereert verder dat het meer in het bijzonder gevoelig was voor klimaatschommelingen geïnduceerd door de 100-kyr en 400-kyr eccentriciteits cyclus. De nieuw verkregen ouderdommen voor de Sinj-, Livno-, en Gacko Meren demonstreren dat de levensduur van deze meren samenvalt met het Midden Miocene Klimaat Optimum. Het is dus aannemelijk dat de toen heersende optimale klimaatomstandigheden de vorming van meren in de Dinariden stimuleerde. Onze nieuwe chronostratigrafie voor deze Dinariden bekkens legt de ouderdom van een aantal molluskensoorten vast, welke belangrijk zijn voor de regionale biostratigrafie. Deze endemische mollusken kunnen nu gebruikt worden als leeftijdsindicatoren voor sedimenten die zich niet lenen voor magnetostratigrafische of 40Ar/39Ar doeleinden. In hoofdstuk 4 combineren we magnetostratigrafische resultaten met een biostratigrafische beschrijving van meer dan 500 tanden van fossiele kleine zoogdieren en bepalen op basis daarvan een laat Oligocene ouderdom voor het Banovići bekken in NE Bosnië en Herzegovina. De 24-23 Ma ouderdom voor het Banovići Meer valt, net als het geval is bij de Midden Miocene meren, samen met optimale klimaatcondities en een wereldwijd verhoogde temperatuur. De samenstelling van de kleine zoogdierenfauna geeft aan dat er kleinschalige uitwisseling van soorten mogelijk was tussen Centraal Europa en West-Azië in het Laat Oligoceen. In hoofdstuk 5 duiken we in de geologische geschiedenis van de noordelijke rand van de Dinariden, gelegen op het grensvlak van de Dinaridenmeren en de Centrale Paratethys. Het terugrollen van de Europese plaat tijdens subductie onder de Karpaten leidde tot grootschalige extensie in het Pannonische Bekken gedurende het Mioceen. Als gevolg van deze extensie verzamelde er zich een meer dan 500 meter dik pakket fluvio-lacustriene afzettingen in het zuidelijk gedeelte van het Pannonische Bekken, gelegen langs de noordrand van de huidige Dinariden. De fossielen in deze afzettingen lijken enorm op de fossielen aangetroffen in het Dinariden Merensysteem. We bepalen in dit hoofdstuk nieuwe 40Ar/39Ar ouderdommen voor drie vulkanische assen aanwezig in het dikke pakket fluvio-lacustrine afzettingen. Op basis van deze dateringen wordt het duidelijk dat extensie in het zuidelijk deel van het Pannonische Bekken 18 Ma geleden begon, gelijktijdig met de aanvang van lacustriene sedimentatie in de intra-montane bekkens van de Dinariden. De accumulatie van fluvio-lacustrine sedimenten ging door tot tenminste 16.0 Ma geleden. Voortdurende rek zorgde er, in combinatie met een stijging in zeeniveau merkbaar in het hele Middellandse Zee gebied, 14.8 Ma geleden voor dat de Centrale Paratethys het zuidelijk deel van het Pannonische Bekken binnen stroomde (Ćorić et al. 2009). Het verschil in ouderdom tussen de top van de fluvio-lacustrine sedimenten en de basis van de marine Paratethys sedimenten impliceert een significant hiaat of ten minste onderbreking van depositie. Een andere uitleg zou zijn dat het binnenstromen van de Paratethys niet gelijktijdig plaatsvond in het scala van lacustrine bekkens langs de noordrand van de Dinariden. In hoofdstuk 6 integreren we alle chronostratigrafische resultaten verkregen in de Dinariden. We voegen hier paleomagnetische rotatie en AMS resultaten aan toe en herzien de paleomagnetische resultaten voor de Dinariden en Adria beschikbaar in de literatuur. We gebruiken het zo verkregen paleomagnetische en geochronologische overzicht om de geodynamische evolutie van de Centrale Dinariden vanaf het Mesozoicum te reconstrueren. We leggen daarbij speciale nadruk op de afgelopen 20 Ma. We concluderen dat een eerste fase van intramontane bekkenvorming plaatsvond in het Laat Oligoceen, toen laterale breuken gerelateerd aan de extrusie van het Alcapa blok doordrongen in het gebergte. Tussen 18 en 13 Ma geleden vond er een tweede fase van intramontane bekkenvorming plaats. In de tijd correspondeert dit, zoals hierboven vermeld, met een periode van grote rek in de korst van de Tisza-Dacia Plaat die het fundament vormt van het Pannonische Bekken. Langs de noordrand van de Dinariden zorgde deze rek voor het ontstaan van een serie bekkens waarin de in hoofdstuk 5 onderzochte fluvio-lacustrine sedimenten werden afgezet. Ook in het gebergte zelf ontstonden bekkens waarin zich de zogenaamde Dinaridenmeren ontwikkelden. Aangezien de bekkens midden in het gebergte, en die gelegen langs de noordrand, zich op een zelfde moment ontwikkelden ten gevolge van de rek in de Tisza-Dacia Plaat, en er bovendien een grote gelijkenis tussen deze bekkens bestaat wat betreft de bewaard gebleven aquatische fauna, kan het geheel als één systeem

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worden beschouwd. Onze chronostratigrafische resultaten maken het aannemelijk dat zowel in het Laat Oligoceen als in het Midden Mioceen een specifieke combinatie van tectonische en klimatologische omstandigheden het mogelijk maakte dat er zich dikke pakketten lacustrine afzettingen verzamelden in de Dinariden. Onze paleomagnetische resultaten wijzen verder uit dat de Dinariden geen significante tectonische rotatie hebben ondervonden sinds het Laat Oligoceen. Dit impliceert dat er weinig koppeling bestond tussen het gebergte en de twee aangrenzende platen, Adria en Tisza-Dacia, welke beiden een flinke rotatie ondergingen gedurende het Mioceen. Het Dinaridengebergte moet in deze periode ten gevolge van de rotatie van Adria een significante hoeveelheid verkorting hebben geaccommodeerd. Een herziening van de paleomagnetische data voorhanden in de literatuur wijst uit dat de Adriatische plaat sinds het Vroege Krijt 48 ± 10° tegen de richting van de klok gedraaid is. Van deze rotatie vond 20° plaats sinds het Mioceen. De hoeveelheid ondervonden rotatie neemt in de Adria-Dinariden collisiezone af naarmate de gemonsterde gesteenten jonger zijn en zich dichter bij het centrum van het Dinariden gebergte bevinden. Deze resultaten verbeteren ons inzicht in de post-orogene evolutie van de Dinariden en lossen de heersende discrepantie tussen structureel geologische en paleomagnetische schattingen voor de hoeveelheid rotatie welke zowel de Dinariden als Adria ondervonden op.

Het tweede gedeelte van dit proefschrift gaat in op de Midden Miocene Evolutie van de Centrale Paratethys. Deze wordt gekarakteriseerd door een aantal momenten van isolatie van de omringende zeeën, waarvan we de ouderdom beter willen bepalen. De eerste keer dat de Paratethys afgesloten wordt in het Midden Mioceen neemt de zoutconcentratie in de epicontinentale zee enorm toe. Deze zogenaamde Badeense Zout Crisis (BSC) resulteerde in de accumulatie van zout in een aantal delen van Zuidoost-Europa en leidde onder meer tot het uitsterven van een groot aantal soorten. In hoofdstuk 7 construeren we een nieuwe chronologie voor deze catastrofale gebeurtenis gebaseerd op 40Ar/39Ar dateringen voor vulkanische assen gevonden net onder, en midden in, de Badeen zouten in het zuiden van Polen. De resultaten wijzen uit dat precipitatie van de evaporieten 13.81 ± 0.08 Ma geleden begon. De gehele zoutcrisis duurde in totaal zo’n 200 tot 600 kyr (duizend jaar). De ca. decimeter dikke cycli in het zoutpakket, welke voorheen als een soort jaarringen beschouwd werden, blijken een periodiciteit van ongeveer 10 jaar te hebben. We tonen met de nieuwe dateringen aan dat het begin van de BSC samenvalt met een van de grootste kelderringen in de temperatuur van de aarde gedurende het Mioceen. Dit suggereert een causaal verband. De nieuwe dateringen maken het aannemelijk dat het plotseling zakken van het zeeniveau als gevolg van opeenhoping van ijs op de polen ten tijden van de zogenaamde Mi-3b glaciatie leidde tot een beperking van de wateruitwisseling tussen de Centrale Paratethys en de Middellandse Zee. Dit zorgde ervoor dat de diepe onderstroom, welke voorheen zorgde voor de uitstroom van zout uit het bekken, geblokkeerd werd. Dit zorgde vervolgens tot een catastrofale toename van het zoutgehalte. In hoofdstuk 8 stellen we een chronostratigrafie op voor de ca. 3 km dikke Midden Miocene invulling van het Transylvanie Bekken in Roemenië. 40Ar/39Ar ouderdommen voor een aantal tuffen aanwezig in de bekkeninvulling fungeren als eerste orde aanknopingspunten voor correlatie naar de geologische tijdschaal. Op basis van het dichte netwerk van seismische lijnen, voorhanden in Transylvanie, kunnen alle gemonsterde secties gecorreleerd worden naar een synthetische doorsnede op een centraal punt in het bekken. Op basis van de verkregen ouderdommen kan een ruwe sedimentatiesnelheid van 1.5 m/kyr berekend worden voor het gedeelte van de bekkeninvulling, wat accumuleerde in het Laat Badeen en Sarmaat. Ten tijden van het Pannoon nam de sedimentatiesnelheid af tot ca. 0.36 m/kyr. Dit komt goed overeen met interpretaties van seismische secties welke een geleidelijke inversie van het bekken laten zien. Aan de hand van de opgestelde chronostratigrafie kan een tweede stap in de voortschrijdende isolatie van de Centrale Paratethys veel nauwkeuriger dan voorheen gedateerd worden. De overgang van het Badeen naar het Sarmaat, twee tijdsperioden van de regionale stratigrafie, wordt gekenmerkt door een grote verandering in de samenstelling van de fauna van de Paratethys. Dit duidt op een toenemende beperking van de uitwisseling van water met de omringende zeeën. Onze resultaten geven aan dat deze overgang in het Transylvanie Bekken 12.80 ± 0.05 Ma geleden gebeurde. De verkregen chronostratigrafisch gekalibreerde paleomagnetische resultaten voorzien ons bovendien van eerste orde inzicht in de tectonische rotatie die het Transylvanie Bekken ondervond sinds

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het begin van het Midden Mioceen. Het bekken draaide van het laat Sarmaat tot het vroeg Pannoon ca. 20° met de klok mee, en is sindsdien nog een extra 6° in die richting gedraaid. De hoeveelheid en richting van de gereconstrueerde rotaties, veroorzaakt door het in oostwaartse richting terugtrekken van de subductiezone gelegen langs de oostrand van de Oost Karpaten, zijn in overeenstemming met een aantal structureel geologische observaties rond het Transylvanie Bekken. Door het nieuwe inzicht in de rotatiegeschiedenis van Transylvanie is het mogelijk de beweging van de Tisza-Dacia plaat gedurende zijn verplaatsing de intra-Karpatische baai in, beter te begrijpen.

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Curriculum Vitae

Johannes Hendricus Wilhelmus Maria de Leeuw (a.k.a. Arjan) was born in Heythuysen (the Netherlands) on September 10th, 1982. During his primary and secondary education Arjan developed a strong interest in the natural sciences. He graduated from the Scholengemeenschap Sint Ursula in Horn in 2000 and subsequently enrolled at University College Utrecht. This undergraduate college had a for the Netherlands unusual curriculum particularly attractive to Arjan because it allowed him to study the full range of Sciences without having to narrow down to one discipline. The Erasmus programme gave Arjan the opportunity to study in Bergen, Norway for one semester in 2002. Here he took his first geological field courses. He obtained his BSc. degree from University College in 2003 and enrolled in a geology masters program at the Faculty of Earth Sciences of Utrecht University. Sedimentology and stratigraphy, disciplines that aim to unravel the sedimentary record, in which so much of the Earth’s history is stored, started to fascinate him. Supervised by Poppe de Boer and Quintijn Clevis, Arjan studied the deposits of ancient sedimentary systems in the field and used numerical modelling experiments to simulate meandering rivers in foreland basin settings. Along the course of his masters, Arjan became aware of the fundamental importance of time constraints for our understanding of geological history. He thus took up a course at paleomagnetic laboratory ‘Fort Hoofddijk’, in which the paleomagicians Cor Langereis, Wout Krijgsman and Mark Dekkers explained him the nitty-gritty of paleomagnetism. Unaware of the consequences, Arjan applied for a PhD position at the same research group after obtaining his MSc. degree in 2005. The following years were turbulent. Many months of fieldwork in Romania, Croatia, and Bosnia & Herzegovina, and just as many months of laboratory work at the Vrije Universiteit in Amsterdam and in the dungeon of ‘Fort Hoofddijk’ eventually provided the results on which this thesis is based.

Arjan currently works as geologist at CASP, Cambridge and investigates the geological history of the northwestern Black Sea region.

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Publication listChapter 1: de Leeuw A., Mandic O., Vranjkovic A., Pavelic D., Harzhauser M., Krijgsman W., and Kuiper K.F. (2010) Chronology and integrated stratigraphy of the Miocene Sinj Basin (Dinaride Lake System, Croatia). Palaeogeography, Palaeoclimatology, Palaeoecology, 292: 155-167.

Chapter 2: de Leeuw A., Mandic O., Krijgsman W., Kuiper K.F., and Hrvatović H., A chronostratigraphy for the Dinaride Lake System deposits of the Livno-Tomislavgrad Basin: the rise and fall of a long-lived lacustrine environment. Submitted to Stratigraphy.

Chapter 3: Mandic O., de Leeuw A., Vuković B., Krijgsman W., Harzhauser M. and Kuiper K.F. (2010) Palaeoenvironmental evolution of Lake Gacko (Bosnia and Herzegovina): impact of the Middle Miocene Climatic Optimum on the Dinaride Lake System. Palaeogeography, Palaeoclimatology, Palaeoecology, 299: 475-492.

Chapter 4: de Leeuw A., Mandic O., de Bruijn H., Marković Z., Reumer J., Wessels W., Šišić, E., and Krijgsman, W. Magnetostratigraphy and small mammals of the Late Oligocene Banovići basin in NE Bosnia and Herzegovina. Submitted to Palaeogeography, Palaeoclimatology, Palaeoecology.

Chapter 5: Mandic O., de Leeuw A., Bulić J., Kuiper K.F., Krijgsman W., and Jurišić-Polšak Z., Paleogeographic evolution of the Southern Pannonian Basin: 40Ar/39Ar age constraints on the Miocene continental series of northern Croatia. Submitted to International Journal of Earth Science.

Chapter 6: de Leeuw, A., Mandic O., Krijgsman W., Kuiper K.F., and Hrvatović H., Paleomagnetic and geochronologic constraints on the geodynamic evolution of the Central Dinarides. Submitted to Tectonophysics.

Chapter 7: de Leeuw A., Bukowski K., Krijgsman W., and Kuiper K.F. (2010) Age of the Badenian salinity crisis; impact of Miocene climate variability on the circum-Mediterranean region. Geology 38: 715-718.

Chapter 8: de Leeuw A., Filipescu S., Maţenco L., Krijgsman W., Kuiper K.F., and Stoica M., Paleomagnetic and chronostratigraphic constraints on the evolution of the middle Miocene Transylvanian Basin and implications for Central Paratethys Stratigraphy and emplacement of the Tisza-Dacia plate. Submitted to Global Planetary Change.

Karami, M. P., de Leeuw, A., Krijgsman, W., Meijer, P. Th. and Wortel, M. J. R., Role of gateways in the evolution of temperature and salinity of semi-enclosed basins: An oceanic box model for the Miocene Paratethys. Submitted to Global Planetary Change.

Filipescu, S., Miclea, A., Wanek, F., de Leeuw, A. and Vasiliev, V. (2011), Micropaleontological response to the changing environment across the Sarmatian-Pannonian boundary in the Transylvanian Basin (Miocene, Oarba de Mureş section, Romania). Geologica Carpathica, 62: 91-102.

Bukowski, K., de Leeuw, A., Gonera, M., Kuiper, K.F., Krzywiec and P., Peryt, D. (2010) Badenian tuffite levels within the Carpathian orogenic front (Gdów – Bochnia area, S Poland), radio-isotopic dating and stratigraphic position. Geological Quarterly of the Polish Geological Institute, 54: 449-464.

Mandić, O., de Leeuw, A., Hrvatović, H., Profil Ostrožac - Jezerski miocen Duvanjskog bazena Section Ostrožac - Lacustrine Miocene of the Duvno basin, Excursion Guide of the 4th Croatian Geological Congress

Vasiliev, I., de Leeuw, A., Filipescu, S., Krijgsman, W., Kuiper, K.F., Stoica, M. and Briceag, A., The age of the Sarmatian-Pannonian transition in the Transylvanian Basin (Central Paratethys). Palaeogeography, Palaeoclimatology, Palaeoecology, v. 297, p. 54-69.

Jiménez-Moreno, G., de Leeuw A., Mandic, O., Harzhauser, M., Pavelić, D., Krijgsman, W., and Vranjković, A., Integrated stratigraphy of the Early Miocene lacustrine deposits of Pag Island (SW Croatia): Palaeovegetation and environmental changes in the Dinaride Lake System. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 280, p. 193-206.

Van Waveren, I. M, F. Hasibuan, Suyoko, Makmur, P.L. de Boer, D. Chaney, K. Ueno, M. Booi, E.P.A. Iskandar, Ch. I. King, A. de Leeuw and J.H.A. van Konijnenburg-van Cittert. Taphonomy, palaeobotany and sedimentology of the Mengkarang Formation (Early Permian, Jambi, Sumatra, Indonesia). In: Spencer G. Lucas, Kate E. Zeigler and Justin A Spielmann (eds.), Permian of Central New Mexico, 176 pages.

de leeuw 2011 thesis

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lake livno

netherlands paleomagnetic

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lakelevel change

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