chronology, correlation and variability in pliocene

XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009

S10_038

Chronology, correlation and variability in Pliocene Glacioeustatic Cyclicites, Tiburon Basin, Mejillones Peninsula,

Chile

Tapia, C.1, Wilson, G.2, 1, Ishman, S.3, Wilke, H.4 (1) Department of Geology, University of Otago, P.O. Box 56, Dunedin, New Zealand. (2) Department of Marine Science, University of Otago, P.O. Box 56, Dunedin, New Zealand. (3) Department of Geology, Southern Illinois University, Carbondale, IL, USA. (4) Departamento de Ciencias Geológicas, Universidad Católica del Norte, Chile. [email protected] Introduction and general setting In this work, marine sequences from Tiburon Basin (Fig. 1), that form part of La Portada Formation [1, 2] at Mejillones Peninsula northern Chile, are examined in order to construct a sea level model with the aim to identify the global versus local eustasy signal. Miocene - Pleistocene inclined to flat lying marine deposits (3 in Fig.1A) fill two half-grabens flanked by the Caleta Herradura and Mejillones normal collapse faults. These faults are related to extensional regime in the costal area due to oceanic plate subduction and subsequent margin uplift [4]. The footwalls west of the structures form the Jorgino and Mejillones horsts that consist of Precambrian to Ordovician metamorphic rocks (1 in Fig.1A) and Jurassic to early Cretaceous volcanic and plutonic rocks (2 in Fig.1A). The Pliocene marine sequences were deposit in a local climatic context of semi aridity before 3 Ma and hyper aridity since 3 Ma [5]. Methodology The outcrops in the Tiburon Basin are exposed in vertical faces, stratification is subhorizontal and no local deformation was identified between or around the sections examined. Five N-S sections (1A, 2B, 5B, 6A and 7A in Fig. 1.B) were used to construct a high resolution composite section (Fig 2.C). Paleomagnetic samples obtained mostly from section 1A, at intervals of between 0.5m and 1m, were measured on a 2G Enterprises cryogenic magnetometer. Two ash layers were sampled one in the middle of section 1A and other at the bottom of section 2B and dated using 40Ar/39Ar. Diatom

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samples were collected from section 1A to identify age and environment of deposition. Magnetic susceptibility was measured at 1 to 5 cm spacing using a Bartington probe. Sedimentology and sequence stratigraphy Six lithofacies were recognized: 1) Laminated & cross-bedded silty diatomaceous clay, bioturbated; 2) Fine diatomaceous silty very fine sandstone, generally laminated, well sorted, burrowed, massively bioturbated; 1 and 2 generally contained whale bones. 3) Stratified, poorly to moderately sorted, unconsolidated fossiliferous medium to fine sand. Diatoms assemblages in 1 to 3 suggest a nearshore environment; 4) Stratified to cross stratified volcanoclastic, grey, silty, unconsolidated, medium, coarse siliclastic sand that becomes moderate to poorly sorted and less fossiliferous towards the top and interpreted to represents the upper shoreface; 5) Reworked fragment shells of turritella, balanus, chalamys delicatula and barnacles with matrix generally medium to coarse sand and minor component of rounded pebbles, usually with calcrete horizon and erosional surface at the base, also interpreted to represent a foreshore environment; 6) Conglomerate containing meta-sedimentary igneous and volcanic clasts, with diameters ranging between 20 cm and 150 cm, poorly sorted, moderately cemented and interpreted to represent fluvial distributary channels. Boundaries of Tiburon Basin sequences are principally represented by either erosional or non-depositional surfaces. The succession can be divided into two subequal parts. The lower part of the succession (fig. 2.A) comprises cycles or motifs with a vertical stacking of sand (facies 3 or 4) overlain by diatomaceous silty sandstone (facies 2) which then grades into a coarser unit (facies 1) often with whale bones fossils marking the transition. The sequence is truncated by a well-developed calcrete burrowed and nodular erosional or non-depositional surface, representing development of hard ground. The upper part of the succession (fig. 2.B) is marked by much coarser cycles or motifs, with stratified volcanoclastic sands (facies 4) coarsening upwards and capped by either broken shellbeds (facies 5) or conglomerates (facies 6). The boundary between the lower and upper parts of the succession is marked by a 1m deep channeled surface (see fig. 2.C). Magnetostratigraphy and age model Magnetozones together with 40Ar/39Ar dates constrain the lower part of the section to between 4.18 and 2.84 Ma equivalent to a 1.34 Ma interval. We recognize 33.5 cyclothems in this interval. The age of the upper part of the composite section is less well constrained and the fact that the trace of the Mejillones Fault pass between the top sections 5B and 6A-7A introduce the possibility of repeated exposed outcrops.

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Conclusions For the lower part of the composite section, the age control implies ~40-kyr periodicity cyclothems (obliquity-paced) in the middle to late Pliocene. Obliquity paced fluctuations in the west Antarctic ice margin have recently been reported by [7]. Ice volume and sea level equivalent fluctuations of up to 10m are suggested by [7]. Accordingly we suggest that despite the relatively thin cyclothems, the lower part of the Tiburon Basin composite section is responding to sea level fluctuation at an obliquity paced timescale. The prominent change from thin fine-grained cycles to thicker coarser cycles between 2.7 and 3.0 Ma. is coincident with a major lowering of base sea level as predicted by the benthic d18O stack for the Plio-Pleistocene [8] Acknowledgements This work was supported by the Foundation for Research Science and Technology of New Zealand, U.S. National Science Foundation and CONICYT (student scholarship to the first author). The authors also thank Ruben Martinez-Pardo for help with fieldwork. References [1] Ibaraki, M. (1992) Geologic age of biosiliceous sediments in Perú and Chile based upon planktonic foraminifera. Revista Geológica de Chile, vol. 19, 1, 61-66. [2] Niemeyer, H., González, G., and Martínez-De Los Ríos, E. (1996) Evolución tectónica cenozoica del margen continental activo de Antofagasta, norte de Chile. Revista Geológica de Chile, vol. 23, 165-186. [3] SERNAGEOMIN. (2003) Mapa Geológico de Chile: versión digital. Servicio Nacional de Geología y Minería, Publicación Geológica Digital Nº4. (CD-ROM, version 1.0, 2003). Santiago. [4] González, G., Cembrano, J., Carrizo, D., Macci, A. and Schneider, H. (2003) The link between forearc tectonics and Pliocene-Quaternary deformation of the Coastal Cordillera, northern Chile. Journal of South American Earth Sciences, vol. 16, 5, 321-342. [5] Hartley, A.J. and Chong, G. (2002) Late Pliocene age for the Atacama Desert: Implications for the desertification of western South America. Geology, vol. 30, 1, 43-46. [6] Cande, S.C. and Kent, D.V. (1995) Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. J. Geophys. Res., vol.100, B4, 6093–6096. [7]. Naish, T., Powell, R., Levy, R., Wilson, G., et al. (2009) Obliquity-paced Pliocene West Antarctic ice sheet oscillations. Nature, vol., 458, 7236, 322-328. [8]. Raymo, M. E. and Huybers, P. (2008) Unlocking the mysteries of the ice ages. Nature, vol. 451, 7176, 284-285.

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chronology, correlation and variability in pliocene

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