Principles ofStratigraphy

Michael E. Brookfield

Principles of Stratigraphy

Principles ofStratigraphy

Michael E. Brookfield

© 2004 by Blackwell Publishing Ltd

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First published 2004 by Blackwell Publishing Ltd

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Brookfield, M. E. (Michael E.)Principles of stratigraphy / Michael E. Brookfield.

p. cm.Includes bibliographical references and index.

ISBN 1-4051-1164-X (pbk. : alk. paper)1. Geology, Stratigraphic. I. Title.

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Contents

Preface viiiAcknowledgments x

1 Introduction 11.1 Stratigraphy – why bother? 11.2 Development of stratigraphy 31.3 Phases of study 8

Part I Basics 11

2 Weathering 132.1 Types of weathering 132.2 Rates of weathering 152.3 Soil formation 162.4 Weathering and soil formation under water 21

3 Sediments and sedimentary rocks 223.1 Transportation and deposition 223.2 Clastic sediments and sedimentary rocks 243.3 Chemical and biochemical sediments and

sedimentary rocks 46

4 Major environmental complexes and their recognition 664.1 Introduction 664.2 Impact and volcanic environments 674.3 Continental environments 674.4 Environments under water 814.5 Mixed environments 994.6 Peculiar environments 99

vi Contents

Part II Tracing environments in space and time 101

5 The vertical dimension 1035.1 The local section 1035.2 Breaks in the record 1055.3 Dividing the local section: the type section 1115.4 Strata and stratification 114

6 The horizontal dimension 1156.1 Physical correlation 1156.2 Lateral changes 1276.3 Mapping 134

7 The time dimension 1407.1 Age equivalence 1427.2 Relative ages 1427.3 Numerical methods (ages in years) 1587.4 Calibration of relative and numerical dates 170

8 Basin analysis 1718.1 Basin-fill architecture 1718.2 Sediment provenance 1758.3 Paleocurrents and sediment dispersal 1758.4 Backstripping 1758.5 Paleothermometry 1808.6 Paleogeographic and paleotectonic maps 184

9 Stratigraphic systems 1869.1 Development of the stratigraphic system 1869.2 Cycle stratigraphy 1909.3 Genetic sequence stratigraphy 1929.4 The current system 197

Part III Interpreting geologic history 207

10 Tectonics 20910.1 Geodesy 20910.2 Hypsometry 21010.3 Gravity 21010.4 Isostasy 21310.5 Tectonics and sedimentary basins 21810.6 Exotic terranes 23510.7 Terrane analysis of orogenic belts 235

11 Sea-level changes 24111.1 Eustatic or “absolute” changes of sea level 24111.2 Relative changes of sea level 246

12 Climate 25512.1 Present distribution and character of climates 25512.2 Identifiable climatic effects on sediments,

biotas, and stable isotopes 25912.3 Controls on climate and climatic change 265

13 Biology 27113.1 Atmosphere and ocean changes 27113.2 Bioclastic sediment changes through time 27113.3 Sediment mixing by organisms 27213.4 Biogeographic changes 272

14 Stratigraphic problem times and places 27614.1 Quaternary stratigraphy 27614.2 Archeological stratigraphy 28114.3 Proterozoic stratigraphy 28314.4 Archean stratigraphy 284

15 Extraterrestrial stratigraphy 28515.1 The Moon 28515.2 Mercury 28715.3 Mars 28715.4 Venus 287

Appendix 1 Imperial/metric conversions 292Appendix 2 Figure legends 293Appendix 3 Geologic time-scale 296

Glossary 297References 307Index 321

Color plates fall between pp. 182 and 183.

Contents vii

Preface“Stratigraphy may be defined as the complete triumph of terminology over facts andcommon sense.”

P.D. Krynine

Stratigraphy is one of the most demanding, funda-mental, and interesting of geologic disciplines. It tries to reconstruct history with a few basic principles, re-quires both careful observation and wide-ranging imagination, and involves many individually fascinat-ing subdisciplines. Yet, as the quotation above shows,this is not how it appears to many people. Stratigraphyhas a poor image because it is sometimes presented onlyas a way of classifying and organizing strata with“codes.”

However, its basic principles are very simple and appli-cable in any study in which a history must be recon-structed from layers. Thus, stratigraphic principles arean essential part of archeology and even forensic science (the sequence of events during a murder, for example). They are now used to work out the history of planetary surfaces in studies that relive the early 19th century excitement in exploratory mapping andcorrelation (Carr et al. 1993).

This book is intended to back up a second-year univer-sity course, so I assume that students using it have already taken a basic Introductory Geology course (orits equivalent), though some material is repeated forconvenience (since students tend to forget things overtime). I wish to emphasize the need to: (i) know howstratigraphy developed and how this constrains currentapproaches and dogmas; (ii) study all aspects of modernprocesses and environments (including extraterrestrialones) as a basis for paleoenvironmental interpretation;(iii) understand the principles of correlation and datingas a basis for stratigraphic reconstructions; (iv) know

some basics of geophysics, tectonics, climatology, andpaleobiology in order to explain the development ofsedimentary basins; and (v) make field observations inperson, to understand the limitations of data and interpretations.

First, it is essential to know the history of ideas in a subject, and how and why choices were made amongdifferent alternatives. The current controversies ofsequence stratigraphy simply recapitulate the disputesbetween Werner and Hutton in the 18th century, between Oppel and Gressly in the 19th century, and between Grabau and Ulrich in the early 20th century.Basic stratigraphic concepts were only slowly estab-lished after much discussion. Perhaps the most im-portant thing to understand is why discredited ideaswere accepted at the time (and vice versa), as this mighthopefully lead to much needed humility and open-mindedness in the face of current dogma (Menard1986; Raup 1986).

Second, the conditions under which ancient stratifiedrocks formed can only be worked out by studying theirmodern counterparts (where possible) and derivingprinciples from sedimentology and ecology that mighthelp in this (Fraser 1989).

Third, the relationships of environments in space andtime require specific concepts of correlation and datingwhich are peculiar to stratigraphy (Berry 1987).

Fourth, it is impossible to interpret the stratigraphicdevelopment of an area, or sedimentary basin, withoutknowing some basic concepts in geophysics, tectonics,climatology, and biology.

Fifth, personal observations on actual rocks are essential, otherwise people get completely out of touchwith their material. Arkell (1933, p. 36) cites the exam-ple of the famous Jurassic geologist, S.S. Buckman:“That Buckman, who had tramped the Cotswolds andthe Sherborne country from end to end and knew everyquarry intimately, whose earlier work was built up sole-ly on sound field-work, could also be the author of hislast paper . . . and some of the later parts . . . of TypeAmmonites is difficult to believe. Without any practicalknowledge of the Cornbrash, without describing somuch as a single section, he proceeded to divide it up into11 brachiopod zones and coined for it 5 new stagenames. Neither zones nor stages have any foundation in fact.”

There has always been disagreement on how to teachstratigraphy, and this reflects the fundamental disagree-ments of earlier geologists. Werner’s historical view of earth history, marked by discrete events in time with a starting point and an end point, contrasted with Hutton’s view of unending and unchanging cyclicalprocesses; time’s arrow versus time’s cycle (Gould1987). Though both traditions are found in moderntextbooks, one or other tends to dominate and each author’s experience naturally colors their individual approaches. The classification and correlation traditionis found in Lemon (1990), Prothero (1990), and Schoch(1989). The environments through time approach isfound in Blatt et al. (1991), Boggs (2001), Brenner and

McHargue (1988), Hallam (1981), Matthews (1984),Nicholls (1999), and Prothero and Schwab (1996). I ammostly sympathetic with the environments throughtime approach; yet this sometimes ignores the essential(perhaps boring) need to actually correlate and daterocks, and understand the problems in doing that. It also tends to minimize irreversible, sometimes rapid and catastrophic changes. Thus, certain lithologies are confined to specific times (Ager 1973): chalks are found only from Cretaceous times onwards (after theevolution of carbonate-secreting plankton). And, as Alvarez et al. (1980) established, large meteorite im-pacts have often affected the stratigraphic record.

I wrote this book hoping to instil in students the use-fulness, wonder, and relevance of stratigraphy. I alsowrote it to re-establish the necessity for tectonic and geophysical knowledge to interpret the stratigraphicrecord. It is truly amazing that some current stratigra-phy texts have nothing on isostasy: a text such as WholeEarth Geophysics (Lillie 1999) should be compulsoryreading for all stratigraphers. I have tried to give refer-ences for all main ideas and also for the figures. Studentsshould not accept statements in which sources are notgiven. It is extremely important that students get intothe habit of questioning and checking both facts andideas, especially from their teachers. Otherwise, un-questioned, authority-derived dogma tends to dominatea subject – remember the history of the continental drifttheory (Oreskes 1999).

Preface ix

Acknowledgments

All books are the result of an author’s experience. I learned most from three rather different teachers – D. Ager, J.R.L. Allen, and A. Hallam – all of whom, how-ever, emphasized the derivation of principles from care-fully chosen observations, rather than the simpleaccumulation of data, or the mindless application of for-mulae. I am grateful to several generations of studentsin my stratigraphy classes who pointed out ways of sim-plifying and clarifying the text: there is not much pointin producing a book that students neither understandnor read. I also owe a debt to all authors of other stratig-raphy texts, whose ideas and methods have helped inwriting my own, and thanks to a variety of anonymous

readers who improved the manuscript with their comments.

Of the individuals who helped me during the writing,I must first thank my wife, Kathleen, for support over many years and for contributing to writing and editing during the early stages. Steve Sadura and DonIrvine at Guelph did wonderful jobs in producing respec-tively the glossary and artwork. The staff at Blackwells,specifically Ian Francis, Delia Sandford, Rosie Hayden,Cee Pike, Linda Auld, and Sue Worrall, managed to beboth prompt and efficient (something I am incapable of)in transforming a rough manuscript into a finishedbook.

Introduction

Stratigraphy, perhaps most importantly, also helpsyou to understand how many economic materialsformed and got distributed in the way they did – and sowill hopefully help you find more. For example, theWembley Field is one of many isolated oil and gas reser-voirs in the Middle Triassic in Alberta, Canada (Fig. 1.1).Finding out why the oil is there, and where other similaroil and gas fields are, requires you to proceed logicallythrough the various phases of stratigraphy.

First, what are the actual oil-bearing rocks and how were they deposited? The local (borehole) sectionsmostly consist of porous sands alternating with claysarranged in coarsening-upwards cycles, deposited bywaves and currents as marine barrier island deposits(Fig. 1.2).

Second, how are these sediments arranged spatiallyand how old are they? In the absence of outcrops, spacecorrelations have to be worked out from borehole logsand seismic sections which show the arrangement andthickness of the strata and environments and that theproductive oil and gas wells are in linear sand bodies of aparticular type of marine barrier bar (Fig. 1.3).

1.1 Stratigraphy – why bother?1.2 Development of stratigraphy1.3 Phases of study

1.1 Stratigraphy – why bother?

Stratigraphy gives you techniques for working out earthhistory: it integrates diverse materials into a coherentview of how the earth and its life forms evolved. Thoughstratigraphy (literally writing about strata) is mostlyabout working out the history of sedimentary rocks, inorder to do this you also need to know the effects of magmatism, metamorphism, tectonism, climaticchange, and sea-level changes, and the effects of organ-ic evolution. So, stratigraphy integrates data and con-cepts from many specialties, and in practice ends up as amuch more comprehensive study than its name implies.

Stratigraphy also lets you test ideas on how varyingcombinations of processes affect the planets throughtime. For example, as evidence for continental drift andchanging climates, Wegener (1915) used the presentlyseparated positions of Carboniferous Mesosaurus-bearing and glacial sediments, which were most plausi-bly explained by an originally compact supercontinent.Together, history and process let you work out how,when, and why environments changed through time.

1

Fig. 1.1 Location of Middle Triassic oil andgas fields in west-central Alberta, Canada(modified from Willis & Moslow 1994a, fig. 3). (AAPG © 1994. Reprinted bypermission of the AAPG whose permissionis required for further use.)

Fig. 1.2 Section of Halfway Formation with barrier-bar interpretation, and porosity and permeability of the oil- and gas-bearingunits (modified from Willis & Moslow 1994a, fig. 5). (AAPG © 1994. Reprinted by permission of the AAPG whose permission isrequired for further use.)

Further detailed studies showed that the oil and gas isconcentrated specifically in the well-sorted and poroussandstones deposited in active tidal inlets and ebb-tidaldeltas whose distribution could be plotted in sections(Fig. 1.4) and on thickness maps (Fig. 1.5).

But why is the oil and gas concentrated in the HalfwayFormation, when many other underlying units alsoshow coarsening-upwards barrier-bar sections? Thereason is the peculiar development of this unit. Smallrises in sea level, which led to the formation of transgres-sive barriers from eroding shoreline sand deposits, alter-nated with periods of stable sea level, during whichseaward migration of barriers led to burial of the trans-gressive barriers with fine-grained impermeable back-barrier and coastal plain sediments (Fig. 1.6). The oiland gas reservoirs can be located by tracing the con-densed radioactive clays at the base of each marinetransgression up-dip into the transgressive barriers.

Third, what is the history of the area? Regional strati-graphic studies show that it formed part of a subsidingpassive margin shelf in western Canada (Fig. 1.7). Inherently unpredictable evolution of discontinuousbarrier islands was controlled by the interactions oftides and sea-level changes. The ages come from marinefossils fitted into the standard geologic time-scale.

Fourth, how does this area fit into the overall interpre-tation of earth history? Although working this out is notalways necessary in a regional study, the results willcontribute to understanding the overall development

and character of the Triassic. This study of the WembleyField used essentially a modern physical stratigraphyapproach to the distribution of strata and their environ-mental interpretation. Biological, climatic, and tectonicfactors were not used (and were probably not needed) tounderstand the field. Nevertheless, all possible factorsand approaches should be considered both before andduring any study, since any individual is usually biasedtowards those approaches made familiar and comfort-able by education and experience. On the one hand, astratigrapher should know why he works in a particularway and realize that other ways may be equally valid:many controversies arise because people do not appreci-ate the aims of different stratigraphic studies. On theother hand, a stratigrapher should criticize inappropri-ate and confusing methods and concepts traditionallyapplied to his area or period: much dead weight and con-fusion could be lifted by applying alternative methodsand concepts.

Furthermore, it is impossible to understand the tradi-tions followed in different areas and periods and evalu-ate their results without knowing how stratigraphydeveloped (Gohau 1991).

1.2 Development of stratigraphy

Geology became a specific discipline at the end of the18th century with the description of strata as its focus,

Introduction 3

Fig. 1.3 Sand thickness map of HalfwayFormation (from Willis & Moslow1994a, fig. 6). (AAPG © 1994.Reprinted by permission of the AAPGwhose permission is required for furtheruse.)

4 Chapter 1

and developed from the Romantic movement’s taste forsavage nature and travels to remote places (as it stilldoes: I work in the mountains of Central Asia). Stratig-raphy, like keeping a journal or collecting beetles, gaveserious purpose to tours that might otherwise seem aim-

less or frivolous (Porter 1977). The first thing to do waswork out the superposition of strata in an area. Suchlocal successions, securely based in limited areas, thenserved as “types” for similar successions elsewhere. Scientific order could then be imposed worldwide on the basis of rock and fossil similarities. Most people spent their time tracing and mapping strata, and in try-ing to classify the geologic column rather than trying toexplain it (Secord 1986).

This is the first tradition of classification and nomen-clature. Within this tradition, the emphasis on the rockstrata and the fossils they contained varied greatly andled to different stratigraphic approaches or styles. Somepeople emphasized the actual rocks (LITHOSTRATIGRAPHY),some the fossils the rocks contained (BIOSTRATIGRAPHY).Though the actual strata determined the aproach, per-sonal style determined the actual field area chosen forstudy. Scientific style was shown in everyday field prac-tice and not in grand generalizations, though theseevolved together. And after formative undergraduatestudy and learning, the rocks studied and the styles chosen tended to narrow and ossify.

The contrast between rock (lithostratigraphic) andfossil (biostratigraphic) approaches is best exemplified in

Fig. 1.4 Dip section across Wembley Field showing tidal channels, ebb-tidal delta environments, and oil- and gas-producingintervals (from Willis & Moslow 1994b, fig. 13).

Fig. 1.5 Distribution of tidal inlet sand units (from Willis &Moslow 1994b, fig. 15).

the work of Sedgwick and Murchison, which led directlyto the controversy over the Cambrian–Silurian bound-ary in the mid-19th century (Secord 1986). Sedgwick’sfame rested on his insight into structure, his ability to visualize rocks in three dimensions and interpret theirrelationships after only a few traverses: he emphasizedthe distribution of rock types, worked on the basicallyunfossiliferous strata of northwestern Wales, and henceused a lithostratigraphic approach. Murchison’s famerested on his development of geologic systems based onfossils: he emphasized the vertical succession of faunas,named the Silurian and Permian periods, worked on thefossiliferous rocks of the Welsh–English border area,and used a biostratigraphic approach. What might have happened if they had changed places? It would bepointless to ask. They chose their work areas preciselybecause of their different approaches and the type ofrocks present. Sedgwick worked in North Wales becauseits unfossiliferous schists and slates were structurallycomplex, and contrasting well-exposed rock types couldbe used to trace the structure. Murchison worked in theWelsh Borders because it was there that biostratigra-phic divisions could be recognized and correlated infossiliferous and structurally simple, but poorly exposed,and lithologically repetitive successions.

In Europe, the biostratigraphic approach eventuallyovershadowed the lithostratigraphic approach because

of the success of d’Orbigny and Oppel in developing biostratigraphy and of Gressly in developing the FACIES

concept. Both concepts evolved together and were basedon the fortunate pecularities of European Jurassic rocks.

The European Jurassic contains widespread, rapidlyevolving, and easily recognizable lineages of swimmingcoiled cephalopods called AMMONITES. These are commonin many different rock types, and biostratigraphic timeunits based on the vertical ranges of different ammonitespecies were erected independent of lithology by d’Orbigny (1842) and particularly by Oppel (1856–8).These “time zones” were often traceable across westernEurope, even where the rock types changed, because thearea is small enough to limit biogeographic effects onfree-living organisms. Oppel was professionally percep-tive (or lucky) in both the fossil group and the area hestudied; he was personally less fortunate in dying oftyphoid at the age of 35.

Also, the European Jurassic was deposited during aperiod of continental rifting and splitting. The changesin rock types from one area to another reflected great lateral changes in environments, both vertically andhorizontally, emphasized by compressional shorteningin the type area of the Jura mountains. Simple tracing ofrock units is difficult or impossible because of the greatlateral variation within even the small sizes of Europeancountries. Thus, geologic surveys recognized from the

Introduction 5

Fig. 1.6 (a–c) Model for the development of a transgressive barrier, and (d) location of successive transgressive barriers (from Willis& Moslow 1994a, figs 12, 13). (AAPG © 1994. Reprinted by permission of the AAPG whose permission is required for further use.)

Fig. 1.7 Thickness (in meters) of Middle Triassic sediments on western Canadian margin (from Willis & Moslow 1994a, fig. 1).(AAPG © 1994. Reprinted by permission of the AAPG whose permission is required for further use.)

precise biostratigraphy available that some rocks were ofthe same age even when they were markedly different in both sediment type and benthic fossils. Gressly(1838–41), working in the Jura mountains of north-eastern France, called these facies changes.

The need to interpret Gressly’s sedimentary and bio-logical facies changes led to the second tradition of usingthe sedimentology and ecology of modern environ-ments to interpret ancient strata and fossils. Based onLyell’s strict PRINCIPLE OF UNIFORMITARIANISM, this funda-mentally modern approach to stratigraphy was estab-lished in Europe by the end of the 19th century and isbest exemplified in the work of J. Walther (1893–4).

No lucky circumstances comparable with the European Jurassic existed in North America. Both faciesand biostratigraphic zonation was practically ignoreduntil the mid-20th century.

In North America, the eastern seaboard was the firstto be studied geologically. Between 1832 and 1851,James Hall, the outstanding American geologist of hisgeneration, moved westwards, describing sections andfossils from the Paleozoic as he went (Dott 1985). Hall’sstudies proceeded in the same way as Murchison’s, ex-cept that lateral changes in lithology were nowhere nearas obvious, practically all the fossils were benthonic andfacies (environmentally) controlled, and no good fossilzonation was possible outside the graptolite-bearingshales of the Appalachians. The uniformity of the exten-sive fossiliferous sedimentary units almost forced theview that widespread lithologies with characteristic fossils succeed one another; and that distinct units bear-ing different fossils imply time differences.

This view was forcefully promoted by Hall’s successor,E.O. Ulrich. Starting in 1885, Ulrich described the LowerPaleozoic of central and eastern North America; whatwe now call the craton interior and passive margin. According to Ulrich, the Paleozoic shelf seas occupiedsmall, shallow, and often disconnected basins. Thesebasins changed their extent and character depending onlocal rhythmic deformations with consequent trans-gressions and regressions. Each individual advance ofthe sea laid down a rock unit with relatively constantlithology and fauna. Each unit was terminated by non-deposition rather than by lateral change into a differentlithology. Each unit was separated by a widespread timebreak marking retreat of the sea. These breaks would becommonplace yet inconspicuous because of the low re-lief of the continental interior, the shallowness of theshelf and interior seas, and the frequency of oscillations.Ulrich interpreted broad contemporary lateral changes

of sediment and fauna, reflecting lateral changes of en-vironment, as distinct sedimentary lenses of differentages. This led to the concept of troughs and barriers, according to which deposits of sandstone, shale, limestone, etc., which formed an orderly contemporaryfacies change, were considered by Ulrich to have beendeposited separately and successively in one or anotherof four or five parallel troughs (Merk 1985). On thisbasis, in 1911, Ulrich proposed two new systems, theOzarkian and the Canadian, between the Cambrian and Ordovician systems.

Ulrich’s ideas have since been castigated as prime examples of reactionary dogmatism that retarded theacceptance of the facies concept in America (Dunbar & Rogers 1957). True enough: but for some of the Paleo-zoic, Ulrich was right. For example, the shallow-waterOrdovician of eastern North America does show sepa-rate and successive overlapping lenses separated by timebreaks in some areas, albeit with more facies changesthan Ulrich would accept (Brookfield & Brett 1988). AndUlrich’s ideas have recently been resurrected in the concept of sequence stratigraphy.

Ulrich’s friend, W.A. Grabau, promoted the oppositefacies view. However, Ulrich’s ideas dominated Ameri-can stratigraphy until the mid-20th century, and thereis a residual tendency to downplay facies at the expenseof layer-cake and cyclical stratigraphy even now; wit-ness the success of cyclical and sequence stratigraphy,“punctuated aggradational cycles,” and “ecostratigra-phy.” (Ulrich and Hall both got a separate chapter in the1985 Geological Society of America Centennial Volumeon the history of North American geology; Grabau is noteven mentioned.) Grabau worked on the Silurian andDevonian of western New York State, on rocks later usedas classic examples of facies change (see Chapter 6).

However, until the early 20th century, rocks couldstill only be indirectly dated. To get a date a known accu-mulation or loss has to be divided by a known and uni-form rate. So, people estimated the amount of salt now inthe sea, divided it by the rate of supply, and got values ofabout 100 million years for the age of the oceans. Orthey estimated the thickness of sediment preserved, divided it by the average rate of supply, and got values ofabout 150 million years since sediment started accumu-lating on the earth. All these efforts foundered on theunreliability of both accumulation values and unifor-mity of rate, and the undoubted removal of both salt andsediment. Estimates based on a molten cooling earthcould not be faulted on 19th century physics, and LordKelvin’s final 1897 estimate of 27 million years for the

Introduction 7

8 Chapter 1

age of the earth was generally accepted. But in 1896 thediscovery of radioactivity by Becquerel gave an addi-tional source of heat, and demolished the basis forKelvin’s short estimate. It also gave, for the first time, areasonably accurate way of dating rocks in years. Withthis discovery, the basic stratigraphic trilogy of rock type distribution, relative time, and absolute time wasestablished.

1.3 Phases of study

The phases of study in stratigraphy essentially followthe original development of stratigraphy in the 19thcentury and are as relevant now as when they first appeared. These phases are followed in this book.

Phase 1: Basics

The basics, needed before starting any stratigraphicstudies, involve first being able to:1 identify and classify minerals, rocks, and fossils accurately;2 infer the processes that formed the minerals, rocks,and fossils from field and laboratory studies of the effectsof modern physical, chemical, and biological processes;3 recognize the ancient depositional (and rarely non-depositional) environments, by comparing the variety,intensity, and periodicity of processes in modern environments with those inferred in ancient rocks successions;4 map the obtained data on maps and sections ofvarious types.

Fig. 1.8 Dynamic stratigraphy (after Aigner 1985, fig. 1).

These basics have to be done well as they form the foun-dation for all further studies. After mastering these youcan then go on to phase 2.

Phase 2: Tracing environments in space and time

Tracing environments in space and time requires foursteps:1 An overview of the area studied involves a prelim-inary survey of what work has already been done, to-gether with an analysis of the type and distribution ofthe rocks and the problems in studying them. The firstcan be done in a library and/or by talking with previousworkers. The second involves areal studies of the surfaceusing remote sensing (e.g. satellite and aerial photo-graphs, although personally I like paragliding), and byactual reconnaissance on foot or by some form of trans-port; and studies of the subsurface with geophysics (e.g.seismic profiles) and boreholes. During this work, anyproblems of access and exposure should become appar-ent. This step overlaps and can help in planning the second step.2 The description of local sections involves meas-uring STRATA, describing their attributes (including com-position, texture, structure, and fossil content), andworking out the processes that formed the sedimentsand the succession of depositional environments pres-ent in the sections. For this you need to know the basicprinciples of sedimentology and ecology.3 The correlation of local sections in space andtime involves the physical tracing and mapping of rockunits, their relative dating by means of fossils and othermethods, and dating by means of RADIOMETRIC DATING orcross-correlation with standard time-scales. You need toknow how environments and organisms vary and differ

in a variety of sedimentary basins; how to plot thesevariations on maps and diagrams (and the advantagesand limitations of different methods); how relative timeunits are constructed; and how radiometric and othermethods of dating are done.4 The reconstruction of sedimentary basin history involves synthesis, often on maps and cross-sections, of trends in rock type, petrology, facies, thick-ness, and so on.

The three steps following the initial overview are mar-vellously summarized by Aigner (1985) as stratinomicanalysis (giving the depositional dynamics of the sedi-ments), facies analysis (showing the lateral and verticalvariability or facies dynamics of the sediments), andBASIN ANALYSIS (explaining how the basin evolved duringsedimentation) (Fig. 1.8).

Phase 3: Interpreting geologic history

Interpreting geologic history involves evaluating the effects of controlling processes such as tectonics, sea-level changes, climate, and biology (the effect of organ-isms) on sedimentary basin history. It requires theability to synthesize large amounts of data from manyfields. On the grand scale it involves the correlation ofhistories of individual basins and intervening areas togive a worldwide picture of the development of a planet,and requires wider consideration of the stratigraphicpeculiarities and problems of different time periods onthese planets.

Each of these phases depends on the competence ofthe preceding phase. Thus, poorly described local sec-tions inevitably result in poor stratigraphy, poor envi-ronmental reconstructions, poor correlations, andlousy reconstructions.

Introduction 9

IBasics

“Only knowledge of facies relationships drawn from study of modern environmentscan save us from the barren cataloging of rock and fossil sequences that sometimespasses for stratigraphy.”

Middleton (1973)

Examining Ordovician quartzites, Jebel Uweinat, NW Sudan.

The first part of this book is thus a necessary outline ofthe physical and biological processes that form, trans-port, and deposit sediments, and the rock and fossil evidence required to reconstruct them. The propertiesmeasured in sediments and sedimentary rocks shouldbe those most useful in determining how they formed.You must understand these processes, and the featuresthey produce, in order to realize the basis for classifica-tion and to be able to recognize environments. For thisreason, the discussion of the way sediments form pre-cedes their classification. Good introductions to thephysical and biological processes that form soils and sediments are Fitzpatrick (1980), Knapp (1979), Taylorand Eggleton (2001), and Weyman and Weyman(1977).

This book uses the metric system. Appendix 1 is a metric/imperial conversion table. Appendix 2 is a legendfor symbols used in most figures. Appendix 3 is a geologictime-scale.

The first phase of stratigraphy, the study of actual strata,and the processes that form, transport, deposit, andmodify strata, is an attempt to infer how ancient strataformed. First, you must understand how physical,chemical, and biological processes produce the varietyof sediments found in modern environments. This requires a broad knowledge of sedimentology.

Second, you must be able to describe and identify sedi-mentary rocks accurately with criteria useful in work-ing out what processes formed them and how they havebeen modified after deposition. This requires knowledgeof various systems of classification and their basis, together with their advantages and defects.

Third, you must be able to recognize major environ-mental complexes from the rather limited clues still leftin ancient rocks. This requires a knowledge of how vari-ous processes combine to define a specific environment.

Only then can you start defining and mapping strati-graphic units. Unfortunately, only too often, descriptionand mapping are done first, before the student has muchidea about how the sediments formed.

Weathering

An important consequence of physical weathering isthat it increases the surface area available for chemicalbreakdown.

Chemical weathering is more complicated. Agentssuch as water, carbon dioxide, oxygen hydrogen ions,bacteria, humic acids, etc., break down minerals intoresidual frameworks, and ions in solution. Even the mostresistant minerals can eventually be decomposed chem-ically, given suitable conditions over a long enough time.The products of weathering are1 unaltered primary minerals and rock fragments;2 new secondary minerals;3 ions in solution.For example, Table 2.1 shows the breakdown of a rock(granite) and a mineral (orthoclase).

Mineral stability at the earth’s surface is the reverse ofBowen’s reaction series for the crystallization of igneousrocks (Fig. 2.1). Minerals formed at high temperaturesand held together by mainly ionic bonds, can be ionized,hydrolyzed, etc., and chemically decomposed muchmore readily than minerals formed at lower tempera-tures and held together by mainly covalent bonds.

2.1 Types of weathering2.2 Rates of weathering2.3 Soil formation2.4 Weathering and soil formation under water

Weathering (alteration in place) determines the natureof the sediment at the source. It begins with the physicaland chemical breakdown of materials exposed at ornear the surface (both on land and under water), and continues during transportation, deposition, and the migration of pore fluids. At the surface, PHYSICAL WEATHERING and CHEMICAL WEATHERING proceed at different rates depending on rock type, climate, andslope.

2.1 Types of weathering

Physical weathering is simply the breaking down ofmaterial into smaller pieces. Softer rocks and mineralscan be worn into small pieces faster than harder miner-als, while fractured rocks and minerals with cleavagesbreak into smaller pieces faster than massive rocks andminerals. Talc (a soft sheet silicate with good cleavages)rapidly breaks down into powder; quartz (a hard frame-work silicate with no cleavage) survives to form themain mineral of sands.

2

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Table 2.1 Chemical breakdown of granite and orthoclase.

Primary constituents Weathering productsMinerals

Colloids Secondary minerals Primary persisting Soluble ions

GraniteAlkali feldspar Si, Al Clays Na, KQuartz Si QuartzMica Si, Al Clays some mica CaFerromagnesium minerals Si, Al Clays, Fe oxides Fe, Mg

OrthoclaseKAlSi3O8+2H++H2O=2K++Al2Si2O5(OH)4+SiO2(orthoclase) (kaolinite) (colloidal silica + quartz)

The formula of kaolinite could be written as Al2O3.2SiO2.2H2O to emphasize the presence of water.

Fig. 2.1 Bowen’s reaction series andtemperature of mineral formation.

Chemical reactants vary greatly in their effects de-pending on the atmosphere, hydrosphere, and rocktypes. The main reactions involve water in some way: direct solution, hydrolysis, hydration, oxidation, and reduction.

Direct solution of primary minerals without chem-ical change is rare, although silica phytoliths often sim-ply dissolve in groundwater. Usually solution removesions produced by other processes.

In hydrolysis, water reacts with silicate minerals toproduce usually clay minerals, ions, and some soluble

silica; for example, the breakdown of orthoclase shownin Table 2.1. Less complex, more soluble minerals pro-duce ions only; for example, the breakdown of calcite:

(2.1)

In carbonation, which is the opposite of hydrolysis,carbon dioxide ions are added to the minerals. The reversible reaction (2.1) goes to the left. Carbonationcauses precipitation of carbonate and cementation of

CaCO H O CO Ca HCOcalcite (carbonic

acid)

2(calciumcation)

(bicarbonateanion)

3 22

32( )

+ -+ + = +

soil horizons, and often occurs during evaporation orphotosynthesis.

In hydration, minerals absorb water, expand, andfall apart.

In oxidation, oxygen ions (usually dissolved inwater) are added to the mineral (or hydrogen ions are re-moved). The oxidation process often transforms solubleminerals into insoluble minerals; for example, the oxida-tion of ferrous to ferric iron, which creates the brownand red colors in soils.

In reduction, hydrogen ions are added to the miner-als (or oxygen ions are removed). Ferric oxides may be re-duced to ferrous oxides, changing soil colors to blue orgreen; and then be removed in solution, bleaching thesoil horizon to a gray color. Oxidation and reduction ofiron compounds are responsible for many of the colorsof sedimentary rocks.

2.2 Rates of weathering

Rates of weathering depend on a host of factors, such asrock type and mineral stability at the earth’s surface, climate and slope, and on chemical reactants present inthe atmosphere and hydrosphere. The proportion ofphysical to chemical weathering and their rates varydue mainly to rock type, temperature and availability ofwater.

Rock types control weathering by their physical andchemical reaction to climate. Igneous rocks tend to formresistant masses in cold and dry climates where physicalbreakdown is faster than chemical breakdown. Meta-morphic rocks vary depending on their structure andmineral stability. Sedimentary rocks also vary. Massivelimestones may form high jagged mountains in high latitudes where the physical breakdown of adjacent layered sandstones and shales is faster. However, similar limestones may form valleys in wetter andwarmer lower latitudes where chemical solution is more important.

Temperature controls weathering because the rateof chemical reactions doubles with every 10 °C rise. So,chemical weathering is faster in hot than in cold areas.Water is required for most chemical reactions, so chem-ical weathering is faster in wet than in dry areas.

As a gross oversimplification we can distinguish threeextreme climatic environments on land that control thetype and rates of weathering: warm and wet areas;warm and dry areas; and cool and mostly wet areas (Fig. 2.2).

Warm and wet areas, with high to moderate rainfall,have running water present at all times. These areas aredominated by chemical weathering, with thick, matureleached soils and lush vegetation. The high tempera-tures and throughflow of water promote rapid chemicalreactions and removal of soluble products. In suchareas, massive unstable rocks such as limestone formlowlands, and hills are low and rolling and covered invegetation (Fig. 2.3a). Even rapidly rising mountains,such as those of the eastern Himalaya, may showrounded profiles covered in vegetation. Modern exam-ples are lowland tropical areas such as the AmazonBasin and much of Southeast Asia.

Warm arid and semi-arid areas, with low and spo-radic rainfall, have running water present only intermit-tently. These areas are characterized by mixed physicaland chemical weathering, with alternation between so-lution and re-precipitation during wet and dry periods.Most also have both diurnal (dew) and seasonal cycles ofwetting and drying, which can lead to the formation of calcretes and other chemical precipitates in subma-ture soils. The relatively high temperatures allow rapidchemical weathering at those places and on those occa-sions when water is present. Massive unstable rockssuch as limestone form hills in such areas, since phy-sical weathering can break down fractured rocks morerapidly (Fig. 2.3b). Modern examples are the deserts and semi-deserts of the American Southwest, northernAfrica (the Sahara), and Central Asia.

Cool glacial and periglacial environments, with variable rainfall, have running water only intermittent-ly present due to the low temperatures. These areas are dominated by physical weathering, includingfreeze–thaw cycles. Even when water is flowing, the low temperatures inhibit chemical reactions. Massiveunstable rocks can form mountains in such areas, with fractured intervening rocks forming valleys (Fig.2.3c). Modern examples are the mountains of theAmerican Northwest, northern Eurasia, Tibet, andAntarctica.

If weathered material is not immediately removed byone of the agents of transportation, then a weatheredmantle or soil accumulates. It is rather unusual for particles to be weathered, transported, and finally deposited without at least spending some time in a soilprofile. So understanding soil processes and formation is important in interpreting sediments and sedimentaryrocks.

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2.3 Soil formation

Where slopes are not too steep, soils accumulate. The soilformed is controlled by the initial parent compositionand the nature and rate of weathering. These, in turn,depend on the original rock type, temperature, availabil-ity of water (summarized by climate), and time. Soils areimportant in stratigraphy: they modify and change thematerials produced by simple weathering; they producenew particles and modify groundwaters; they can pro-vide information of sedimentary processes and envi-ronments; and they can be important stratigraphichorizons in their own right (Taylor & Eggleton 2001).

Soil formation involves three processes: (i) the pro-duction of inorganic soil material by the weathering of the parent bedrock or sediment; (ii) the incorporation

of organic matter formed by the decomposition of plantand animal tissues; and (iii) the reorganization of thesecomponents by aggregation and translocation to formSOIL HORIZONS. These processes produce a bewildering variety of soils, which nevertheless usually show (atleast roughly) the well-known threefold division into Ahorizons of organic accumulation and leaching; B hori-zons of accumulation; and C horizons of partially al-tered bedrock (Fig. 2.4).

Weathering is often the only one of these threeprocesses that is covered in stratigraphy texts. However,equally important are the processes that transform or-ganic matter and cause reorganization within soils.These processes not only produce new minerals and ag-gregates but can even produce new rocks and particlesthat will undergo further weathering and transport. The

Fig. 2.2 Regions of differing climatic regimes (after Weyman & Weyman 1977, fig. 2). (Reproduced with permission ofHarperCollins.)

Weathering 17

(a) (c)

Fig. 2.3 Examples of the main regimes: (a) warm and wet; (b) warm and dry; and (c) cool and wet.

(b)

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rocks and particles produced by soil processes may easi-ly be confused with those produced in other ways.

Many plants produce sand-sized colloidal silica parti-cles called phytoliths (which are indistinguishable fromthe small chert particles produced during diagenesis oflimestones). The massive silica beds (silcretes) that canprecipitate in stable semi-arid soils are difficult to dis-tinguish from cherts formed by the replacement oflimestones (Fig. 2.5).

The nodular and massive carbonates (calcretes) thatform in semi-arid soils are difficult to distinguish frommarine limestones. Thus, some supposed Cretaceousmarine limestones of Central India are now reinterpret-ed as thick soil calcrete horizons, requiring majorchanges in Indian Cretaceous stratigraphy and paleo-geography (Brookfield & Sahni 1987). Furthermore, silcretes and calcretes may then be weathered and trans-ported; and such particles may be difficult to identify.

Unless you can recognize that these particles were pro-duced in soils, you may infer a non-existent limestoneand/or chert source.

Organic matter is decomposed by soil micro-organ-isms, dominantly bacteria and fungi, but sometimes inthe guts of larger organisms such as earthworms, producing humus. Mixing of this partially decomposedorganic matter with altered rock and minerals forms soilA horizons. Decomposition is favored by alkaline andneutral conditions, and both larger organic and bacteri-al activity drop sharply with increasing acidity. In neu-tral or alkaline conditions, such as under grassland, thecomplete breakdown of organic matter is accompaniedby reworking of the upper soil layers, producing a mullhumus of almost completely decomposed organic mate-rial in a well-aerated spongy fabric of clay minerals withsilica phytoliths, held together with clay minerals,sesquioxides, and polysaccharides. With increasingly

(a) (b)

Fig. 2.4 Highly weathered tropical ferrasol: (a) an example of a highly chemically weathered tropical ferrasol; (b) typical profile of atropical ferrasol (from Fitzpatrick 1980, fig. 1.2). (Reproduced with permission of Pearson Education Limited.)