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Sedimentology and Stratigraphy


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Sedimentology and Stratigraphy

Second Edition

Gary Nichols

A John Wiley & Sons, Ltd., Publication


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Nichols, Gary.

Sedimentology and stratigraphy / Gary Nichols. 2nd ed.

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ISBN 978-1-4051-3592-4 (pbk. : alk. paper) ISBN 978-1-4051-9379-5 (hardcover : alk. paper) 1. Sedimentation and

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Contents

Preface, ix

Acknowledgements, xi

1 INTRODUCTION: SEDIMENTOLOGYAND STRATIGRAPHY, 11.1 Sedimentary processes, 11.2 Sedimentary environments and facies, 21.3 The spectrum of environments and facies, 31.4 Stratigraphy, 31.5 The structure of this book, 4Further reading, 4

2 TERRIGENOUSCLASTIC SEDIMENTS: GRAVEL,SAND AND MUD, 52.1 Classification of sediments and sedimentary

rocks, 52.2 Gravel and conglomerate, 72.3 Sand and sandstone, 102.4 Clay, silt and mudrock, 212.5 Textures and analysis of terrigenous clastic

sedimentary rocks, 232.6 Terrigenous clastic sediments: summary, 27Further reading, 27

3 BIOGENIC, CHEMICAL ANDVOLCANOGENIC SEDIMENTS, 283.1 Limestone, 283.2 Evaporite minerals, 363.3 Cherts, 383.4 Sedimentary phosphates, 383.5 Sedimentary ironstone, 383.6 Carbonaceous (organic) deposits, 403.7 Volcaniclastic sedimentary rocks, 41Further reading, 43

4 PROCESSES OF TRANSPORTAND SEDIMENTARY STRUCTURES, 444.1 Transport media, 444.2 The behaviour of fluids and particles

in fluids, 454.3 Flows, sediment and bedforms, 504.4 Waves, 584.5 Mass flows, 604.6 Mudcracks, 644.7 Erosional sedimentary structures, 654.8 Terminology for sedimentary structures

and beds, 664.9 Sedimentary structures and sedimentary

environments, 68Further reading, 68

5 FIELD SEDIMENTOLOGY, FACIESAND ENVIRONMENTS, 695.1 Field sedimentology, 695.2 Graphic sedimentary logs, 705.3 Palaeocurrents, 755.4 Collection of rock samples, 785.5 Description of core, 795.6 Interpreting past depositional

environments, 805.7 Reconstructing palaeoenvironments in space

and time, 845.8 Summary: facies and environments, 85Further reading, 86

6 CONTINENTS: SOURCES OFSEDIMENT, 876.1 From source of sediment to formation

of strata, 876.2 Mountain-building processes, 886.3 Global climate, 88


6.4 Weathering processes, 896.5 Erosion and transport, 936.6 Denudation and landscape evolution, 956.7 Tectonics and denudation, 996.8 Measuring rates of denudation, 1006.9 Denudation and sediment supply: summary, 101Further reading, 101

7 GLACIAL ENVIRONMENTS, 1027.1 Distribution of glacial environments, 1027.2 Glacial ice, 1047.3 Glaciers, 1057.4 Continental glacial deposition, 1077.5 Marine glacial environments, 1107.6 Distribution of glacial deposits, 1127.7 Ice, climate and tectonics, 1127.8 Summary of glacial environments, 113Further reading, 113

8 AEOLIAN ENVIRONMENTS, 1148.1 Aeolian transport, 1148.2 Deserts and ergs, 1168.3 Characteristics of wind-blown particles, 1168.4 Aeolian bedforms, 1188.5 Desert environments, 1208.6 Aeolian deposits outside deserts, 1268.7 Summary, 127Further reading, 127

9 RIVERS AND ALLUVIAL FANS, 1299.1 Fluvial and alluvial systems, 1299.2 River forms, 1319.3 Floodplain deposition, 1399.4 Patterns in fluvial deposits, 1399.5 Alluvial fans, 1419.6 Fossils in fluvial and alluvial

environments, 1469.7 Soils and palaeosols, 1469.8 Fluvial and alluvial fan deposition:

summary, 149Further reading, 149

10 LAKES, 15110.1 Lakes and lacustrine environments, 15110.2 Freshwater lakes, 15310.3 Saline lakes, 15710.4 Ephemeral lakes, 15810.5 Controls on lacustrine deposition, 16010.6 Life in lakes and fossils in lacustrine

deposits, 160

10.7 Recognition of lacustrine facies, 161Further reading, 161

11 THE MARINE REALM: MORPHOLOGYAND PROCESSES, 16311.1 Divisions of the marine realm, 16311.2 Tides, 16511.3 Wave and storm processes, 16911.4 Thermo-haline and geostrophic

currents, 17011.5 Chemical and biochemical sedimentation

in oceans, 17011.6 Marine fossils, 17211.7 Trace fossils, 17311.8 Marine environments: summary, 178Further reading, 178

12 DELTAS, 17912.1 River mouths, deltas and estuaries, 17912.2 Types of delta, 17912.3 Delta environments and successions, 18212.4 Variations in delta morphology

and facies, 18412.5 Deltaic cycles and stratigraphy, 19312.6 Syndepositional deformation

in deltas, 19512.7 Recognition of deltaic deposits, 196Further reading, 198

13 CLASTIC COASTS ANDESTUARIES, 19913.1 Coasts, 19913.2 Beaches, 20113.3 Barrier and lagoon systems, 20313.4 Tides and coastal systems, 20613.5 Coastal successions, 20713.6 Estuaries, 20713.7 Fossils in coastal and estuarine

environments, 212Further reading, 214

14 SHALLOW SANDY SEAS, 21514.1 Shallow marine environments of terrigenous

clastic deposition, 21514.2 Storm-dominated shallow clastic seas, 21714.3 Tide-dominated clastic shallow seas, 22014.4 Responses to change in sea level, 22214.5 Criteria for the recognition of sandy

shallow-marine sediments, 223Further reading, 224

vi Contents


15 SHALLOW MARINECARBONATE AND EVAPORITEENVIRONMENTS, 22515.1 Carbonate and evaporite depositional

environments, 22515.2 Coastal carbonate and evaporite

environments, 22815.3 Shallow marine carbonate environments, 23315.4 Types of carbonate platform, 23715.5 Marine evaporites, 24215.6 Mixed carbonateclastic environments, 245Further reading, 245

16 DEEP MARINE ENVIRONMENTS, 24716.1 Ocean basins, 24716.2 Submarine fans, 25016.3 Slope aprons, 25616.4 Contourites, 25716.5 Oceanic sediments, 25816.6 Fossils in deep ocean sediments, 26116.7 Recognition of deep ocean deposits:


17 VOLCANIC ROCKS ANDSEDIMENTS, 26317.1 Volcanic rocks and sediment, 26317.2 Transport and deposition of volcaniclastic

material, 26517.3 Eruption styles, 26817.4 Facies associations in volcanic

successions, 26917.5 Volcanic material in other environments, 27117.6 Volcanic rocks in Earth history, 27117.7 Recognition of volcanic deposits:


18 POST-DEPOSITIONAL STRUCTURESAND DIAGENESIS, 27418.1 Post-depositional modification of

sedimentary layers, 27418.2 Diagenetic processes, 27918.3 Clastic diagenesis, 28518.4 Carbonate diagenesis, 28718.5 Post-depositional changes to

evaporites, 29118.6 Diagenesis of volcaniclastic sediments, 29118.7 Formation of coal, oil and gas, 292Further reading, 296

19 STRATIGRAPHY: CONCEPTSAND LITHOSTRATIGRAPHY, 29719.1 Geological time, 29719.2 Stratigraphic units, 30119.3 Lithostratigraphy, 30219.4 Applications of lithostratigraphy, 306Further reading, 310

20 BIOSTRATIGRAPHY, 31120.1 Fossils and stratigraphy, 31120.2 Classification of organisms, 31220.3 Evolutionary trends, 31420.4 Biozones and zone fossils, 31520.5 Taxa used in biostratigraphy, 31820.6 Biostratigraphic correlation, 32120.7 Biostratigraphy in relation to other

stratigraphic techniques, 322Further reading, 323

21 DATING AND CORRELATIONTECHNIQUES, 32421.1 Dating and correlation techniques, 32421.2 Radiometric dating, 32521.3 Other isotopic and chemical techniques, 32921.4 Magnetostratigraphy, 33021.5 Dating in the Quaternary, 332Further reading, 334

22 SUBSURFACE STRATIGRAPHYAND SEDIMENTOLOGY, 33522.1 Introduction to subsurface stratigraphy and

sedimentology, 33522.2 Seismic reflection data, 33622.3 Borehole stratigraphy and sedimentology, 34122.4 Geophysical logging, 34322.5 Subsurface facies and basin analysis, 348Further reading, 348

23 SEQUENCE STRATIGRAPHY ANDSEA-LEVEL CHANGES, 34923.1 Sea-level changes and sedimentation, 34923.2 Depositional sequences and systems

tracts, 35723.3 Parasequences: components of systems

tracts, 36223.4 Carbonate sequence stratigraphy, 36523.5 Sequence stratigraphy in non-marine

basins, 36823.6 Alternative schemes in sequence

stratigraphy, 368

Contents vii


23.7 Applications of sequence stratigraphy, 36923.8 Causes of sea-level fluctuations, 37323.9 Sequence stratigraphy: summary, 380Further reading, 380

24 SEDIMENTARY BASINS, 38124.1 Controls on sediment accumulation, 38124.2 Basins related to lithospheric extension, 38424.3 Basins related to subduction, 38724.4 Basins related to crustal loading, 390

24.5 Basins related to strike-slip tectonics, 39124.6 Complex and hybrid basins, 39224.7 The record of tectonics in stratigraphy, 39324.8 Sedimentary basin analysis, 39324.9 The sedimentary record, 397Further reading, 397

References, 398

Index, 411

viii Contents


Preface

There is pleasing symmetry about the fact that thebackbone of the first edition of this book was writtenwithin the Antarctic Circle in gaps between fieldworkwith the British Antarctic Survey, while the bulk ofthis second edition has been written from within theArctic Circle during my tenure of a 2-year position asProfessor of Geology at the University Centre on Sval-bard. It is not that I have any great affinity for thepolar regions, it just seems that I have almost literallygone to the ends of the Earth to find the peace andquiet that I need to write a book. Between mysojourns in these polar regions 10 years have passed,and both sedimentology and stratigraphy have movedon enough for a thorough update of the material to berequired. Just as importantly, technology has movedon, and I can provide a much more satisfying range ofillustrative material in digital form on a CD includedwith the text. Geology is a wonderfully visual science,and it is best appreciated at first hand in the field, butphotographs of examples can also aid understanding.I am an unashamed geo-tourist, always looking foryet another example of a geological phenomenon,whether on fieldwork or on holiday. The photographsused in this book and accompanying CD-ROM weretaken over a period of 20 years and include examplesfrom many corners of the globe.

AN UNDERGRADUATE TEXT

This book has been written for students who arestudying geology at university and it is intended toprovide them with an introduction to sedimentologyand stratigraphy. It is hoped that the text is accessibleto those who are completely new to the subject andthat it will also provide a background in concepts andterminology used in more advanced work. Theapproach is largely descriptive and is intended to

complement the more numerical treatment of thetopics provided by books such as Leeder (1999). Sedi-mentary processes are covered in more detail in textssuch as Allen (1997) and a much more detailed anal-ysis of sedimentary environments and facies is pro-vided by Reading (1996). For a more comprehensivetreatment of some aspects of stratigraphy books suchas Coe (2003) are recommended.

DEFINITIONS OF TERMS

This book does not include a glossary, but instead it isintended that terminology is explained and, wherenecessary, defined in context within the text. Thefirst occurrence of a technical term is usually cast inbold italics, and it is at this point that an explanationis provided. To find the meaning of a term, the readershould consult the index and go to its first listedoccurrence. There are differences of opinion aboutsome terminology, but it is beyond the scope of thistext to provide discussion of the issues: in most casesthe most broadly accepted view has been adopted; inothers simplicity and consistency within the bookhave taken precedence.

REFERENCES

The references chosen are not intended to be compre-hensive for a topic, but merely a selection of a fewrelatively recent publications that can be used as astarting point for further information. Older sourcesare cited where these provide important primaryaccounts of a topic. At the end of each chapter thereis a list of suggested further reading materials: theseare mainly recent textbooks, compilations of papersin special publications and key review papers and


are intended as a starting point for further generalinformation about the topics covered in the chapter.

CROSS-REFERENCINGAND THE CD-ROM

To reduce duplication of material, there is quite exten-sive cross-referencing within the text, indicated by thesection number italicised in parentheses, for example(2.3.4). Relevant figures are indicated by, for exam-ple, Fig. 2.34. The accompanying CD-ROM containsmore illustrative material, principally photographs,

than is provided within the book: specific referenceto this material has not been made in the text, as thebook is intended to be stand-alone. In contrast, theCD-ROM is intended for use in conjunction withthe book, and so the diagrams and photographs onit are not fully captioned or explained. An index onthe CD-ROM contains information about each slide.All photographs used in the book and CD-ROM weretaken by the author and all diagrams drafted by theauthor. A list of the locations of each of the photo-graphs in the text is provided in an appendix on theCD-ROM.

x Preface


Acknowledgements

Thanks to Phil Chapman for casually suggesting tothe teenage younger brother of his friend RogerNichols that he might like to study geology at Alevel: this turned out to be the best piece of adviceI ever received. The late Doug Shearman was aninspirational lecturer in sedimentology when I was astudent at Imperial College, London, and he unwit-tingly made me committed to the idea of being anacademic sedimentologist. (The greatest professionalcompliment ever paid to me was by Rick Sibson, aformer colleague of Doug, who, after I had given apresentation at a conference nearly 20 years later,

said there were shades of Doug Shearman in thetalk you gave today.) Peter Friend provided under-stated guidance to me as PhD project supervisor:I could not have a better academic pedigree than asa former research student of Peter. It has been a greatpleasure to work with many different people in manydifferent countries, all of whom have in some wayprovided me with some inspiration. Most importantly,they have made my whole experience of 25 years ingeology a lot of fun. Thanks also to Davina for justabout everything else.

1

Introduction: Sedimentologyand Stratigraphy

Sedimentology is the study of the processes of formation, transport and deposition ofmaterial that accumulates as sediment in continental and marine environments andeventually forms sedimentary rocks. Stratigraphy is the study of rocks to determine theorder and timing of events in Earth history: it provides the time frame that allows us tointerpret sedimentary rocks in terms of dynamic evolving environments. The strati-graphic record of sedimentary rocks is the fundamental database for understandingthe evolution of life, plate tectonics through time and global climate change.

1.1 SEDIMENTARY PROCESSES

The concept of interpreting rocks in terms of modernprocesses dates back to the 18th and 19th centuries(the present is the key to the past). Sedimentologyhas existed as a distinct branch of the geologicalsciences for only a few decades. It developed as theobservational elements of physical stratigraphybecame more quantitative and the layers of stratawere considered in terms of the physical, chemicaland biological processes that formed them.The nature of sedimentary material is very varied in

origin, size, shape and composition. Particles such asgrains and pebbles may be derived from the erosion ofolder rocks or directly ejected from volcanoes. Organ-isms form a very important source of material, rangingfrom microbial filaments encrusted with calcium car-bonate towhole or broken shells, coral reefs, bones andplant debris. Direct precipitation of minerals from solu-

tion in water also contributes to sediments in somesituations.Formation of a body of sediment involves either the

transport of particles to the site of deposition by grav-ity, water, air, ice or mass flows or the chemical orbiological growth of the material in place. Accumula-tion of sediments in place is largely influenced by thechemistry, temperature and biological character of thesetting. The processes of transport and deposition canbe determined by looking at individual layers of sedi-ment. The size, shape and distribution of particles allprovide clues to the way in which the material wascarried and deposited. Sedimentary structures such asripples can be seen in sedimentary rocks and can becompared to ripples forming today, either in naturalenvironments or in a laboratory tank.Assuming that the laws that govern physical and

chemical processes have not changed through time,detailed measurements of sedimentary rocks can be

used to make estimates (to varying degrees of accu-racy) of the physical, chemical and biological condi-tions that existed at the time of sedimentation. Theseconditions may include the salinity, depth and flowvelocity in lake or seawater, the strength and directionof the wind in a desert and the tidal range in a shallowmarine setting.

1.2 SEDIMENTARY ENVIRONMENTSAND FACIES

The environment at any point on the land or under thesea can be characterised by the physical and chemicalprocesses that are active there and the organisms thatlive under those conditions at that time. As an exam-ple, a fluvial (river) environment includes a channelconfining the flow of fresh water that carries anddeposits gravelly or sandy material on bars in thechannel (Fig. 1.1). When the river floods, waterspreads relatively fine sediment over the floodplainwhere it is deposited in thin layers. Soils form andvegetation grows on the floodplain area. In a succes-sion of sedimentary rocks (Fig. 1.2) the channel maybe represented by a lens of sandstone or conglomeratethat shows internal structures formed by deposition onthe channel bars. The floodplain setting will be repre-sented by thinly bedded mudrock and sandstone withroots and other evidence of soil formation.In the description of sedimentary rocks in terms of

depositional environments, the term facies is oftenused. A rock facies is a body of rock with specifiedcharacteristics that reflect the conditions underwhich it was formed (Reading & Levell 1996).Describing the facies of a body of sediment involvesdocumenting all the characteristics of its lithology,texture, sedimentary structures and fossil contentthat can aid in determining the processes of forma-tion. By recognising associations of facies it is possibleto establish the combinations of processes that weredominant; the characteristics of a depositional envi-ronment are determined by the processes that arepresent, and hence there is a link between faciesassociations and environments of deposition. Thelens of sandstone in Fig. 1.2 may be shown to be ariver channel if the floodplain deposits are found asso-ciated with it. However, recognition of a channel formon its own is not a sufficient basis to determine thedepositional environment because channels filledwith sand exist in other settings, including deltas,

Fig. 1.1 A modern depositional environment: a sandyriver channel and vegetated floodplain.

Fig. 1.2 Sedimentary rocks interpreted as the deposits of ariver channel (the lens of sandstones in the centre right of the

view) scoured into mudstone deposited on a floodplain (the

darker, thinly bedded strata below and to the side of the

sandstone lens).

2 Introduction: Sedimentology and Stratigraphy

tidal environments and the deep sea floor: it is theassociation of different processes that provides the fullpicture of a depositional environment.

1.3 THE SPECTRUM OFENVIRONMENTS AND FACIES

Every depositional environment has a unique combi-nation of processes, and the products of these pro-cesses, the sedimentary rocks, will be a similarlyunique assemblage. For convenience of description andinterpretation, depositional environments are classi-fied as, for example, a delta, an estuary or a shoreline,and subcategories of each are established, such as wave-dominated, tide-dominated and river-dominated del-tas. This approach is in general use by sedimentarygeologists and is followed in this book. It is, however,important to recognise that these environments ofdeposition are convenient categories or pigeonholes,and that the description of them tends to be of typicalexamples. The reality is that every delta, for example, isdifferent from its neighbour in space or time, that everydeltaic deposit will also be unique, and although wecategorise deltas into a number of types, our deposit islikely to fall somewhere in between these pigeon-holes. Sometimes it may not even be possible to con-clusively distinguish between the deposits of a deltaand an estuary, especially if the data set is incomplete,which it inevitably is when dealing with events of thepast. However, by objectively considering each bed interms of physical, chemical and biological processes, itis always possible to provide some indication of whereand how a sedimentary rock was formed.

1.4 STRATIGRAPHY

Use of the term stratigraphy dates back to dOrbingyin 1852, but the concept of layers of rocks, or strata,representing a sequence of events in the past is mucholder. In 1667 Steno developed the principle of super-position: in a sequence of layered rocks, any layer isolder than the layer next above it. Stratigraphy can beconsidered as the relationship between rocks and timeand the stratigrapher is concerned with the observa-tion, description and interpretation of direct and tan-gible evidence in rocks to determine the history of theEarth. We all recognise that our planet is a dynamicplace, where plate tectonics creates mountains and

oceans and where changes in the atmosphere affectthe climate, perhaps even on a human time scale. Tounderstand how these global systems work, we need arecord of their past behaviour to analyse, and this isprovided by the study of stratigraphy.Stratigraphy provides the temporal framework for

geological sciences. The relative ages of rocks, andhence the events that are recorded in those rocks, canbe determined by simple stratigraphic relationships(younger rocks generally lie on top of older, as Stenorecognised), the fossils that are preserved in strata andby measurements of processes such as the radioactivedecay of elements that allowus to date some rock units.At one level, stratigraphy is about establishing anomenclature for rock units of all ages and correlatingthem all over the world, but at another level it is aboutfinding the evidence for climate change in the past orthe movements of tectonic plates. One of the powerfultools we have for predicting future climate change isthe record in the rock strata of local and global changesover periods of thousands to millions of years. Further-more our understanding of evolutionary processes is inpart derived from the study of fossils found in rocks ofdifferent ages that tell us about how forms of life havechanged through time. Other aspects of stratigraphyprovide the tools for finding new resources: for exam-ple, sequence stratigraphy is a predictive technique,widely used in the hydrocarbon industry, that can beused to help to find new reserves of oil and gas.The combination of sedimentology and stratigraphy

allows us to build up pictures of the Earths surface atdifferent times in different places and relate them toeach other. The character of the sedimentary rocksdeposited might, for example, indicate that at onetime a certain area was an arid landscape, with desertdunes andwithwashes of gravel coming from anearbymountain range. In that same place, but at a later time,conditions allowed the formation of coral reefs in ashallow sea far away from any landmass, and we canfind the record of this change by interpreting the rocksin terms of their processes and environments of deposi-tion. Furthermore, wemight establish that at the sametime as there were shallow tropical seas in one place,there lay a deep ocean a few tens of kilometres awaywhere fine sediment was deposited by ocean currents.We can thus build up pictures of the palaeogeogra-phy, the appearance of an area during some time inthe past, and establish changes in palaeogeographythrough Earth history. To complete the picture, thedistribution of different environments and their

Stratigraphy 3

changes through time can be related to plate tec-tonics, because mountain building provides thesource for much of the sediment, and plate move-ments also create the sedimentary basins where sedi-ment accumulates.

1.5 THE STRUCTURE OF THIS BOOK

Sedimentology and stratigraphy can be consideredtogether as a continuum of processes and products,both in space and time. Sedimentology is concernedprimarily with the formation of sedimentary rocks butas soon as these beds of rock are looked at in terms oftheir temporal and spatial relationships the study hasbecome stratigraphic. Similarly if the stratigrapherwishes to interpret layers of rock in terms of environ-ments of the past the research is sedimentological. It istherefore appropriate to consider sedimentology andstratigraphy together at an introductory level.The starting point taken in this book is the smallest

elements, the particles of sand, pebbles, clay minerals,pieces of shell, algal filaments, chemical precipitatesand other constituents that make up sediments (Chap-ters 2 and 3). An introduction to the petrographicanalysis of sedimentary materials in hand specimenand under the microscope is included in these chap-ters. In Chapter 4 the processes of sediment transportand deposition are considered, followed by a section onthe methodology of recording and analysing sedimen-tary data in the field in Chapter 5. Weathering anderosion is considered in Chapter 6 as an introduction tothe processes which generate the clastic material thatis deposited in many sedimentary environments. Thefollowing chapters (7 to 17) deal largely with differentdepositional environments, outlining the physical,chemical and biological processes that are active, thecharacteristics of the products of these processes andhow they may be recognised in sedimentary rocks.Continental environments are covered in Chapters 8

to 10, followed by marine environments in Chapters12 to 16 the general theme being to start at the top,with the mountains, and end up in the deep oceans.Exceptions to this pattern are Chapter 7 on glacialenvironments and Chapter 17 on volcanic processesand products. Post-depositional processes, includinglithification and the formation of hydrocarbons, areconsidered in Chapter 18. Chapters 19 to 23 are ondifferent aspects of stratigraphy and are intended toprovide an introduction to the principles of stratigraphicanalysis using techniques such as lithostratigraphy,biostratigraphy and sequence stratigraphic correlation.The final chapters in the book provide a brief introduc-tion to sedimentary basins and the large-scale tectonicand climatic controls on the sedimentary record.Sedimentology and stratigraphy cannot be consid-

ered in isolation from other aspects of geology, and inparticular, plate tectonics, petrology, palaeontologyand geomorphology are complementary topics. Refer-ence is made to these subjects in the text, but only abasic knowledge of these topics is assumed.

FURTHER READING

The following texts provide a general background to geology.

Chernicoff, S. & Whitney, D. (2007) Geology: an Introduction

to Physical Geology (4th edition). Pearson/Prentice Hall,

New Jersey.

Grotzinger, J., Jordan, T.H., Press, F. & Siever, R. (2007)

Understanding Earth (5th edition). Freeman and Co., New

York.

Lutgens, F.K. & Tarbuck, E.J. (2006) Essentials of Geology

(9th edition). Pearson/Prentice Hall, New Jersey.

Smith, G.A. & Pun, A. (2006) How Does the Earth Work?

Physical Geology and the Process of Science. Pearson/Pren-

tice Hall, New Jersey.

Summerfield, M.A. (1991) Global Geomorphology: an Introduc-

tion to the Study of Landforms. Longman/Wiley, London/

New York.

4 Introduction: Sedimentology and Stratigraphy

2

Terrigenous Clastic Sediments:Gravel, Sand and Mud

Terrigenous clastic sediments and sedimentary rocks are composed of fragments thatresult from the weathering and erosion of older rocks. They are classified according to thesizes of clasts present and the composition of the material. Analysis of gravels andconglomerates can be carried out in the field and can reveal where the material camefromand how it was transported. Sands and sandstones can also be described in the field,but for a complete analysis examination under a petrographic microscope is required toreveal the composition of individual grains and their relationships to each other. The finestsediments, silt and clay, can only be fully analysed using scanning electron microscopesand X-ray diffractometers. The proportions of different clast sizes and the textures ofterrigenous clastic sediments and sedimentary rocks can provide information about thehistory of transport of the material and the environment of deposition.

2.1 CLASSIFICATION OF SEDIMENTSAND SEDIMENTARY ROCKS

A convenient division of all sedimentary rocks isshown in Fig. 2.1. Like most classification schemesof natural processes and products it includes anoma-lies (a deposit of chemically precipitated calciumcarbonate would be classified as a limestone, notan evaporite) and arbitrary divisions (the definitionof a limestone as a rock having more than 50%calcium carbonate), but it serves as a general frame-work.

Terrigenous clastic material This is material thatis made up of particles or clasts derived from

pre-existing rocks. The clasts are principally detrituseroded from bedrock and are commonly made uplargely of silicate minerals: the terms detrital sedi-ments and siliciclastic sediments are also used forthis material. Clasts range in size from clay particlesmeasured in microns, to boulders metres across.Sandstones and conglomerates make up 2025% ofthe sedimentary rocks in the stratigraphic record andmudrocks are 60% of the total.

Carbonates By definition, a limestone is any sedi-mentary rock containing over 50% calcium carbo-nate (CaCO3). In the natural environment aprincipal source of calcium carbonate is from thehard parts of organisms, mainly invertebrates such

Nichols/Sedimentology and Stratigraphy 9781405193795_4_002 Final Proof page 5 26.2.2009 8:13pm Compositor Name: ARajuNichols/Sedimentology and Stratigraphy 9781405193795_4_002 Final Proof page 5 26.2.2009 8:13pm Compositor Name: ARajuNichols/Sedimentology and Stratigraphy 9781405193795_4_002 Final Proof page 5 26.2.2009 8:13pm Compositor Name: ARaju

as molluscs. Limestones constitute 1015% of thesedimentary rocks in the stratigraphic record.

Evaporites These are deposits formed by the precipi-tation of salts out of water due to evaporation.

Volcaniclastic sediments These are the products ofvolcanic eruptions or the result of the breakdown ofvolcanic rocks.

Others Other sediments and sedimentary rocks aresedimentary ironstone, phosphate sediments, organicdeposits (coals and oil shales) and cherts (siliceoussedimentary rocks). These are volumetrically lesscommon than the above, making up about 5% ofthe stratigraphic record, but some are of considerableeconomic importance.In this chapter terrigenous clastic deposits are consid-ered: the other types of sediment and sedimentaryrock are covered in Chapter 3.

2.1.1 Terrigenous clastic sedimentsand sedimentary rocks

A distinction can be drawn between sediments(generally loose material) and sedimentary rocks

which are lithified sediment: lithification isthe process of turning into rock (18.2). Mud,silt and sand are all loose aggregates; theaddition of the suffix -stone (mudstone, siltstone,sandstone) indicates that the material has beenlithified and is now a solid rock. Coarser, loose gravelmaterial is named according to its size as granule,pebble, cobble and boulder aggregates, whichbecome lithified into conglomerate (sometimeswith the size range added as a prefix, e.g. pebbleconglomerate).A threefold division on the basis of grain size is

used as the starting point to classify and name terri-genous clastic sediments and sedimentary rocks:gravel and conglomerate consist of clasts greaterthan 2mm in diameter; sand-sized grains are between2mm and 1/16mm (63 microns) across; mud(including clay and silt) is made up of particles lessthan 63mm in diameter. There are variants on thisscheme and there are a number of ways of providingsubdivisions within these categories, but sedimen-tologists generally use the Wentworth Scale(Fig. 2.2) to define and name terrigenous clasticdeposits.

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Fig. 2.1 A classification scheme for sediments and sedimentary rocks.

6 Terrigenous Clastic Sediments: Gravel, Sand and Mud


2.1.2 The UddenWentworthgrain-size scale

Known generally as the Wentworth Scale, this is thescheme in most widespread use for the classification ofaggregates particulate matter (Udden 1914; Went-worth 1922). The divisions on the scale are made onthe basis of factors of two: for example, medium sandgrains are 0.25 to 0.5mm in diameter, coarse sandgrains are 0.5 to 1.0mm, very coarse sand 1.0 to2.0mm, etc. It is therefore a logarithmic progression,but a logarithm to the base two, as opposed to the baseten of the more common log scales. This scale hasbeen chosen because these divisions appear to reflect

the natural distribution of sedimentary particles and ina simple way it can be related to starting with a largeblock and repeatedly breaking it into two pieces.Four basic divisions are recognised:

clay (2.0mm)

The phi scale is a numerical representation of theWentworth Scale. The Greek letter f (phi) is oftenused as the unit for this scale. Using the logarithmbase two, the grain size can be denoted on the phiscale as

f log2 (grain diameter in mm)The negative is used because it is conventional torepresent grain sizes on a graph as decreasing fromleft to right (2.5.1). Using this formula, a grain diam-eter of 1mm is 0f: increasing the grain size, 2mmis 1f, 4mm is 2f, and so on; decreasing the grainsize, 0.5mm is 1f, 0.25mm is 2f, etc.

2.2 GRAVEL AND CONGLOMERATE

Clasts over 2mm in diameter are divided into gran-ules, pebbles, cobbles and boulders (Fig. 2.2). Consol-idated gravel is called conglomerate (Fig. 2.3) andwhen described will normally be named according tothe dominant clast size: if most of the clasts arebetween 64mm and 256mm in diameter the rockwould be called a cobble conglomerate. The termbreccia is commonly used for conglomerate madeup of clasts that are angular in shape (Fig. 2.4). In

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Fig. 2.3 A conglomerate composed of well-rounded pebbles.

Gravel and Conglomerate 7


some circumstances it is prudent to specify that adeposit is a sedimentary breccia to distinguish itfrom a tectonic breccia formed by the fragmentationof rock in fault zones. Mixtures of rounded and angu-lar clasts are sometimes termed breccio-conglomer-ate. Occasionally the noun rudite and the adjectiverudaceous are used: these terms are synonymouswith conglomerate and conglomeratic.

2.2.1 Composition of gravel andconglomerate

Amore complete description of the nature of a gravel orconglomerate can be provided by considering the typesof clast present. If all the clasts are of the samematerial(all of granite, for example), the conglomerate is con-sidered to bemonomict. A polymict conglomerate isone that contains clasts of many different lithologies,and sometimes the term oligomict is used wherethere are just two or three clast types present.Almost any lithologymay be foundas a clast in gravel

and conglomerate. Resistant lithologies, thosewhich are less susceptible to physical and chemicalbreakdown, have a higher chance of being preservedas a clast in a conglomerate. Factors controlling theresistance of a rock type include the minerals presentand the ease with which they are chemically or phys-ically broken down in the environment. Some sand-stones break up into sand-sized fragments wheneroded because the grains are weakly cementedtogether. The most important factor controlling thevarieties of clast found is the bedrock being eroded inthe area. Gravel will be composed entirely of lime-stone clasts if the source area is made up only of

limestone bedrock. Recognition of the variety of clastscan therefore be a means of determining the source ofa conglomeratic sedimentary rock (5.4.1).

2.2.2 Texture of conglomerate

Conglomerate beds are rarely composed entirely ofgravel-sized material. Between the granules, pebbles,cobbles and boulders, finer sand and/or mud will oftenbe present: this finer material between the large clastsis referred to as thematrix of the deposit. If there is ahigh proportion (over 20%) of matrix, the rock maybe referred to as a sandy conglomerate or muddyconglomerate, depending on the grain size of thematrix present (Fig. 2.5). An intraformational con-glomerate is composed of clasts of the same materialas the matrix and is formed as a result of reworking oflithified sediment soon after deposition.The proportion of matrix present is an important fac-

tor in the texture of conglomeratic sedimentary rock,that is, the arrangement of different grain sizes withinit. A distinction is commonly made between conglom-erates thatare clast-supported (Fig. 2.6), that is, withclasts touching each other throughout the rock, andthose which are matrix-supported (Fig. 2.7), inwhich most of the clasts are completely surroundedby matrix. The term orthoconglomerate is some-times used to indicate that the rock is clast-supported,and paraconglomerate for a matrix-supported tex-ture. These textures are significant when determiningthe mode of transport and deposition of a conglomer-ate (e.g. on alluvial fans: 9.5).The arrangement of the sizes of clasts in a conglom-

erate can also be important in interpretation of deposi-tional processes. In a flow of water, pebbles are movedmore easily than cobbles that in turn require less energyto move them than boulders. A deposit that is made upof boulders overlain by cobbles and then pebbles may beinterpreted in some cases as having been formed from aflow that was decreasing in velocity. This sort of inter-pretation is one of the techniques used in determiningthe processes of transport and deposition of sedimentaryrocks (4.2).

2.2.3 Shapes of clasts

The shapes of clasts in gravel and conglomerate aredetermined by the fracture properties of the bedrockthey are derived from and the history of transport.

Fig. 2.4 A conglomerate (or breccia) made up of angularclasts.



Rocks with equally spaced fracture planes in all direc-tions form cubic or equant blocks that form sphericalclasts when the edges are rounded off (Fig. 2.8). Bed-rock lithologies that break up into slabs, such as awell-bedded limestone or sandstone, form clasts withone axis shorter than the other two (Krumbein &Sloss 1951). This is termed an oblate or discoidform. Rod-shaped or prolate clasts are less common,forming mainly from metamorphic rocks with astrong linear fabric.

When discoid clasts are moved in a flow of waterthey are preferentially oriented and may stack up in aform known as imbrication (Figs 2.9 & 2.10). Thesestacks are arranged in positions that offer the leastresistance to flow, which is with the discoid clastsdipping upstream. In this orientation, the water canflow most easily up the upstream side of the clast,whereas when clasts are oriented dipping downstream, flow at the edge of the clast causes it to bereoriented. The direction of imbrication of discoid

Fig. 2.6 A clast-supported conglomerate: the pebbles are allin contact with each other.

Fig. 2.7 A matrix-supported conglomerate: each pebble issurrounded by matrix.

Fig. 2.5 Nomenclature used for mixturesof gravel, sand and mud in sediments and

sedimentary rock.

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pebbles in a conglomerate can be used to indicatethe direction of the flow that deposited the gravel.If a discoid clast is also elongate, the orientation ofthe longest axis can help to determine the mode ofdeposition: clasts deposited by a flow of water willtend to have their long axis oriented perpendicularto the flow, whereas glacially deposited clasts (7.3.3)will have the long axis oriented parallel to theice flow.

2.3 SAND AND SANDSTONE

Sand grains are formed by the breakdown of pre-existing rocks by weathering and erosion (6.4 &6.5), and from material that forms within the deposi-tional environment. The breakdown products fallinto two categories: detrital mineral grains, erodedfrom pre-existing rocks, and sand-sized pieces of rock,or lithic fragments. Grains that form within thedepositional environment are principally biogenic inorigin, that is, they are pieces of plant or animal,but there are some which are formed by chemicalreactions.Sand may be defined as a sediment consisting pri-

marily of grains in the size range 63mm to 2mmand a sandstone is defined as a sedimentary rockwith grains of these sizes. This size range is dividedinto five intervals: very fine, fine, medium, coarseand very coarse (Fig. 2.2). It should be noted thatthis nomenclature refers only to the size of the parti-cles. Although many sandstones contain mainlyquartz grains, the term sandstone carries no implica-tion about the amount of quartz present in the rockand some sandstones contain no quartz at all.Similarly, the term arenite, which is a sandstonewith less than 15% matrix, does not imply any parti-cular clast composition. Along with the adjectivearenaceous to describe a rock as sandy, arenite hasits etymological roots in the Latin word for sand,arena, also used to describe a stadium with a sandyfloor.

2.3.1 Detrital mineral grains in sandsand sandstones

A very large number of different minerals may occurin sands and in sandstones, and only the most com-mon are described here.

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Fig. 2.8 The shape of clasts can be considered in terms offour end members, equant, rod, disc and blade. Equant and

disc-shaped clasts are most common.

Fig. 2.9 A conglomerate bed showing imbrication of clastsdue to deposition in a current flowing from left to right.

Fig. 2.10 The relationship between imbrication and flowdirection as clasts settle in a stable orientation.



Quartz

Quartz is the commonest mineral species found asgrains in sandstone and siltstone. As a primarymineral it is a major constituent of granitic rocks,occurs in some igneous rocks of intermediate compo-sition and is absent from basic igneous rock types.Metamorphic rocks such as gneisses formed fromgranitic material and many coarse-grained metasedi-mentary rocks contain a high proportion of quartz.Quartz also occurs in veins, precipitated by hot fluidsassociated with igneous and metamorphic processes.Quartz is a very stable mineral that is resistant tochemical breakdown at the Earths surface. Grains ofquartz may be broken or abraded during transport butwith a hardness of 7 on Mohs scale of hardness,quartz grains remain intact over long distances andlong periods of transport. In hand specimen quartzgrains show little variation: coloured varieties such assmoky or milky quartz and amethyst occur but mostlyquartz is seen as clear grains.

Feldspar

Most igneous rocks contain feldspar as a major com-ponent. Feldspar is hence very common and is releasedin large quantities when granites, andesites, gabbrosas well as some schists and gneisses break down. How-ever, feldspar is susceptible to chemical alteration dur-ing weathering and, being softer than quartz, tends tobe abraded and broken up during transport. Feldsparsare only commonly found in circumstances where thechemical weathering of the bedrock has not been toointense and the transport pathway to the site of deposi-tion is relatively short. Potassium feldspars are morecommon as detrital grains than sodium- and calcium-rich varieties, as they are chemically more stable whensubjected to weathering (6.4).

Mica

The two commonest mica minerals, biotite andmuscovite, are relatively abundant as detrital grainsin sandstone, although muscovite is more resistant toweathering. They are derived from granitic to inter-mediate composition igneous rocks and from schistsand gneisses where they have formed as metamorphicminerals. The platy shape of mica grains makes themdistinctive in hand specimen and under the micro-scope. Micas tend to be concentrated in bands on

bedding planes and often have a larger surface areathan the other detrital grains in the sediment; this isbecause a platy grain has a lower settling velocitythan an equant mineral grain of the same mass andvolume so micas stay in temporary suspension longerthan quartz or feldspar grains of the same mass.

Heavy minerals

The common minerals found in sands have densitiesof around 2.6 or 2.7 g cm3: quartz has a density of2.65 g cm3, for example. Most sandstones contain asmall proportion, commonly less than 1%, of mineralsthat have a greater density. These heavy mineralshave densities greater than 2.85 g cm3 and are tra-ditionally separated from the bulk of the lighterminerals by using a liquid of that density which thecommon minerals will float in but the small propor-tion of dense minerals will sink. These minerals areuncommon and study of them is only possible afterconcentrating them by dense liquid separation. Theyare valuable in provenance studies (5.4.1) becausethey can be characteristic of a particular source areaand are therefore valuable for studies of the sources ofdetritus. Common heavy minerals include zircon,tourmaline, rutile, apatite, garnet and a range ofother metamorphic and igneous accessory minerals.

Miscellaneous minerals

Other minerals rarely occur in large quantities insandstone. Most of the common minerals in igneoussilicate rocks (e.g. olivine, pyroxenes and amphiboles)are all too readily broken down by chemical weath-ering. Oxides of iron are relatively abundant. Localconcentrations of a particular mineral may occurwhen there is a nearby source.

2.3.2 Other components of sandsand sandstones

Lithic fragments

The breakdown of pre-existing, fine- to medium-grained igneous, metamorphic and sedimentaryrocks results in sand-sized fragments. Sand-sized lithicfragments are only found of fine to medium-grainedrocks because by definition the mineral crystal andgrains of a coarser-grained rock type are the size ofsand grains or larger. Determination of the lithology

Sand and Sandstone 11


of these fragments of rock usually requires petro-graphic analysis by thin-section examination (2.3.5)to identify the mineralogy and fabric.Grains of igneous rocks such as basalt and rhyolite

are susceptible to chemical alteration at the Earthssurface and are only commonly found in sandsformed close to the source of the volcanic material.Beaches around volcanic islands may be blackbecause they are made up almost entirely of lithicgrains of basalt. Sandstone of this sort of compositionis rare in the stratigraphic record, but grains of vol-canic rock types may be common in sediments depos-ited in basins related to volcanic arcs or rift volcanism(Chapter 17).Fragments of schists and pelitic (fine-grained) meta-

morphic rocks can be recognised under the micro-scope by the strong aligned fabric that theselithologies possess: pressure during metamorphismresults in mineral grains becoming reoriented orgrowing into an alignment perpendicular to the stressfield. Micas most clearly show this fabric, but quartzcrystals in a metamorphic rock may also display astrong alignment. Rocks formed by the metamorph-ism of quartz-rich lithologies break down to relativelyresistant grains that can be incorporated into a sand-stone.Lithic fragments of sedimentary rocks are generated

when pre-existing strata are uplifted, weathered anderoded. Sand grains can be reworked by this processand individual grains may go through a number ofcycles of erosion and redeposition (2.5.4). Finer-grained mudrock lithologies may break up to formsand-sized grains although their resistance to furtherbreakdown during transport is largely dependent onthe degree of lithification of the mudrock (18.2).Pieces of limestone are commonly found as lithicfragments in sandstone although a rock made uplargely of calcareous grains would be classified as alimestone (3.1). One of the most common lithologiesseen as a sand grain is chert (3.3), which being silicais a resistant material.

Biogenic particles

Small pieces of calcium carbonate found in sandstoneare commonly broken shells of molluscs and otherorganisms that have calcareous hard parts. Thesebiogenic fragments are common in sandstonedeposited in shallow marine environments wherethese organisms are most abundant. If these calcar-

eous fragments make up over 50% of the bulk of therock it would be considered to be a limestone (thenature and occurrence of calcareous biogenic frag-ments is described in the next chapter: 3.1.3). Frag-ments of bone and teeth may be found in sandstonesfrom a wide variety of environments but are rarelycommon. Wood, seeds and other parts of land plantsmay be preserved in sandstone deposited in continen-tal and marine environments.

Authigenic minerals

Minerals that grow as crystals in a depositional envi-ronment are called authigenic minerals. They aredistinct from all the detrital minerals that formed byigneous or metamorphic processes and were subse-quently reworked into the sedimentary realm. Manycarbonate minerals form authigenically and anotherimportant mineral formed in this way is glauconite/glaucony (11.5.1), a green iron silicate that forms inshallow marine environments.

Matrix

Fine-grained material occurring between the sandgrains is referred to as matrix (2.2.2). In sands andsandstone the matrix is typically silt and clay-sizedmaterial, and it may wholly or partly fill the spacesbetween the grains. A distinction should be drawnbetween the matrix, which is material depositedalong with the grains, and cement (18.2.2), whichis chemically precipitated after deposition.

2.3.3 Sandstone nomenclatureand classification

Full description of a sandstone usually includes someinformation concerning the types of grain present.Informal names such as micaceous sandstone areused when the rock clearly contains a significantamount of a distinctive mineral such as mica. Termssuch as calcareous sandstone and ferruginoussandstone may also be used to indicate a particularchemical composition, in these cases a noticeable pro-portion of calcium carbonate and iron respectively.These names for a sandstone are useful and appro-priate for field and hand-specimen descriptions, butwhen a full petrographic analysis is possible with athin-section of the rock under a microscope, a more



formal nomenclature is used. This is usually the Pet-tijohn et al. (1987) classification scheme (Fig. 2.11).The Pettijohn sandstone classification combines

textural criteria, the proportion of muddy matrix,with compositional criteria, the percentages of thethree commonest components of sandstone: quartz,feldspar and lithic fragments. The triangular plot hasthese three components as the end members to form aQ, F, L triangle, which is commonly used in clasticsedimentology. To use this scheme for sandstone clas-sification, the relative proportions of quartz, feldsparand lithic fragments must first be determined byvisual estimation or by counting grains under amicroscope: other components, such as mica or bio-genic fragments, are disregarded. The third dimensionof the classification diagram is used to display thetexture of the rock, the relative proportions of clastsand matrix. In a sandstone the matrix is the silt andclay material that was deposited with the sand grains.The second stage is therefore to measure or estimatethe amount of muddy matrix: if the amount of matrixpresent is less than 15% the rock is called an arenite,between 15% and 75% it is a wacke and if most of thevolume of the rock is fine-grained matrix it is classifiedas a mudstone (2.4.1).Quartz is the most common grain type present in

most sandstones so this classification emphasises thepresence of other grains. Only 25% feldspar need bepresent for the rock to be called a feldspathicarenite, arkosic arenite or arkose (these three

terms are interchangeable when referring to sand-stone rich in feldspar grains). By the same token,25% of lithic fragments in a sandstone make it alithic arenite by this scheme. Over 95% of quartzmust be present for a rock to be classified as a quartzarenite; sandstone with intermediate percentages offeldspar or lithic grains is called subarkosic areniteand sublithic arenite. Wackes are similarly dividedinto quartz wacke, feldspathic (arkosic) wackeand lithic wacke, but without the subdivisions. If agrain type other than the three main components ispresent in significant quantities (at least 5% or 10%),a prefix may be used such as micaceous quartz aren-ite: note that such a rock would not necessarily con-tain 95% quartz as a proportion of all the grainspresent, but 95% of the quartz, feldspar and lithicfragments when they are added together.The term greywacke has been used in the past for a

sandstone that might also be called a feldspathic orlithic wacke. They are typically mixtures of rock frag-ments, quartz and feldspar grains with a matrix ofclay and silt-sized particles.

2.3.4 Petrographic analysis of sandsand sandstones

In sand-grade rocks, the nature of the individual grainsand the relationship between these grains and thematerial between them is best seen in a thin-section

Fig. 2.11 The Pettijohn classification ofsandstones, often referred to as a

Toblerone plot (Pettijohn 1975).

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of the rock, a very thin (normally 30 microns) slice ofthe rock, which can be examined under a petrologi-cal/petrographic microscope (Fig. 2.12). Thin-section examination is a standard technique for theanalysis of almost all types of rock, igneous and meta-morphic as well as sedimentary, and the proceduresform part of the training of most geologists.

The petrographic microscope

A thin-section of a rock is cemented onto a glassmicroscope slide and it is normal practice to cementa thin glass cover slip over the top of the rock slice toform a sandwich, but there are circumstances wherethe thin-section is left uncovered (3.1.2). The slide isplaced on the microscope stage where a beam of whitelight is projected through the slide and up through thelenses to the eyepiece: this transmitted light micro-scopy is the normal technique for the examination ofrocks, the main exceptions being ore minerals, whichare examined using reflected light (this is because ofthe optical properties of the minerals concerned seebelow). The majority of minerals are translucentwhen they are sliced to 30 microns thick, whatevertheir colour or appearance in hand specimen: this isparticularly true of silicate and carbonate minerals,which are the groups of prime interest to the sedimen-tary geologist. It is therefore possible to view theoptical properties of the minerals, the way theyappear and interact with the light going throughthem, using a petrographic microscope.Underneath the microscope stage the light beam

passes through a polarising filter, which only

allows light waves vibrating in one plane to passthrough it and hence through the thin-section.Toward the bottom of the eyepiece tube there is asecond polarising filter that is retractable. This polar-ising filter is mounted perpendicular to the one belowthe stage, such that it only allows through lightwaves that are vibrating at ninety degrees to thelower one. If this second filter, known as the analys-ing filter, is inserted across the lenses when there isno thin-section, or just plain glass, on the stage, thenall the light from the beam will be cut out and itappears black. The same effect can be achieved withPolaroid sunglasses: putting two Polaroid lenses atninety degrees to each other should result in theblocking out of all light.Other standard features on a petrographic micro-

scope are a set of lenses at the end of the eyepiece tubethat allow different magnifications of viewing to beachieved. The total magnification will be a multiple ofone of these lenses and the eyepiece magnification.The eyepiece itself has a very fine cross-wire mountedin it: this acts as a frame of reference to be used whenthe orientation of the thin-section is changed by rotat-ing the stage. The stage itself is graduated in degreesaround the edge so that the amount of rotation canbe measured. An optional feature within the eyepieceis a graticule, a scale that allows measurements offeatures of the thin-section to be made if the magnifi-cation is known.There are usually further tools for optical analyses

on the microscope, such as additional lenses that canbe inserted above and below the stage, and plates thatcan be introduced into the eyepiece tube. These areused when advanced petrographic techniques areemployed to make more detailed analyses of minerals.However, at an introductory level of sedimentary pet-rography, such techniques are rarely used, and anal-ysis can be carried out using only a limited range ofthe optical properties of minerals, which are describedin the following sections.

2.3.5 Thin-section analysis of sandstones

Use of the following techniques will allow identifica-tion of the most frequently encountered minerals insedimentary rocks. Only a very basic introduction tothe principles and application of thin-section analysisis provided here. For more detailed and advancedpetrographic analysis, reference should be made to

Fig. 2.12 A photomicrograph of a sandstone: the grains areall quartz but appear different shades of grey under crossed

polars due to different orientations of the grains.



an appropriate book on optical mineralogy (e.g. Grib-ble & Hall 1999; Nesse 2004), which should be usedin conjunction with suitable reference books on sedi-mentary petrography, particularly colour guides suchas Adams et al. (1984).

Grain shape

A distinctive shape can be a characterising feature ofa mineral, for example members of the mica family,which usually appear long and thin if they have beencut perpendicular to their platy form. Minerals mayalso be elongate, needle-like or equant, but in all casesit must be remembered that the shape depends on theangle of the cut through the grain. Grain shape alsoprovides information about the history of the sedi-ment (2.5.4) so it is important to distinguish betweengrains that show crystal faces and those that showevidence of abrasion of the edges.

Relief

Relief is a measure of how strong the lines that markthe edges of the mineral, or minerals that comprise agrain, are and how clearly the grain stands outagainst the glass or the other grains around it. It is avisual appraisal of the refractive index of the mineral,which is in turn related to its density. A mineral suchas quartz has a refractive index that is essentially thesame as glass, so a grain of quartz 30 microns thickmounted on a microscope slide will only just be visible(the mounting medium glue normally has thesame optical properties as the glass slide): it is there-fore considered to have low relief. In contrast, agrain of calcite against glass will appear to have verydistinct, dark edges, because it is a denser mineralwith a higher refractive index and therefore has ahigh relief. Because a sedimentary grain will oftenbe surrounded by a cement (18.2.2) the contrast withthe cement is important, and a quartz grain will standout very clearly if surrounded by a calcite cement.Certain heavy minerals, such as zircon, can readilybe distinguished by their extremely high relief.

Cleavage

Not all minerals have a regular cleavage, a preferredfracture orientation determined by the crystal latticestructure, so the presence or absence of a cleavagewhen the mineral is viewed in thin-section can be a

useful distinguishing feature. Quartz, for example,lacks a cleavage, but feldspars, which otherwisehave many optical properties that are similar toquartz, commonly show clear, parallel lines of clea-vage planes. However, the orientation of the mineralin the thin-section will have an important effectbecause if the cut is parallel to the cleavage planes itwill appear as if the mineral does not have a cleavage.The angle between pairs of cleavage planes can beimportant distinguishing features (e.g. betweenminerals of the pyroxene family and the amphibolegroup of minerals). The cleavage is usually best seenunder plane-polarised light and often becomes clearerif the intensity of the light shining through is reduced.

Colour and opacity

This property is assessed using plane-polarised light(i.e. without the analysing filter inserted). Someminer-als are completely clear while others appear slightlycloudy, but are essentially still colourless: mineralsthat display distinct colours in hand specimen do notnecessarily show any colours in thin-section (e.g. pur-ple quartz or pink feldspar). Coloursmaybe faint tints ormuch stronger hues, themost common being shades ofgreen and brown (some amphiboles and micas), withrarer yellows and blues. (A note of caution: if a rock israther poorly lithified, part of the process of manufac-ture of the thin-section is to inject a resin into the porespaces between the grains to consolidate it; this resin iscommonly dyed bright blue so that it can easily bedistinguished from the original components of therock it is not a blue mineral!)Some grains may appear black or very dark brown.

The black grains are opaque minerals that do notallow any light through them even when cut to athin slice. Oxides and sulphides are the commonestopaque minerals in sedimentary rocks, particularlyiron oxides (such as haematite) and iron sulphide(pyrite), although others may occur. Black grainsthat have a brown edge, or grains that are darkbrown throughout, are likely to be fragments oforganic material.

Pleochroism

A grain of hornblende, a relatively common memberof the amphibole group, may appear green or brownwhen viewed under plane-polarised light, but whatis distinctive is that it changes from one colour to the



other when the grain is moved by rotating themicroscope stage. This phenomenon is known aspleochroism and is also seen in biotite mica and anumber of other minerals. It is caused by variationsin the degree of absorption of different wavelengthsof light when the crystal lattice is at differentorientations.

Birefringence colours

When the analysing lens is inserted across the objec-tive/eyepiece tube, the appearance of the minerals inthe thin-section changes dramatically. Grains thathad appeared colourless under plane-polarised lighttake on a range of colours, black, white or shades ofgrey, and this is a consequence of the way thepolarised light has interacted with the minerals.Non-opaque minerals can be divided into two groups:isotropic minerals have crystal lattices that do nothave any effect on the pathway of light passingthrough them, whatever orientation they are in(halite is an example of an isotropic mineral); whenlight passes through a crystal of an anisotropicmineral, the pathway of the light is modified, andthe degree to which it is affected depends on theorientation of the crystal. When a crystal of an iso-tropic mineral is viewed with both the polarising andanalysing filters inserted (under cross-polars), itappears black. However, an anisotropic mineral willdistort the light passing through it, and some of thelight passes through the analyser. The mineral willthen appear to have a colour, a birefringencecolour, which will vary in hue and intensity depend-ing on the mineral type and the orientation of theparticular grain (and, in fact, the thickness of theslice, but thin-sections are normally cut to 30microns, so this is not usually a consideration).For any given mineral type there will be a max-

imum birefringence colour on a spectrum of coloursand hues that can be illustrated on a birefringencechart. In a general sense, minerals can be described ashaving one of the following: low birefringence col-ours, which are greys (quartz and feldspars are exam-ples), first order colours (seen in micas), which arequite intense colours of the rainbow, and high ordercolours, which are pale pinks and greens (common incarbonate minerals). Petrology reference books (e.g.Gribble & Hall 1999; Nesse 2004) include chartsthat show the birefringence colours for commonminerals.

Angle of extinction

When the stage is rotated, the birefringence colour ofa grain of an anisotropic mineral will vary as thecrystal orientation is rotated with respect to theplane-polarised light. The grain will pass through amaximum colour (although this may not be themaximum colour for this mineral, as this will dependon the three-dimensional orientation of the grain) andwill pass through a point in the rotation when thegrain is dark: this occurs when the crystal lattice is inan orientation when it does not influence the path ofthe polarised light. With some minerals the grain goesblack goes into extinction when the grain isoriented with the plane of the polarised light parallelto a crystal face: this is referred to as parallel extinc-tion. When viewed through the eyepiece of the micro-scope the grain will go into extinction when thecrystal face is parallel to the vertical cross-wire.Many mineral types go into extinction at an angle tothe plane of the polarised light: this can be measuredby rotating a grain that has a crystal face parallel tothe vertical cross-wire until it goes into extinction andmeasuring the angle against a reference point on theedge of the circular stage. Different types of feldsparcan be distinguished on the basis of their extinctionangle.

Twinning of crystals

Certain minerals commonly display a phenomenonknown as twinning, when two crystals have formedadjacent to each other but with opposite orientationsof the crystal lattice (i.e. mirror images). Twinnedcrystals may be difficult to recognise under plane-polarised light, but when viewed under crossed polarsthe two crystals will go into extinction at 1808 to eachother. Multiple twins may also occur, and in fact are acharacteristic of plagioclase feldspars, and these areseen as having a distinctive striped appearance undercrossed polars.

2.3.6 The commonest minerals insedimentary rocks

Almost any mineral which is stable under surfaceconditions could occur as a detrital grain in a sedi-mentary rock. In practice, however, a relatively smallnumber of minerals constitute the vast majority of



grains in sandstones. The common ones are brieflydescribed here, and their optical properties sum-marised in Fig. 2.13.

Quartz

Most sandstones and siltstones contain grains ofquartz, which is chemically the simplest of the silicateminerals, an oxide of silicon. In thin-section grainsare typically clear, low relief and do not show anycleavage; birefringence colours are grey. Quartzgrains from a metamorphic source (and occasionallysome igneous sources) may show a characteristicundulose extinction, that is, as the grain is rotated,the different parts go into extinction at differentangles, but there is no sharp boundary betweenthese areas. This phenomenon, known as strainedquartz, is attributed to deformation of the crystallattice, which gives the grain irregular optical proper-ties and its presence can be used as an indicator ofprovenance (5.4.1).

Feldspars

Feldspars are silicate minerals that are principal com-ponents of most igneous and many metamorphic

rocks: they are also relatively common in sandstones,especially those made up of detritus eroded directlyfrom a bedrock such as a granite. Feldspar crystalsare moderately elongate, clear or sometimes slightlycloudy andmay showawell-developed cleavage. Reliefis variable according to chemical composition, but isgenerally low, and birefringence colours are weak,shades of grey. Feldspars fall into two main groups,potash feldspars and the plagioclase feldspars.Potash feldspars such as orthoclase are the most

common as grains in sedimentary rocks. It can bedifficult to distinguish orthoclase from quartz at firstglance because the two minerals have a similar reliefand lowbirefringence colours, but the feldsparwill showa cleavage in some orientations, twinning may be seenunder cross-polars, and it is often slightly cloudy underplane-polarised light. The cloudiness is due to chemicalalteration of the feldspar, something that is not seen inquartz. Another mineral in this group is microcline,which is noteworthy because, under plane-polarisedlight, it shows a very distinctive cross-hatch pattern offine, black and white stripes perpendicular to eachother: although less common than orthoclase, it isvery easy to recognise in thin-section.Plagioclase feldspars are a family of minerals that

have varying proportions of sodium and calcium in

Fig. 2.13 The optical properties of the minerals most commonly found in sedimentary rocks.



their composition: albite is the sodium-rich form, andanorthite the calcium-rich, with several others inbetween. The most characteristic distinguishing fea-ture is the occurrence of multiple twins, which givethe grains a very pronounced black and white stripedappearance under crossed polars. The extinctionangle varies with the composition, and is used as away of distinguishing different minerals in the plagi-oclase group (Gribble & Hall 1999; Nesse 2004).

Micas

There are many varieties of mica, but two of the mostfrequently encountered forms are the white mica,muscovite, and the brown mica, biotite. Micas arephyllosilicates, that is, they have a crystal structureof thin sheets, and have a very well developed platycleavage that causes the crystals to break up into verythin grains. If the platy grains lie parallel to the planeof the thin-section, they will appear hexagonal, but itis much more common to encounter grains that havebeen cut oblique to this and therefore show the clea-vage very clearly in thin-section. The grains alsoappear elongate and may be bent: mica flakes arequite delicate and can get squeezed between hardergrains when a sandstone is compacted (18.3.1). Bio-tite is usually very distinctive because of its shape,cleavage, brown colour and pleochroism (whichmay not always be present). It has bright, first-orderbirefringence colours, but these are often masked bythe brown mineral colour: the extinction angle is 08to 38. The strong, bright birefringence colours ofmuscovite flakes are very striking under cross-polars,which along with the elongate shape and cleavagemake this a distinctive mineral.

Other silicate minerals

In comparison to igneous rocks, sedimentary rockscontain a much smaller range of silicate minerals ascommon components. Whereas minerals belonging tothe amphibole, pyroxene and olivine groups areessential minerals in igneous rocks of intermediateto mafic composition (i.e. containing moderate torelatively low proportions of SiO2), these mineralsare rare in sediments. Hornblende, an amphibole, isthe most frequently encountered, but would normallybe considered a heavy mineral (see below), as wouldany minerals of the pyroxene group. Olivine, so com-mon in gabbros and basalts, is very rare as a detrital

grain in a sandstone. This is because of the suscep-tibility of these silicate minerals to chemical break-down at the Earths surface, and they do not generallysurvive for long enough to be incorporated into asediment.

Glauconite

This distinctive green mineral is unusual because,unlike other silicates, it does not originate fromigneous or metamorphic sources. It forms in sedimenton the sea floor and can accumulate to form signifi-cant proportions of some shallow marine deposits(11.5.1). Under plane-polarised light glauconitegrains have a distinctive, strong green colour that ispatchy and uneven over the area of the grain: thiscolour mottling is because the mineral normallyoccurs in an amorphous form, and other crystal prop-erties are rarely seen.

Carbonate minerals

The most common minerals in this group are thecalcium carbonates, calcite and aragonite, while dolo-mite (a magnesiumcalcium carbonate) and siderite(iron carbonate) are also frequently encountered insedimentary rocks. Calcium carbonate minerals areextremely common in sedimentary rocks, being themain constituents of limestone. Calcite and aragoniteare indistinguishable in thin-section: like all sedimen-tary carbonates, these minerals have a high relief andcrystals show two clear cleavage planes present at 758to each other. Birefringence colours are pale, high-order greens and pinks. The form of calcite in a sedi-mentary rock varies considerably because much of ithas a biogenic origin: the recognition of carbonatecomponents in thin-section is considered in section3.1.2.Most dolomite is a diagenetic product (18.4.2), the

result of alteration of a limestone that was originallycomposed of calcium carbonate minerals. When indi-vidual crystals can be seen they have a distinctiveeuhedral rhombic shape, and cleavage planes parallelto the crystal faces may be evident. The euhedralmorphology can be a good clue, but identification ofdolomite cannot be confirmed without chemical testson the material (3.1.2). Siderite is very difficult todistinguish from calcite because most of its opticalproperties are identical. The best clue is often a slightyellow or brownish tinge to the grain, which is a



result of alteration of some of the iron to oxides andhydroxides.

Oxides and sulphides

The vast majority of natural oxide and sulphide miner-als are opaque, and simply appear as black grainsunder plane-polarised light. The iron oxide haematiteis particularly common, occurring as particles thatrange down to a fine dust around the edges of grainsand scattered in the matrix. The edges of haematitegrains will often look brownish-red. Magnetite, also aniron oxide, occurs as a minor component of manyigneous rocks and is quite distinctive because it occursas euhedral, bipyramidal crystals, which appear asfour or eight-sided, equant black grains in thin-section.Iron hydroxides, limonite and goethite, which are yel-lowish brown in hand specimen, appear to have brownedges in thin-section.Pyrite is an iron sulphide that may crystallise

within sediments. Although a metallic gold colour asa fully-formed crystal, fine particles of pyrite appearblack, and in thin-section this mineral often appearsas black specks, with the larger crystals showing thecubic crystal shape of the mineral. Locally, othersulphides and oxides can be present, for example thetin ore, cassiterite, which occurs as a placer mineral(minerals that concentrate at the bottom of a flow dueto their higher density).

Heavy minerals

A thin-section of a sandstone is unlikely to containmany heavy mineral grains. Zircon is the most fre-quently encountered member of this group: it is anextremely resistant mineral that can survive weath-ering and long distances of transport. Grains areequant to elongate, colourless and easily recognisedby their very high relief: the edges of a zircon grainwill appear as thick, black lines. Other relatively com-mon heavy minerals are rutile, apatite, tourmalineand sphene.

2.3.7 Lithic grains

Not every grain in a sandstone is an individualmineral: the breakdown of bedrock by weatheringleads to the formation of sand-sized fragments of theoriginal rock that can be incorporated into a sedi-

ment. The bedrock must itself be composed of crystalsor particles that are smaller than sand-size: graniteconsists of crystals that are sand-sized or larger, andso cannot occur as lithic clasts in sands, but its fine-grained equivalent, rhyolite, can occur as grains.Lithic fragments of fine-grained metamorphic andsedimentary rocks can also be common.

Chert and chalcedony

Under plane-polarised light, chert (3.3) looks verymuch like quartz, because it is also composed of silica.The difference is that the silica in chert is in an amor-phous or microcrystalline form: under cross-polars ittherefore often appears to be highly speckled black,white and grey, with individual crystals too small tobe resolved under a normal petrographic microscope.Chalcedony is also a form of silica that can readily beidentified in thin-section because it has a radial struc-ture when viewed under cross-polars; fine black andwhite lines radiate from the centre, becoming lighterand darker as the grain is rotated.

Organic material

Carbonaceous material, the remains of plants, isbrown in colour, varying from black and opaque totranslucent reddish brown in thin-section. The palergrains can resemble a mineral, but are always blackunder cross-polars. The shape and size is extremelyvariable and some material may appear fibrous. Coalis a sedimentary rock made up largely of organicmaterial: the thin-section study of coal is a specialisedsubject that can yield information about the vegeta-tion that it formed from and its burial history.

Sedimentary rock fragments

Clasts of claystone, siltstone or limestone may be pres-ent in a sandstone, and a first stage of recognition ofthem is that they commonly appear rather dirtyunder plane-polarised light. Very fine particles ofclay and iron oxide in a lit