Sedimentary Environments E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Sedimentary Environments is one of the most distinguished and influential textbooks in the earth sciences published in the last 20 years. The first and second editions both won universal praise and became classic works in sedimentology. Since the publication of the last edition, the study of sedimentary environments and facies has made great strides, with major advances in facies modelling, sequence stratigraphy and basin modelling. The 3rd edition of this classic text will likely set the benchmark even higher, and needless to say, will continue being the textbook of choice for sedimentology students.
- The latest edition of a classic text.
- Incorporates all the latest advances in dynamic stratigraphy.
- Will remain the textbook of choice for upper level undergraduate and graduate students in sedimentology.
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Seitenzahl: 2094
Veröffentlichungsjahr: 2013
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Contents
Contributors
Preface
1 Introduction
1.1 Development of sedimentology and sedimentary geology
1.2 Scope and philosophy of this book
1.3 Organization of the book
Further reading
2 Controls on the sedimentary rock record
2.1 Controlling factors
2.2 Facies and sequences
2.3 Facies in the subsurface
2.4 Sequence stratigraphy
2.5 Models
Further reading
3 Alluvial sediments
3.1 Introduction
3.2 Alluvial processes
3.3 Present-day alluvial settings
3.4 Ancient alluvial sediments
3.5 Alluvial facies
3.6 Larger-scale geometry, organization and controls
Further reading
4 Lakes
4.1 Introduction
4.2 Diversity of present-day lakes
4.3 Properties of lake water
4.4 Kinetics of lake water
4.5 Chemistry of lake waters
4.6 Clastic sedimentation
4.7 Chemical and biochemical sedimentation
4.8 Rhythmites
4.9 Lake-level changes
4.10 Ancient lake deposits
4.11 Ancient clastic-dominated basins
4.12 Ancient carbonate-dominated basins
4.13 Mixed clastic-carbonate basins
4.14 Evaporitic lake basins
4.15 Organic-matter-dominated basins
4.16 Cycles in lake deposits
4.17 Economic importance of lake deposits
Further reading
5 Desert aeolian systems
5.1 Introduction
5.2 The desert aeolian system
5.3 Aeolian processes and theory
5.4 Present-day aeolian systems
5.5 Ancient aeolian systems
Further reading
6 Clastic coasts
6.1 Introduction
6.2 Shoreline processes
6.3 Coastal models and classifications
6.4 Rocky coasts
6.5 Coarse-grained gravel-rich coasts
6.6 River deltas
6.7 Non-deltaic siliciclastic coasts
Further reading
7 Shallow clastic seas
7.1 Introduction
7.2 Clastic shelf models and classification
7.3 Modern tide-dominated shallow seas
7.4 Modern wave- and storm-dominated shallow seas
7.5 Modern oceanic current-dominated shelves
7.6 Modern mud-dominated shelves
7.7 Ancient shallow clastic seas: facies recognition and interpretation
7.8 Ancient offshore shallow marine clastic sedimentation patterns: interaction of physical processes and relative sealevel fluctuations
Further reading
8 Marine evaporites: arid shorelines and basins
8.1 Introduction
8.2 Conditions of marine evaporite formation
8.3 General features of evaporites
8.4 Sabkhas
8.5 Shallow-water evaporites
8.6 Deep-water evaporites
8.7 Ancient sabkha/salina evaporites
8.8 Ancient basin-marginal platform evaporites
8.9 Ancient basin-central evaporites
8.10 Evaporites and sequence stratigraphy
Further reading
9 Shallow-water carbonate environments
9.1 Introduction
9.2 Controls on carbonate production and sedimentation
9.3 Carbonate platforms
9.4 Carbonate depositional environments
9.5 Carbonate platforms and relative sea-level changes
Further reading
10 Deep seas
10.1 Introduction
10.2 Processes and products
10.3 Deep-water clastic systems
10.4 Deep-water pelagic and hemipelagic systems
Further reading
11 Glacial sediments
11.1 Introduction
11.2 Characteristics of glaciers
11.3 Processes
11.4 Modern glacial environments and facies
11.5 Ancient glacial facies
11.6 Ice ages in Earth history
Further reading
12 Volcanic environments
12.1 Introduction
12.2 Distribution and products of volcanism
12.3 Magmatic processes and their effect
12.4 Eruption processes and facies
12.5 Sedimentary processes in volcanic terranes
12.6 The stratigraphic record of volcanism
12.7 Classification of volcaniclastic deposits
12.8 Volcanic landforms
12.9 Monogenetic basaltic volcanoes
12.10 Monogenetic silicic volcanoes
12.11 Polygenetic basaltic volcanoes
12.12 Polygenetic intermediate volcanoes
12.13 Polygenetic silicic volcanoes
12.14 Analysis of ancient volcanic successions
Further reading
Additional information
13 Problems and perspectives
13.1 Historical review
13.2 Economic aspects
13.3 Environmental aspects
13.4 Future studies
References
Index
©1978, 1986, 1996 by Blackwell Science Ltd,a Blackwell Publishing company
BLACKWELL PUBLISHING350 Main Street, Malden, MA 02148-5020, USA9600 Garsington Road, Oxford OX4 2DQ UK550 Swanston Street, Carlton, Victoria 3053, Australia
The right of the Author to be identified as the Author of this Work has been asserted in accordance with the UK Copyright, Designs, and Patents Act 1988.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs, and Patents Act 1988, without the prior permission of the publisher.
First published 1978Second edition 1986Third edition 1996
10 2009
Library of Congress Cataloging in Publication Data
Sedimentary environments: processes, facies, and stratigraphy /edited by H. G. Reading.–3rd ed.
p.cm.
Rev. ed. of: Sedimentary environments and facies. 1978.Includes bibliographical references and index.ISBN 978-0-6320-3627-1
1. Rocks, Sedimentary. 2. Facies (Geology). 3. Sedimentation anddeposition. 1. Reading, H. G. II. Sedimentary environments and facies.
QE471.5378 1996539.7′4–dc200
95-48457CIP
A catalogue record for this title is available from the British Library.
For further information onBlackwell Publishing, visit OUT website:www.blackwellpublishing.com
Contributors
Philip A. Allen
Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK
Christopher T. Baldwin
College of Arts and Sciences, Sam Houston State University, Box 2209, Huntsville, Texas 77341, USA
Trevor P. Burchette
BP Exploration Operating Company, Uxbridge One, 1 Harefield Road, Uxbridge, Middlesex UB8 1PD, UK
John D. Collinson
Barrow Cottage, Marchamley, Shrewsbury SY4 5LD, UK
Gillian M. Harwood
Died on 12th March 1996
Howard D. Johnson
Department of Geology, Imperial College, Prince Consort Road, London SW7 2BP, UK
Alan C. Kendall
Department of Environmental Sciences, University of East Anglia, Norwich NR4 7Tf, UK
Gary A. Kocurek
Department of Geological Sciences, University of Texas, PO Box 7909, Austin, Texas 78713, USA
Bruce K. Levell
Shell UK Exploration and Production, Shell-Mex House, Strand, London WC2R ODX, UK
Julia M.G. Miller
Department of Geology, Vanderbilt University, Box 7-B, Nashville, Tennessee 37235, USA
Geoffrey J. Orton
Geology Department, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4MI, Canada
Harold G. Reading
Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK
Dorrik A.V. Stow
Department of Geology, University of Southamptom, Empress Dock, European Way, Southampton S014 3ZH, UK
Michael R. Talbot
Geologisk Institutt, Universitet i Bergen, Allégt. 41, 5007 Bergen, Norway
V. Paul Wright
Postgraduate Research Institute for Sedimentology, University of Reading, Whiteknights, Reading RG6 2AB, UK
Preface
The first edition of this book was conceived in 1974 to provide a comprehensive text, covering modern and ancient environments, suitable for advanced university students, research workers and professional geologists. To cover all environments and facies with the authority of an active research worker, we formed a group of authors who knew each other well and shared a similar philosophical view. We could criticize, amend and integrate each other’s contributions, while retaining individual styles and responsibility for each chapter.
The outlines of the second edition remained the same as the first edition, although each chapter was extensively rewritten, with many new figures, and a few changes of authors. The third edition has also been changed substantially. This is primarily because of the need to add sequence stratigraphic models to those of facies sedimentology. Two chapters have been dropped (‘Pelagic Environments’ and ‘Sedimentation and Tectonics’); ‘Volcanic Environments’ has been added and the two chapters previously devoted to ‘Deltas’ and ‘Siliciclastic Shorelines’ have been combined under ‘Clastic Coasts’.
As with all textbooks, our main problems have been the selection of material and the need to strike a balance between comprehensiveness and cost. In the first edition many chapters had to be reduced to half of their original length and references pruned. In this edition the problem has been exacerbated by the growth of sedimentological material and concepts in the past 20 years. Almost every chapter now has at least one textbook devoted to it, as well as several special publications. In addition, we have tried to incorporate some of the concepts and ideas of sequence stratigraphy, yet retain the fundamentals of sedimentology, without which no sequence stratigraphic model can be validated.
No book is solely the product of its authors. In this book we have incorporated facts, ideas, philosophies and prejudices of many others; some are quoted and acknowledged; others have been absorbed by us over many years from teachers, colleagues, friends and students. It was the late Maurits de Raaf who taught many of us to combine careful facies analysis and an examination of every detail in a rock with an unceasing search for the processes which formed it, to doubt any hypothesis we may be defending and to be aware of becoming too dogmatic. This philosophy, we hope, will be followed by all who read this book, look at rocks or develop models.
We want first to thank those authors of previous editions without whose insights this edition would not have been possible, Marc M. Edwards, Trevor Elliott, Hugh Jenkyns, Andrew H.G. Mitchell, Nicholas A. Rupke, Charlotte Schreiber, Bruce W. Sellwood, Maurice E. Tucker and Roger Till. Individual chapters or parts of chapters were read by Benjamin F. Adams, Gail M. Ashley, Bernard M. Besly, Ronald C. Blakey, Michael D. Blum, Michael J. Branney, Stephen P. Carey, Ray A. Cas, John C. Crowell, Cristino J. Dabrio, Robert W. Dalrymple, José M.L. Dominguez, Marc B. Edwards, Richard V. Fisher, Peter Francis, Bibek Ghosh, Michael J. Hambrey, Karen G. Havholm, Markes E. Johnson, Nicholas Lancaster, Francesco Massari, Torn Pierson, Guy Plint, George Postma, Greg Valentine, Jonathon E. Verlander. We wish to thank all those who typed mdividual chapters and in particular Mrs P. McNiff who typed many drafts and much of the final manuscript. Responsibility, however, for omissions, lack of balance or for errors must remain with us. We wish also to acknowledge the many societies and publishing houses who permitted illustrations from articles in their journals and books to be used as the basis for our figures; to the Geological Society of America, the Geological Society of London, the Society for Sedimentary Geology (SEPM), the International Association of Sedimentologists, the American Association of Petroleum Geologists, Canadian Society of Petroleum Geologists, International Glaciological Society, Norges Geologiske Undersøkelse, Scott Polar Research Institute, Elf Aquitaine, American Geophysical Union, US Geological Survey, American Association for the Advancement of Science, The Royal Society, Geological Association of Canada and Smithsonian Institution; to Academic Press, American Journal of Science, Annual Reviews Inc., A.A. Balkema Publishers, Cambridge University Press, Chapman and Hall, Economic Geology Publishing Co., Edward Arnold Publishers, Elsevier Science, Geological Society Publishing House, Graham and Trotman Ltd., John Wiley and Sons, Longmans Group, Reidel, Macmillan Magazines Ltd, Springer-Verlag, Oxford University Press, University of Chicago Press and Unwin Hyman.
The book could never have corne to fruition without Blackwell Science, in particular Simon Rallison and Jonathan Rowley, always supported by Robert Campbell. Not only do more figures in this book emanate from the publications of Blackwell Science than any other publisher, but their constant encouragement has supported us, especially when we ran into difficulties.
Finally, no book can be completed without families and friends whose forebearance and help has sustained us over the years that this book has been in preparation.
Harold G. ReadingJune, 1996 Oxford
Gill Harwood died on 12th March 1996 after a long struggle against cancer. We trust that her chapter with Alan Kendall will remain a permanent tribute to her devotion to science and research on evaporites.
1
Introduction
H.G. Reading
1.1 Development of sedimentology and sedimentary geology
Sedimentology is concerned with the composition and genesis of sediments and sedimentary rocks, and the creation of predictive models. It includes sedimentary petrology, which is the study of the nature and relationships of the constituent particles and their diagenesis. It differs from stratigraphy in that time is not of prime importance except in so far as it deals with sequences and the law of Superposition is fundamental. It overlaps with other geological disciplines such as geochemistry, mineralogy, palaeontology and tectonics. In addition, sedimentology takes from and contributes to chemistry, biology, physics, geomorphology, oceanography, soil science, civil engineering, climatology, glaciology and fluid dynamics. When sedimentology and stratigraphy are combined they become the science of sedimentary geology, a term that is generally used in a wider sense than sedimentology. However, since the genesis of sedimentary rocks cannot be understood without reference to the time framework within which they were deposited, sedimentology has always, and still does, embrace a large element of stratigraphy as well.
Modern sedimentology, characterized by the study of processes, can be said to have started with the publication of Kuenen and Migliorini’s (1950) paper on turbidity currents as a cause of graded bedding (see Sect. 10.2.3). Before 1950, the sciences of stratigraphy, concerned primarily with correlation and broad palaeogeographic reconstructions, and sedimentary petrology, concerned primarily with the microscopic examination of sedimentary rocks, had evolved more or less independently, with the exception of a few notable contributions such as those of Sorby (1859, 1879).
The turbidite concept developed from Daly’s (1936) hypothesis that turbidity currents might be the agent of erosion of submarine canyons and from the model flume experiments of Kuenen (1937, 1950). Under the impact of this concept, geologists, who for years had been working on ‘flysch’ began to realize that an actual mechanism of flow could be envisaged as the agent of transport and deposition of graded sand beds. Geologists could now look at sedimentary rocks as sediments that had modern analogues, some aspects of which could be simulated by experiment. Familiar rocks could be examined with new insight and such features as sole marks, previously largely undetected, because they were not understood, could be described and perhaps explained.
As data on the composition, texture and structures of sedimentary rocks have grown, models have been developed that lean on a comparison with processes observed in modern environments and in experiments. Although many of these comparative models are founded on observations that can be made in now-active environments, others are the result of a creative blend of experience and imagination. Matching process with the corresponding sedimentary product is often difficult. In present-day shallow-water environments, processes are readily studied and measured, but data on their products are difficult to collect. In the Ancient, composition, texture and sedimentary structures arc normally easily observed, but the processes which produced the observed features cannot be directly measured. A prime aim of sedimentology is to narrow the gap between modern process and past product, in some cases aided by an understanding of diagenesis.
The many books on sedimentology which appeared in the 1960s and 1970s reflected the surge of new concepts. Sedimentary structures and their use in basinal reconstruction were emphasized by Potter and Pettijohn (1963). Physical processes of sedimentation and their importance in understanding sedimentary structures were first brought to the attention of geologists in the volume edited by Middleton (1965) and a deeper understanding of some of the processes has been developed by J.R.L. Allen (1968, 1984). A succinct description and explanation of the processes of formation of sedimentary structures is that of Collinson and Thompson (1982). In the carbonate field the first book to reflect the progress in matching process with product was that edited by Ham (1962). However, it scarcely mentioned diagenesis, understanding of which advanced rapidly in the 1960s to culminate in the most important book in the limestone field, that by Bathurst (1971, 1975). The importance of biological processes was for long underestimated by most sedimentologists and few textbooks mentioned organisms except as disturbers of sediment. German authors, however, such as Seilacher and the Tübingen school, and Schäfer (1972) cultivated the science of analysing faunas and the effect of their life and death patterns upon sediments. The genesis of sediments was stressed particularly by Blatt, Middleton and Murray (1972, 1980) who emphasized the mechanisms and processes of physical and chemical sedimentation.
In these early books, environmental analysis was not discussed at length. However, Reineck and Singh (1973) covered both physical and biological sedimentary processes and structures and also modern clastic sedimentary environments, with particular emphasis on the shallow-marine. J.L. Wilson (1975) did the same for carbonate facies, emphasizing the impact of organic evolution on carbonate build-ups.
These were followed by Reading (1978) and R.G. Walker (1979). The former started from a consideration of modern environments and moved through process to facies. R.G. Walker (1979, 1984) emphasized facies using modern environments as an aid to their interpretation. The latest edition (R.G. Walker & James, 1992), which is the best general introduction, expands to include sequence stratigraphic concepts, especially sea-level controls. Galloway and Hobday (1983) stressed terrigenous clastic sediments, especially their economic aspects while the Association of American Petroleum Geologists published two magnificently illustrated volumes, one on sandstone depositional environments (Scholle and Spearing, 1982) and the other on carbonate depositional environments (Scholle, Bebout & Moore, 1983). The applied elements of sedimentology were also emphasized by Brenchley and Williams (1985). The Spanish equivalent to the present book is that edited by Arche (1989).
Meanwhile there was a growing number of special publications from societies and associations and of textbooks. The most important special publications are those published by the Society of Economic Paleontologists and Mineralogists (SEPM) and the International Association of Sedimentologists (IAS). Many of these deal with specific environments and are listed under Further Reading in the appropriate chapters. In addition, the economic importance, especially to the petroleum industry, of sedimentology has resulted in a number of special publications from the American Association of Petroleum Geologists (AAPG) and the Canadian Association of Petroleum Geologists (CAPG).
Throughout these decades some geologists kept a broader perspective alive by considering the relationship between sedimentation and tectonics, firstly through the concept of the geosyncline (Kay, 1951; Aubouin, 1965) and then through that of plate tectonics (Mitchell & Reading, 1969; Dewey & Bird, 1970; Dickinson, 1971a). Many books were devoted to tectonics and sedimentation (e.g. Burke & Drake, 1974; Dickinson, 1974a; Dott & Shaver, 1974). However, it was not until the late 1970s that two major developments took place. One was the consideration of sedimentary rocks on a broader basis than had been done by the rather narrowly focused sedimentologists of the 1960s and 1970s. This was to look at basins in their entirety, using both geophysical and stratigraphical methods and data acquired from extensive outcrops and the subsurface. This led to a number of books (e.g. Miall, 1984; P.A. Allen & J.R. Allen, 1990) on basin analysis. Meanwhile, petroleum geologists developed seismic stratigraphy, turning it into sequence stratigraphy (Sect. 2.2) (Payton, 1977; Wilgus, Hastings et al., 1988), a philosophy rooted in the concepts of stratigraphy and sedimentation of Sloss (1950, 1963) (Sect. 2.2). Thus there has been a return, in part, to the stratigraphical concepts of the early part of this century that were put into abeyance by the process, facies sedimentologists of the 1960s and 1970s.
1.2 Scope and philosophy of this book
The prime purpose of this book is to show how ancient environments may be reconstructed by interpreting first the process or processes which gave rise to sedimentary rocks and then the environment in which these processes operated. To achieve this, an understanding of the factors, such as climate, tectonics and changing base level, that control the environment, both modern and ancient, is essential.
The reconstruction of environments requires the following.
Thus the emphasis of the book will be on:
Only a fraction of the material available has been incorporated in this book. Innumerable examples, ideas and alternative models have had to be eliminated to save space. We hope we have brought out the more important ones but judgement is subjective, and none of us can claim to have read everything, let alone understood all that has been published on the subjects covered in our chapter. The chapters are not comprehensive reviews of the subject. To achieve that would require at least one textbook for each chapter; in most cases, these have already been published. What we have tried to do is to produce a readable text, covering the essentials, which is a starting point for the subject, in particular for those either just entering sedimentology, or for those who want an introduction into those areas of sedimentology that they are not working on.
1.3 Organization of the book
There is no unique division of environments and there is no simple match between environment, processes and facies. An environment is a particular set of physical, chemical and biological variables; a facies is a body of rock with specified characteristics, and many processes operate in more than one environment.
Matching environment, process and facies is seldom easy, and frequently decisions have had to be made between dividing the book on a basis of environment, process or of facies. Division of most chapters was on environment, because that is the prime emphasis of the book and most major environments are dominated by a particular suite of processes, which are then covered in that chapter. Many facies, however, cut across several chapters. A particular difficulty is presented by evaporites which are now known to have occurred in almost as many environments as have sandstones, ranging from deserts to lakes, coastal flats and deep seas, and therefore the chapter on arid shorelines includes deep-water evaporites as well as those found in sabkhas. Inevitably, because there is a continuum from subaerial through coastal to deep seas, environments have had to be arbitrarily divided. Artificial boundaries have had to be placed between individual chapters. Although chapters have been selected mainly on geographical environment, in some cases emphasis has been given to the facies, as in the separation of ‘Shallow clastic seas’ from ‘Shallow-water carbonate environments’. Here the processes which transport and deposit the sediments are essentially the same but, because the sediment is derived, in one case from the erosion of mainly extrabasinal sources and in the other case from biochemical intrabasinal sources, the facies types and facies patterns are very different, especially the relationship of sequences to relative sea-level changes. In the case of ‘Desert aeolian systems’ and ‘Glacial sediments’ climate, with consequent distinctive processes, is the prime factor in division. The same is true for ‘Clastic coasts’ and ‘Marine evaporites: arid shorelines and basins’ which have been separated on the basis of their distinctive climates and, therefore, facies. ‘Volcanic environments’, like ‘Glacial sediments’, have unique sources of sediment and embrace all other environments.
Within chapters, the organization of sections is no less difficult. Should the prime section headings be based on controlling factors, environment, process, facies or even grain size? How closely should modern and ancient be intertwined, especially when there is a complete gradation from a process and product that can be measured and observed today through those that we can observe within a generally known environment that was formed a few years ago, to the clearly ancient rock record where all the controlling processes and factors have to be inferred? In many chapters, the section headings and their organization are substantially different from previous editions, and during revisions they were changed more than once. No pattern is ideal, and the end is a compromise that must result in some overlap. Because of this overlap between chapters and sections within chapters, the reader needs to make the links by reference to the Contents and the Index. Cross referencing between chapters has been kept to the minimum to avoid breaking the text.
Further reading
Arche A. (Ed.) (1989) Sedimentología, 1, 541 pp.; 2, 526 pp. Nuevas tendencias 11, 12, Consejo Superior de Investigaciones Científica, Madrid.
Brenchley P.J. & Williams B.P.J. (Eds) (1985) Sedimentology: Recent Developments and Applied Aspects, 342 pp. Spec. Publ. geol. Soc. Lond., 18.
Einsele G., Ricken W. & Seilacher A. (Eds) (1991) Cycles and Events in Stratigraphy, 955 pp. Springer-Verlag, Berlin.
Friedman G.M., Sanders J.E. & Kopaska-Merkel D.C. (1992) Principles of Sedimentary Deposits: Stratigraphy and Sedimentology, 717 pp. Macmillan, New York.
Galloway W.E. & Hobday D.K. (1983) Terrigenous Clastic Depositional Systems, 480 pp. Springer-Verlag, New York.
Leeder M.R. (1982) Sedimentology: Process and Product, 344 pp. George, Allen & Unwin, London.
Scholle P.A. & Spearing D. (Eds) (1982) Sandstone Depositional Environments, 410 pp. Mem. Am. Ass. petrol. Geol., 31, Tulsa.
Scholle P.A., Bebout D.G. & Moore C.H. (Eds) (1983) Carbonate Depositional Environments, 708 pp. Mem. Am. Ass. petrol. Geol., 33, Tulsa.
Walker R.G. & James N.P. (Eds) (1992) Facies Models: Response to Sea Level Change, 409 pp. Geol. Ass. Can., Waterloo, Ontario.
2
Controls on the sedimentary rock record
H.G. Reading & B.K. Levell
2.1 Controlling factors
Sedimentation results from the interaction of the supply of sediment, its reworking and modification by physical, chemical and biological processes, and accommodation space – that is, the space available for potential sediment accumulation. In many settings reworking allows only a small proportion of the delivered sediment to be preserved. Most is removed either almost immediately by an increase in physical energy, such as that produced by a storm or tidal current, by sediment instability as with deposition on a slope, by chemical dissolution, or, over a longer period of time, by environmental changes such as channel migration or shoreline advance or retreat. The accommodation space is controlled largely by external processes such as changes in sea level, climate, tectonic movements, volcanic activity, compaction and longer-term subsidence rates which together define a depositional base level.
2.1.1 Sediment supply
The supply of sediment varies in volume, composition and grain size, as well as in the mechanism and rate of delivery. These variations are the result of the climate, basinal water chemistry and the tectonics and bedrock geology of the source area. Where the supply of land-derived (terrigenous) sediment is abundant, siliciclastic sediments predominate. Where this is low or absent, physical erosion may be effective and chemical, biochemical and biological processes have a chance to produce or modify sediments. Evaporites, carbonates, diatomites, cherts, ironstones, phosphorites and carbonaceous sediments then predominate. There is thus a fundamental distinction between terrigenous extrabasinal particles derived ultimately, if not immediately, from outside the sedimentary basin, and biochemical intra-basinal particles generated within the depositional basin. Erupting volcanoes, however, upset this general rule and may supply large volumes of intrabasinal terrigenous volcaniclastic material of unusual compositions. In addition, substantial amounts of ‘intrabasinal’ terrigenous sediment may be supplied, for instance, from uplifted fault blocks or pre-rift thermal domes within large basins.
TERRIGENOUS SYSTEMS
A knowledge of the source or provenance of detrital sediment adds substantially to our understanding of depositional basins. The approximate grain size and composition of the lighter fraction, the heavy mineral population, and isotopic signatures can yield invaluable information on the nature of the bedrock and weathering processes in the source area.
Each depositional system has its own immediate source area. Deep-water systems such as submarine fans are supplied from an adjacent shelf or delta whose morphology, size, tectonics and climate they reflect. Shelves are supplied from coasts or coastal plains, which may be in part deltaic; deltas are supplied from alluvial systems which themselves reflect the features of the hinterland. Thus contemporaneous depositional systems are linked together as ‘systems tracts’ (Sect. 2.4). The downcurrent systems are directly controlled by the sediment supplied, or not supplied, by the upcurrent systems.
The rate of sediment supply is generally controlled more by the volume of sediment available in a given time interval than by transport capacities. Rates vary by many orders of magnitude, even within those systems dominated by terrigenous sediment.
The sizes and gradients of terrigenous depositional systems are related to the grain size or calibre of the sediment (Reading & Orton, 1991). Coarse-grained or gravel-rich alluvial, deltaic and deep-sea systems are all relatively small and steep with delta plain areas of 1–100 km2 and gradients of >5 m km−1 (Table 6.1). Deep-water coarse-grained submarine fans have radii of 1–50 km and slope gradients of 20–250 m km−1 (1°–14°). Owing to the high competence required, sediment transport and deposition are largely by short-lived, but frequent, catastrophic events such as floods initiated by rain storms or slumping caused by seismic shocks.
Medium-grained or sand-rich systems tend to be intermediate in size, with moderate gradients. Delta plains have areas of 100–25 000 km2 and gradients of 5.0–0.1 m km−1. Deep-water fan systems have radii of 10–100 km and gradients of 18–6 m km−1. The range of grain sizes available means that physical processes of transport and deposition operate over a wide range of energy levels. The resulting facies tend to be well differentiated, reflecting the full range of basin processes – river flow, tidal, storm, wave and wind energy of the basin as well as its morphological pattern. These systems are therefore particularly suitable both for modelling sedimentary dynamics and for the application of process models to environmental interpretation.
Although some fine-grained mud-rich systems can be small, the majority are very large, with low gradients. Delta plains have areas of 20 000–460 000 km2 and gradients of 0.1–0.001 m km−1. Submarine fans have radii of 100–3000 km and gradients of 5–1 m km−1. The very low gradients of muddy shorelines make them very sensitive to sea-level changes caused either by tides or by longer-term rises and falls. Where sediment supply is high, the rapid deposition of mud and silt causes frequent slumping on delta slopes and submarine fans, in spite of the low gradients. On the other hand, slope aprons, where sediment accumulation is slow, are characterized by infrequent but very large slumps. The size of most mud-rich systems means that there is a considerable time lag between major changes in the controlling factors and the sedimentary response. Changes in one system may eventually affect patterns of sedimentation in the next system downcurrent, though with delay times of perhaps millions of years.
The pattern of sediment delivery to the basin is also important. Sediment may be supplied from a single point source, from multiple sources, from a linear source, from all around the basin, or from one end or the side of the basin. Such patterns may change with time, with important consequences. At one instant there may be a single point source and, in some situations, this may stay fixed for a long period. However, more commonly, sources change over time and thus over the longer, geological time periods in which rock successions accumulate, sediment sources are likely to have switched on one or more occasions. Modern siliciclastic depositional models, based on ‘instantaneous’ present-day examples, tend to emphasize single point sources. Ancient examples more commonly suggest multiple sources.
Volcanoes are also major contributors of sediment, yielding the whole range of grain sizes both below and above any water level and with variable composition (Sect. 2.1.10; Chapter 12).
BIOCHEMICAL AND CHEMICAL SYSTEMS
The production of biochemical sediment in lakes and the sea is controlled primarily by the nature and productivity of the biota which, in turn, depend on temperature, water chemistry, and the penetration of light into the water. In shallow water, although non-skeletal grains such as ooids, peloids and some lime mud can be important, carbonate production is primarily from the skeletal parts of animals and plants (algae) (Sect. 9.2). Thus the productivity of the ‘carbonate factory’ depends not only on the appropriate conditions of salinity, nutrients and temperature, but especially on light intensity. This is because some producers such as red and green algae are phototrophic. Others, such as mixotrophs (hermatypic corals and larger benthic forams), are light dependent because they use symbiont algae. Yet others, such as molluscs (especially bivalves), bryozoans, crinoids and brachiopods, are or were suspension feeders that ultimately depend on phytoplankton.
In deep basins only near surface waters penetrated by light produce significant sediment. Planktonic foraminifera and coccoliths yield calcareous material; Radiolaria, diatoms, siliciflagellates and some sponges yield siliceous material; upwelling phosphorus-rich waters yield phosphates. Productivity varies substantially (Sect. 10.2.1), with nutrient-rich waters in zones of oceanic upwelling producing 10 times as much pelagic sediment as nutrient-poor oceanic waters. However, of equal importance to productivity in determining sediment accumulation is the rate of dissolution of the particles as they descend through the water column. Calcareous particles are particularly susceptible to dissolution such that below a certain depth, the calcite compensation depth (CCD), few calcareous particles survive and the bottoms of deeper basins are covered by siliceous rather than calcareous oozes.
Evaporites are precipitated directly from sea or lake waters that have become concentrated to form brines. Their composition depends not only on the salinity of the brine but on its ionic make up, which, in lake waters, may vary considerably. Rates of deposition can be faster than for any other normal process of sedimentation, with vertical accretion up to 100 m ky−1 (Table 8.3).
2.1.2 Climate
The two main aspects of climate are temperature and precipitation, but, locally, wind regimes may also be significant. Not only are mean annual temperature and precipitation important, but also their fluctuations, both seasonal and non-seasonal, and the magnitude and frequency of extreme events.
The meteorological patterns of the Earth are primarily a consequence of the interaction of the Sun’s radiation, the rotation of the Earth and the distribution of continents and oceans. If the Earth were to stop rotating, rising hot air at the equator would blow towards the poles where it would be cooled, become heavier, descend, and then return to the equator. However, this simple convection pattern (Fig. 2.1) can be considered to be disturbed by the Earth’s rotation, which sets up a powerful deflecting effect, known as the Coriolis force. At the equator the Earth rotates at a velocity of 1600 km h−1 in an easterly direction; this velocity decreases away from the equator until it reaches zero at the poles. If a mass of air or water moves away from the equator it starts with the Earth’s equatorial velocity and passes to places where the rotational velocity is lower. The moving mass therefore tends to travel eastward faster than the Earth beneath it and the further north or south it goes the more it turns towards the east (Fig. 2.1). If a mass moves towards the equator, where the rotational velocity is higher, it tends to be left behind by the movement of the Earth and is deflected towards the west. Thus in the northern hemisphere the deflection is towards the right; in the southern hemisphere it is towards the left. This produces the anticlockwise rotation of water entering lakes and oceans such as the North Atlantic, in the northern hemisphere, and clockwise rotation in the southern hemisphere.
Hot air rising at the equator is responsible for the humid tropical rain belt. This is because, for precipitation to occur, an air mass must rise sufficiently for cooling by expansion to produce condensation of moisture. Although the hot air that rises at the equator and blows towards the poles is deflected towards the east, surface winds in the tropical belts either side of the equator blow towards the equator and westwards as the NE and SE trade winds. This is because the heated air from the tropics passes into latitudes that have a smaller circumference than at the equator, and the air becomes crowded, thus raising the pressure and forcing air downwards to give the high-pressure subtropical calm belt where deserts form. Secondary Hadley circulation cells known as the Hadley, Ferrel and Polar cells (Fig. 2.1) are created with winds blowing towards the equator deflected westwards by the Coriolis force and those blowing towards the poles deflected towards the east. In this way belts of trade winds, and of westerlies, develop either side of, the high pressure belt. Meanwhile, the descending air at the high pressure arid polar regions is deflected westwards as cold surface air wedges under the warmer westerlies along the Polar front. These moisture-laden winds flow up above the cold wedge to cause the relatively steady precipitation of the temperate regions. Such idealized cell patterns are perturbed by the distribution of areas of land and ocean. They also migrate on seasonal and longer-term cycles.
Figure 2.1 General planetary circulation of the lower atmosphere showing the Hadley circulation pattern of the three main cells in the northern hemisphere. (a) Present-day zonal pattern of high and low pressure belts and associated pattern of prevailing winds (after Duff, 1992). (b) Predicted position of circulation patterns and climatic belts at both climatic maximum and climatic minimum due to changes in the amount and distribution of heat received from the Sun (after Mathews & Perlmutter, 1994).
The climates of land areas have been divided in many different ways on the basis of average temperatures and rainfall, and their fluctuations and extremes, and there is no simple method of classification based on a single parameter. A practical zonation that relates continental geomorphic processes, environments and the supply of sediment to temperature and rainfall patterns, and one which is quoted in many text books (e.g. Chorley, Schumm & Sugden, 1984; Summerfield, 1991), is that based on Tricart and Cailleux (1972) (Fig. 2.2; Table 2.1). In that scheme eight morphoclimatic regions are defined – that is, large areas where distinctive associations of geomorphic processes occur. The classification is based on: (i) mean annual temperature; (ii) mean annual precipitation; (iii) mean number of wet (>50 mm precipitation) months; and (iv) mean temperature of the warmest month. In addition, there are azonal mountain belts that do not fit the spatial pattern because they have a suite of climatic zones dependent on altitude and on the presence of orographic barriers.
At the extremes are the relatively simple non-seasonal climatic zones polar glacial, arid and humid tropical. In polar glacial zones frost weathering is at a maximum, other types of mechanical weathering are moderate and chemical weathering at a minimum. There is little mass wasting or river flow, but glacial scour and wind action are at a maximum, the latter removing most of the silt and leaving gravel, sand and boulder clay. In arid desert zones, mechanical weathering by salt and diurnal temperature fluctuations and wind action are at a maximum; chemical weathering is at a minimum. Sand and silt are the dominant mobile sediments. In humid tropical zones covered by rainforests, chemical weathering may give rise to a thick blanket (up to 100 m) of fine-grained debris and deep soil development. This prevents mechanical weathering and the consequent lack of coarse sediment limits mechanical erosion. Fine-grained clay-rich sediments are dominant.
The other zones have distinct seasons. The periglacial zone, characterized by permafrost and tundra vegetation, has a very wide seasonal temperature range and extensive frost, mechanical weathering and wind action. Chemical weathering and glacial scour are minimal but on slopes mass wasting by talus creep and solifluction are ubiquitous. Fluvial processes are very strongly seasonal with a short sharp peak discharge in the spring. Though dominated by gravel and sand, there is more clay than in the glacial zone. The dry continental zone, with its grassland steppes, also has a very wide seasonal temperature range. This leads to highly seasonal flooding and mass wasting processes. There is low to moderate mechanical (frost and wind) and chemical weathering. The tropical semi-arid zone, composed of dry, thorny savanna, marginal to hot deserts, is similar to the dry continental zone, except that it lacks the low temperatures. There is no frost weathering and both mechanical and chemical weathering are low to moderate. Fluvial processes are episodic and may be powerful where vegetation is sparse. Wind is locally important. The tropical wet-dry zone or wet savanna is a very broad type that embraces quite a range of climates, which includes both those that have extremely seasonal wet-dry periods and those that have long wet periods interspersed with short dry ones (e.g. Cecil, 1990). Mechanical weathering is low but mass wasting and fluvial processes are considerable. The dominant weathering process is chemical with penetration deep below the surface giving a range of surface duricrusts, bauxite (alcrete), laterite (ferricrete), and silcrete, depending on the degrees of temperature and rainfall, and the composition of the bedrock. The humid temperate mid-latitude zone, covered by well-developed soils and deciduous broadleaved trees, is also a very broad group. It includes the whole of subarctic Europe and the eastern half of North America. Most processes operate to a moderate extent. Frost action is very varied and mechanical weathering somewhat less important than chemical weathering. Erosion is modest in this zone.
Figure 2.2 Present world distribution of climates and corresponding morphoclimatic zones (from Tricart & Cailleux, 1972; Summerfield, 1991). Notice latitudinal zonal pattern is modified by azonal factors, especially the distribution of land masses and oceans and the effects of prevailing winds on coasts.
Table 2.1 Principal features of morphoclimatic zones (based partly on data in Chorley, Schumm & Sugden, 1984; Summerfield, 1991).
Although climatic zones broadly reflect latitude, the distribution of continents and oceans, as well as mountains, substantially modifies this general pattern. Owing to the circulation of ocean currents, similar temperatures are found today on either side of the Atlantic at latitudes that differ by as much as 20°. Extremes of rainfall may occur at the same latitude. For example, in the subtropical trade wind belts the westerly flow of winds draws heavy moisture from the oceans on to eastward-facing coastlines, while the complementary west-facing coasts are where all the present-day coastal deserts lie. Southern Asia sees a transition from some of the highest rainfalls in the world in the east (Bangladesh) to the lowest (Arabia) (Fig. 2.2). In addition, it must be remembered that the local relief is crucial in governing rainfall amounts, and in a land of rugged relief, whether due to tectonic activity or volcanoes, arid basins may pass over distances of less than 50 km into mountains with substantial precipitation.
Climatic zones are never stationary and there is abundant evidence in the sedimentary record that climatic changes occur on a variety of scales. The thermal equator shifts seasonally, interacting with large landmasses to intensify summer heating and winter cooling of the atmosphere relative to the ocean. This seasonal differential heating leads to a significant poleward shift of the junction between the northern and southern hemispheric Hadley cells (the intertropical convergence zone). It creates monsoonal climates near the equator by drawing in monsoonal rains during the summer as the Asian continent heats up.
Longer-term climatic change is inevitable. This is in part due to plate tectonic movements resulting in the creation of new oceans and mountains, and variations in the amount of volcanic dust and CO2 in the atmosphere. It is also due to extraterrestrial factors such as variations in insolation – that is, in the strength and distribution of incoming solar radiation as predicted by Milankovitch theory (Sect. 2.1.5).
Migration of the three Hadley circulation cells by as much as 30° latitude has been postulated (Perlmutter & Mathews, 1989; Mathews & Perlmutter, 1994). This results in a similar movement of latitudinal climatic belts, termed cyclostratigraphic belts, within which climatic and environmental end members can be defined (Fig. 2.1b). These are the extremes that can occur. More commonly the climatic shift is less. For example, at the present day, the subtropical high pressure zone, where deserts form, is between 20° and 30° latitude. However, during phases of climatic minimum (maximum cooling) it might have lain between 10° and 20° and at phases of climatic maxima between 30° and 40° (Fig. 2.1b). The high pressure zone also probably contracted during the climatic minimum with a consequent increase in wind power far beyond that of the present day. Oscillation between the two is over any of the time scales of Milankovitch cycles, but may be as little as half a precession cycle (less than 10 ky).
Each cyclostratigraphic belt has a different sequence of climatic change. In general, during a climatic minimum the Earth is expected to be cooler and drier than during a climatic maximum, and this is true for those areas affected by the Hadley and Polar cells. However, for the mid-latitude Ferrel cell, which moves towards the equator during a climatic minimum, belts between latitudes 35° and 50° become wetter (Fig. 2.1b).
Climate is a prime control on many sedimentary facies, and therefore they can be excellent palaeoclimatic indicators. The facies of lagoons and lakes, as well as soils, are particularly valuable as indicators of past climates. Temperature is a major control on the formation of evaporites, glacial tills, some oolites, palaeosols, vegetation, and many faunas. Rainfall affects evaporites, aeolian dunes, clay mineral provinces, palaeosols, vegetation, and fluvial, lagoonal and lacustrine morphology. In lakes, climatic changes are recorded not only in facies changes, but also in base-level and salinity changes which can affect their stratigraphy as much as tectonic movements.
Climate also governs sediment yield. After the evolution of land plants in Devonian times, equable climates, as in the present-day humid tropical belt and temperate humid mid-latitude regions, have promoted a good cover of vegetation, and erosion is kept to a minimum with clays being the dominant sediment. Inequable climates tend to yield greater volumes of sediment, especially of sands and gravels. In pre-Devonian times, the absence of land plants promoted highly variable discharges and the development of braided rivers even in regions of relatively constant and abundant rainfall.
2.1.3 Tectonic movements and subsidence
Tectonic movements affect sedimentation in a number of different ways and on many different scales. Isostatic movements are essentially an attempt by the lithosphere to balance spatially variable loads: for example, vertical adjustments in the lithosphere arising from its loading or unloading by water, sediment or ice, or by thermal or dynamic changes in the mantle. Sediment loading may enhance crustal subsidence by a factor of 3 when compared with an exactly similar basin that is sediment starved and loaded only with water. However, it is important to stress that the loading or unloading of one part of the crust also has substantial effects outside the immediate area as a result of the flexural rigidity of the lithosphere. Extended discussion of basin analysis can be found in Miall (1990) and P.A. Allen and Allen (1990).
On a global scale, the distribution and movements of lithospheric plates lead to the changing pattern of oceans and continents that controls the size and nature of the larger source areas, sediment transport paths and sedimentary depocentres. Continental collision zones, such as the Himalayas, produce the largest volumes of sediment. Many mountain belts, such as the American cordillera, the Pyrenees, the Alps, and Zagros mountains of Iran have an adjacent foreland basin, formed as a result of crustal loading by the nearby mountain belt. These are ready receptacles for sediments derived from the mountains. Inboard from subduction zones there is a discernible pattern of accretionary wedge, forearc, volcanic arc and backarc basin. Strike-slip belts are characterized by a linear series of rather small basins and highs of great local complexity. Extensional rifts also have relatively small scale basins and highs but these are generally of a larger size (50–70 km × 20–40 km) than those of strike-slip basins (20–40 km × 10–20 km) In both cases there is much seismic activity and small-scale but frequent mass flows are deposited close to fault scarps. Individual fault blocks may be important local sources of sediment within larger basins. In contrast, the broad passive continental margins show very large-scale asymmetric facies patterns that, although segmented by transform faults, may extend for thousands of kilometres along depositional strike. They have an early phase of rapid rift-related, fault-controlled subsidence, creating very large accommodation space, followed by slower subsidence due to thermal cooling of the lithosphere. On continents, large, gently subsiding sag basins give relatively slowly deposited, extensive, thin sedimentary sequences in which individual progradational and transgressive sequences migrate laterally over hundreds of kilometres, resulting in rather rapid changes in the vertical sequence and many hiatuses. The large stable cratons of the North American, Russian or Australian shield areas have little accommodation space and have been relatively starved of sediment. Deposition on their margins is consequently very responsive to relative sea-level changes whether eustatic or due to gentle tectonic warping. Overall sedimentation rates have been so slow that sedimentary processes were dominated by erosion and non-deposition and the formation of glauconite and phosphorite when the area was flooded. Unconformities, discontinuities and hiatuses abound.
On a smaller scale, movements along faults, the growth of folds, block tilting, differential subsidence and uplift on scales of 1-100s m provide a critical and delicate control on the type, thickness and distribution of sedimentary facies. The migration of rivers and deltas, the location of fluvial and deltaic depocentres and of carbonate banks, may all be governed by differential tectonic movements. These may be a consequence of differences in the underlying basement, in particular of heat distribution and buoyancy of crust, of active tectonic movements across faults, of differential compaction in the underlying sediment, or of lateral and vertical movements of mobile salt and shale.
2.1.4 Sea-level changes
Local changes in water depth (the distance between the water surface and water bottom) may be caused by changes in deposition: for example, in the terrigenous sediment budget, the production of biochemical sediment, or by variations in the rate of removal of sediment by marine processes.
Sea-level changes are of two types. Eustatic sea-level change is a function of sea-surface movement relative to some fixed point such as the centre of the Earth. Relative sea-level change is measured relative to some moving point in the underlying subsiding crust, or near the sea floor. It is therefore a function of both sea-surface movement and sea-floor movement in the fixed frame (Posamentier, Jervey & Vail, 1988; Posamentier & James, 1993).
Sea-level changes occur on a variety of scales. Short-term variations include those due to waves, tides, either daily (diurnal), or more commonly twice daily (semi-diurnal), so-called annual tides, more or less instantaneous storm surges, and tsunamis (giant sea waves produced by earthquake shocks and volcanic phenomena such as caldera collapse or entry of pyroclastic flows into the sea). Waves may be up to 20 m high. Semi-diurnal tides can also have ranges of 20 m, but most tidal ranges are much less, with a tidal range over 4 m considered to be macrotidal. No seas are completely tideless. The so-called tideless seas, such as the Mediterranean and the semi-enclosed Gulf of Mexico, have a tidal rise and fall of a few tens of centimetres. Even Lake Michigan has a tidal range of 10 cm. Slightly longer-term, seasonal or annual tides also raise and lower sea level. Sea level may be lowered by a decrease in temperature and/or rise in salinity (which increases the density of sea water), by an increase in atmospheric pressure, or by an offshore wind. Sea level may be raised by an increase in temperature, a drop in salinity such as when melting of winter snow may add substantially to the water budget, by a decrease in atmospheric pressure, by oceanic currents that impinge on a coastline or by strong seasonal winds (e.g. the monsoons) that drive water on to the windward coasts of South East Asia, especially in the Bay of Bengal. Episodic catastrophic events that alter sea level include storm surges associated with onshore winds. These are often combined with sudden falls in atmospheric pressure during the passage of storms or hurricanes that raise sea level several metres above normal. Exceptional events such as tsunamis and/or major sedimentary slides into the ocean may cause sea level to rise instantaneously 27 m or more above the normal on the margin of the open ocean, and up to 500 m when amplified in enclosed fjords.
Longer-term variations in relative sea level arise through the interplay of changes in global sea level and basin floor subsidence and uplift. The most important effects on a regional scale are mountain building, volcanism, sediment compaction and isostatic movements due to sediment loading, ice loading and unloading (glacioisostasy), water loading and unloading (hydroisostasy) and thermal mechanisms. Glacioisostatic oscillations are a consequence of loading of crust by ice followed by crustal rebound as the ice melts and the load is removed. Rates and amounts of uplift are enormous, with rises of over 250 m in less than 10 000 years in northern Canada and Scandinavia (Sect. 11.5.2). Here the relative sea-level falls far outpace any rise of sea level due to the melting of the ice. Around this zone of subsidence and rebound is a peripheral flexural forebulge which operates in a reverse fashion – that is, it rises as ice loads the centre and sinks as the ice melts, so adding to the sea-level fall and rise in the peripheral region (Lambeck, Cloetingh & McQueen, 1987). Hydroisostasy is a consequence of any form of sea-level change. A fall in sea level reduces the water load on the sea floor or continental margin and causes uplift. A rise in sea level increases the water load and causes subsidence. The amount of movement reduces the effect of the sea-level change by about a third. This effect can be significant on continental margins and even more so during large desiccating and refilling events in restricted basins such as the Mediterranean during Messinian times when many hundreds of metres of sea-level movement may have taken place (Sect. 8.1).
Global (eustatic) sea-level changes may be brought about by changes in the volume of the ocean basins, changes in volumes of water in the world’s oceans, changes to the hypsometric curve (the aerial distribution of global elevations) and in changes to the geoid (an equipotential surface of the Earth’s gravitational field corresponding to mean sea level in the oceans). This latter surface today has a relief of 180 m relative to the centre of the Earth and must have fluctuated in the past, perhaps by as much as 50–250 m on a My time scale and 60 m over the last 20 ky. These fluctuations are a result of shifting distributions of plates, ice or water masses, as well as changes in gravity forces such as those set up during Milankovitch effects (Sect. 2.1.5) (Mörner, 1994).
Changes in the volume of the ocean basins may have many causes (Donovan & Jones, 1979; Harrison, Brass et al., 1981; Pitman & Golovchenko, 1983).
All these changes are long lived, lasting for millions of years, and slow, only about 1 m My−1 (1.0 mm ky−1).
Changes in the volume of water in the world’s oceans may be caused by changes in mean ocean temperature (Donovan & Jones, 1979), by the waxing and waning of ice sheets, or by the sudden flooding or desiccation of isolated ocean basins such as the Miocene Mediterranean or the Cretaceous South Atlantic. The first mechanism may produce sea-level changes of as much as 10 m, but these are very slow. The second and third mechanisms, however, are several orders of magnitude faster. As the ice melted after the last Ice Age, sea level rose at a rate of 10 m ky−1 from about 15 000 to 6000 BP with a maximum rate of 2.4 m per century. Total rise due to melting of ice was about 100–130 m, but this figure varied around the world due to geoidal changes. If the remaining ice sheets in Antarctica and Greenland were to melt fully, the resulting rise should be about another 65–80 m. However, the compensatory effects of hydroisostasy on the ocean floor and continental margins would reduce the rise of sea level to about 40–50 m. If glacioeustatic falls have similar rates, we have to envisage sea-level rises and falls of up to 150 m at rates that average about 10 m ky−1 and are occasionally more rapid. However, as mentioned above, these rates may be counteracted on a more local scale by glacioisostatic effects.
Desiccation of small ocean basins and their reflooding, as has been documented for the Miocene of the Mediterranean, is another means by which substantial volumes of water could be added to the oceans during desiccation and lost during the reflooding. Although the amount of rise and fall is perhaps only a tenth of that due to glaciation (10–15 m if the whole present Mediterranean were desiccated) the rate would be very fast, perhaps as little as 1000 years if the present Mediterranean were suddenly cut off at the Straits of Gibraltar.
