Sedimentary Environments

Processes, Facies and Stratigraphy

Harold G. Reading, Harold G. Reading

• Editore: Wiley-Blackwell
• Anno: 2013
• ISBN:
  9781118687635
• Edizione: 3
• Argomento: Sciences physiques
• Categoria: Géologie et sciences de la Terre

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.

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.

1 A thorough description of the rocks, either in the field or in core, with additional laboratory data obtained from samples collected to answer specific questions. Since time is limited, rock description is inevitably selective, emphasizing some features, underplaying others and rejecting yet others as quite unimportant. The selection depends on the judgement, experience and purpose of the investigator. Judgement and experience take time to acquire and can only be gained by seeing lots of rocks. The absence of certain features is often as important as their presence. For example, the consistent absence of shallow-water features, rather than any positive evidence for great depth, leads sedimentologists to infer that most turbidites were deposited in deep water. The utilization of negative evidence requires a familiarity with a wide range of sedimentary rocks and environments.

2 An awareness of processes so that, simultaneously with rock description, the strength or direction of the current or the type of flow which carried and deposited each grain is being considered. Such questions as ‘What was the oxidation state, salinity or pH of the water?’ or ‘What forms of life were extant?’ can also be asked. We also have to consider the later alteration or diagenetic processes which may have changed not only the colour of the rocks but also their grain size and composition. Particular processes are seldom confined to one environment, though they may be absent from some, and therefore similar rocks may form in different environments.

3 A knowledge of present-day environments, the processes which operate within them and the factors that control them. We need to know how environments evolve both under stable conditions and as sea level, climate, tectonic activity or sediment supply change. Our understanding of environments is bounded not only by the limits on knowledge of the present day, whole regions still being virtually unexplored, but also by the uniqueness of the present. For example, the recent rise of sea level allows us readily to develop models of transgressive sedimentation in shallow seas but makes it difficult to develop models for periods of relatively stable or falling sea level. It is salutary to consider how difficult it would be to conceive a model of glacial sedimentation had the human race developed in an entirely non-glacial period. It would have taken a courageous scientist to postulate, from a limited knowledge of sea ice and snow falls, the hypothesis of large ice caps and glaciers which could erode and deposit large quantities of sediment.

Thus the emphasis of the book will be on:

1 environments, reviewing modern environments, with their associated processes and products;

2 processes, concentrating on those that occur in each environment and showing how they relate to the resultant sediment: they are not discussed for their own sake as there are already several good textbooks on processes (e.g. Pye, 1994) and the genesis of sedimentary rocks and structures (e.g. Collinson & Thompson, 1982; J.R.L. Allen, 1984, 1985);

3 facies, stressing field data, facies relationships, sequences and associations;

4 sequence stratigraphy, or facies analysis using a chronostratigraphic framework defined by extensive correlative surfaces;

5 controls, emphasizing the interplay of sediment supply, tectonics and sea-level changes; and

6 geological applications, illustrating how sedimentary rocks are related to their geological background and how the recognition of sedimentary processes and environments illuminates our understanding of past climates, the chemistry of the oceans and the land, the development of life and world tectonics.

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 p...

Indice dei contenuti

• Cover
• Contents
• Title Page
• Copyright
• Contributors
• Preface
• 1 Introduction
• 2 Controls on the sedimentary rock record
• 3 Alluvial sediments
• 4 Lakes
• 5 Desert aeolian systems
• 6 Clastic coasts
• 7 Shallow clastic seas
• 8 Marine evaporites: arid shorelines and basins
• 9 Shallow-water carbonate environments
• 10 Deep seas
• 11 Glacial sediments
• 12 Volcanic environments
• 13 Problems and perspectives
• References
• Index