©1996 by Blackwell Science Ltd
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First published 1996
12 2009
Library of Congress Cataloging-in-Publication Data
Sequence stratigraphy / edited by Dominic Emery and Keith Myers; with contributions from George Bertram ... [et al].
p. cm.
Includes bibliographical references and index.
ISBN 978-0-632-03706-3
1. Geology, Stratigraphic.
I. Emery, Dominic II. Myers, Keith. III. Bertram, George T.
QE651.S458 1996
551.7 – dc20
A catalogue record for this title is available from the British Library.
For further information on
Blackwell Publishing, visit our website:
www.blackwellpublishing.com
List of Contributors
Preface
Historical Perspective
CHAPTER ONE Historical Perspective
1.1 What is sequence stratigraphy?
1.2 The evolution of sequence stratigraphy
Concepts and Principles
CHAPTER TWO Concepts and Principles of Sequence Stratigraphy
2.1 Introduction
2.2 Relative sea-level, tectonics and eustasy
2.3 Sediment supply
2.4 Sequences and systems tracts
2.5 High-resolution sequence stratigraphy and parasequences
Sequence Stratigraphie Tools
CHAPTER THREE Seismic Stratigraphy
3.1 Seismic interpretation
3.2 Seismic reflection termination patterns
3.3 Recognition of systems tracts on seismic data
3.4 Pitfalls in interpretation
CHAPTER FOUR Outcrop and Well Data
4.1 Introduction and historical perspective
4.2 Resolution of well data
4.3 Sequence stratigraphy of outcrops and cores
4.4 Sequence stratigraphy of wireline logs
CHAPTER FIVE Chronostratigraphic Charts
5.1 The purpose of chronostratigraphic charts
5.2 Construction of chronostratigraphic charts from seismic data
5.3 Interpreting a chronostratigraphic chart
5.4 Coastal onlap curves and relative sea-level curves
5.5 Constructing chronostratigraphic charts from other data
CHAPTER SIX Biostratigraphy
6.1 Introduction
6.2 Fossil groups and zonal schemes
6.3 Palaeoenvironmental analysis
6.4 Biostratigraphy and sequence stratigraphy
6.5 Conclusions
Applications to Depositional Systems
CHAPTER SEVEN Fluvial Systems
7.1 Introduction
7.2 Fluvial processes and channel styles
7.3 The concept of the graded stream profile
7.4 Fluvial architecture
7.5 Reconstructing fluvial architecture
CHAPTER EIGHT Paralic Successions
8.1 Introduction
8.2 Paralic depositional systems
8.3 Sequences in paralic successions
8.4 Parasequences in paralic successions
8.5 The sequence stratigraphy of distinct paralic systems
8.6 Correlation procedure
8.7 An example: the Viking Formation, Western Canadian Basin
8.8 Reservoirs in paralic successions
8.9 Paralic systems at a seismic scale
8.10 Variations in paralic systems within a sea-level cycle
8.11 Summary
CHAPTER NINE Deep-marine Clastic Systems
9.1 Introduction
9.2 Deep-marine clastic systems – depositional processes and classification
9.3 Fan development during lowstands
9.4 Fan development during highstand and transgression
9.5 Conclusions
CHAPTER TEN Carbonate Systems
10.1 Introduction
10.2 Controls on carbonate sedimentation
10.3 Carbonate slopes, platform classification and facies belts
10.4 Sequence stratigraphic models for carbonate platforms
10.5 Cyclicity and parasequences on carbonate platforms
10.6 Conclusions
CHAPTER ELEVEN Organic-rich Facies and Hydrocarbon Source Rocks
11.1 Introduction
11.2 Delta/coastal plain organic-rich facies and source rocks
11.3 Organic-rich facies and systems tracts in clastic systems
11.4 Marine Carbonate Source Rocks
11.5 Conclusions
CHAPTER TWELVE Computer Modelling of Basin Fill
12.1 Introduction
12.2 Forward model types
12.3 Examples of stratigraphic use of computer modelling
12.4 Practical forward computer modelling
12.5 Conclusions
References
Index
List of Contributors
GEORGE BERTRAM Stratigraphic Research International, Braehead Avenue, Milngavie, Glasgow, UK
DOMINIC EMERY BP Exploration, Uxbridge One, 1 Harefield Road, Uxbridge, London, UK
CEDRIC GRIFFITHS NCPGG, Thebarton Campus, University of Adelaide, Adelaide, Australia
NICK MILTON BP Norge, Forus, Stavanger, Norway
KEITH MYERS BP Exploration, Uxbridge One, 1 Harefield Road, Uxbridge, London, UK
TONY REYNOLDS BP Exploration, Sunbury-on-Thames, London, UK
MARCUS RICHARDS BP Exploration, Anchorage, Alaka, USA
SIMON STURROCK 56 Gloucester Court, Kew Road, Kew, London, UK
Preface
In 1989, the chief geologist of BP Exploration and his senior colleagues recognized the need to expand the company’s resource of sequence stratigraphers, and created the Stratigraphic Studies Group. This group initially included a few experts, but was composed chiefly of a mixture of willing geophysicists, sedimentologists and biostratigraphers, who were to train as sequence stratigraphers, but more importantly, were to bring the expertise from their own disciplines to bear on sequence stratigraphy. This merging of geological disciplines with sequence stratigraphic principles first saw the light of day as BP’s ‘Introduction to Sequence Stratigraphy’ course. This course has been given internally since 1991, and has been presented in whole or in part to over a dozen national oil companies and at several international geological and geophysical conferences. The ‘Introduction to Sequence Stratigraphy’ course manuals formed the basis for this book, because, as we naively thought, it would not be too much trouble to recast the manuals in the form of a textbook, which could form the basis of university and professional courses. Inevitably, it was more difficult than we had imagined. Sequence stratigraphy is a rapidly evolving subject, new terminology was being added as we wrote, and the jargon we sought to demystify continues to grow. This book must thus be seen as a sequence stratigraphic synthesis for the mid-1990s, and not the final word on the subject. It is above all a practical guide and contains tools and techniques that the authors have found useful in their daily work.
We have arranged the book to cover four main themes; a brief history of sequence stratigraphy, concepts and principles, sequence stratigraphic tools and finally the application of sequence stratigraphy to different depositional systems. For the last theme, we have tried to emphasize the importance of seeing sequence stratigraphy in its sedimentological context, and it is recommended that the reader should have some familiarity with sedimentological processes before tackling the last five chapters. Otherwise, the book covers all the basics of sequence stratigraphy, and is intended to be a broad text suitable for undergraduate geologists of all years, MSc and PhD sedimentologists and stratigraphers, and for oil company geoscientists who wish to broaden their knowledge of the stratigraphie methods available for solving problems with which they are routinely faced. We hope you find it interesting and useful.
Bob Jones contributed several sections on biostratigraphy and Neil Parkinson contributed to Chapter 2. We are grateful to the following reviewers who have helped improve the book at various stages of its development: David Roberts, Henry Posamentier, Maurice Tucker, Dan Bosence, Mike Bowman, Andy Horbury and Andy Fleet.
We also acknowledge the support of BP Exploration, particularly David Roberts, Bob Rosenthal, John Wills and Peter Melville and we are grateful to BP’s partners for permission to publish information on many of the areas and fields mentioned in the text. The following companies are also acknowledged for their assistance in providing seismic data; Lynx Information Systems, Trans-Asia Oil and Mineral Development Corp., Balabac Oil Exploration and Drilling Co. Inc., Crestone Energy Corp., Coplex (Palawan) Ltd., Oriental Petroleum and Minerals Corp., The Philodrill Corp., Seafront Resources Corp., Unioil and Gas Development Co. Inc., Vulcan Industrial and Mining Corp.
George Bertram, Dominic Emery,
Cedric Griffiths, Nick Milton,
Keith Myers, Tony Reynolds,
Marcus Richards and Simon Sturrock
BP Exploration
London, Sunbury-on-Thames,
Glasgow, Stavanger and Anchorage
Sequence stratigraphy is a sub-discipline of stratigraphy, the latter being defined broadly as ‘the historical geology of stratified rocks’. There have been many definitions of sequence stratigraphy over the years, but perhaps the simplest, and that preferred by the authors, is ‘the subdivision of sedimentary basin fills into genetic packages bounded by unconformities and their correlative conformities’. Sequence stratigraphy is used to provide a chronostratigraphic framework for the correlation and mapping of sedimentary facies and for stratigraphic predictiorr.
Several geological disciplines contribute to the sequence stratigraphic approach, including seismic stratigraphy, bio-stratigraphy, chronostratigraphy and sedimentology. These are discussed in more detail in forthcoming chapters. Note that lithostratigraphy is not considered to contribute usefully to sequence stratigraphy. Lithostratigraphy is the correlation of similar lithologies, which are commonly diachronous and have no time-significance (Fig. 1.1). Lithostratigraphic correlation is useful provided the sequence stratigraphic boundaries enveloping the interval of interest are constrained.
Sequence stratigraphy is often regarded as a relatively new science, evolving in the 1970s from seismic stratigraphy. In fact sequence stratigraphy has its roots in the centuries-old controversies over the origin of cyclic sedimentation and eustatic versus tectonic controls on sea-level. Much of this early debate has been summarized recently in a set of historical geological papers edited by Dott in 1992 (1992a), entitled ‘Eustasy: the Ups and Downs of a Major Geological Concept’, and the interested reader is referred to this volume for more detail. Other historically important collections of sequence stratigraphic papers include American Association of Petroleum Geologists (AAPG) Memoir 26, published in 1977, and Society of Economic Paleontologists and Mineralogists (SEPM) Special Publication 42, published in 1988.
The Deluge and the story of Noah is the most well-known of the earliest references to sea-level change. To the early investigators of sea-level change, the veracity of the Deluge was not in question, but its origin was the subject of considerable debate by scientists and clergy alike. Perhaps the most popular of several theories were Burnet’s Sacred Theory of the Earth, published in 1681, and the Telliamed of de Maillet, published in 1748 (and recently revisited by Carozzi in 1992). De Maillet proposed that following the formation of the Earth by the accretion of the ashes of burning suns over the cortex of an extinguished sun, a water envelope which developed around the planet gradually diminished in volume through time, and in so doing created the topography we see today. In effect, de Maillet interpreted sea-level changes on Earth as a ‘single falling limb of a cosmic eustatic cycle’ (Carozzi, 1992). This concept of a one-way sea-level fall was known as Neptunian theory. The erosion of primitive mountains by marine processes and the development of a series of offlapping sediment packages as implied by de Maillet and other Neptunists is illustrated schematically in Fig. 1.2.
The eighteenth century also saw the beginning of detailed stratigraphic analysis of rock units, and the recognition of unconformities as primary bounding surfaces. In 1788, Hutton first appreciated the significance of unconformities separating cycles of ‘uplift, erosion and deposition’, and unconformities were used by stratigraphers such as Sedgwick and Murchison in the following century to establish physical boundaries for geological periods (Sedgwick and Murchison, 1839). As the great stratigraphers continued with their practical approach, William Buckland (1823) proposed the concept of Diluvialism which was to eclipse Neptunian theory. In diluvial theory, the geological products immediately preceding the flood were referred to as antediluvial, and those following the flood were referred to as post-diluvial or alluvial. However, the attraction of diluvial theory also soon waned as further geological evidence served to counter the simplistic notion of a single dramatic flooding event.
In the middle of the nineteenth century, the eustatic versus tectonic controls on sea-level change debate began in earnest with the glacial theories of Lyell and Agassiz. Lyell and others (including Celsius and Linnaeus) observed raised beaches along the coastline of Scandinavia and noted evidence of falling sea-levels from centuries-old marks on shoreline outcrops. Lyell concluded that the land was being slowly and differentially elevated (Lyell, 1835), a fact confirmed by Bravais in 1840 who had observed tilted beaches along fjords of the Scandinavian Arctic coast. At about the same time, Agassiz (1840) was developing his theories of glaciation, and MacLaren, on reviewing Agassiz’s glacial theory in 1842, saw the potential of melting ice-caps as a major control on global sea-level. Unfortunately, neither Agassiz nor MacLaren received acceptance for their ideas for at least two more decades, until Croll (1864), in a forerunner of Milankovitch theory (1920), published the concept of orbitally forced glaciations.
By the late nineteenth century, glacial theory was thus able to explain eustatic sea-level change and isostatic uplift. However, it was to be several decades before glacial eustasy was resurrected as a control on sedimentary rhythmicity; other explanations of global eustasy took precedence, notably the work of Eduard Suess. Suess first coined the term eustasy in 1906, when he attributed the patterns of onlap and offlap of sedimentary units to global sea-level changes. Suess favoured a mechanism whereby sea-level was lowered by subsidence of the sea-floor, and raised by the displacement of sea water by oceanic sedimentation. He refused to believe the evidence for differential land uplift from Scandinavia, concluding that the Baltic was ‘gradually emptying’ (Suess, 1888). However, the majority of geologists in the early twentieth century still held the Lyellian view that the major control on sea-level at any point along the coast was the movement of the land. Despite the general lack of support for Suess’ ideas, a number of American geologists began to develop concepts of global controls on unconformity development. Foremost amongst these was Chamberlin, who in 1898 and 1909 published his theory on the ‘diastrophic control of stratigraphy by world-wide sea level changes’. Three diagrams from his first paper show this to be a precursor of modern sequence stratigraphic concepts (Fig. 1.3).
Chamberlin’s ideas were developed by several American geologists in the following decades, particularly in Palaeozoic systems of the Mid-west. Most notable amongst these were Ulrich and Schuchert, the latter using early palaeogeographic concepts and facies theory to re-create past environments bounded by global unconformities. However, the single most important publication from the ‘eustatic’ school was that of Grabau, a contemporary of Ulrich and Schuchert, whose ‘pulsation theory’ postulated rhythmic transgressions and regressions caused by changing heat flow from inside the Earth. The resulting ‘pulse beat of the Earth’, published in Grabau’s ‘The Rhythm of the Ages’ in 1940, had a periodicity of about 30 million years and caused the development of global unconformities, which could be used to divide the stratigraphic record. Prior to Grabau’s work, European geologists, notably Stille (1924) had begun to develop ideas about global unconformities caused by global tectonism with resulting eustatic effects, akin to modern low-order eustatic cycles.
On a smaller scale, sedimentary rhythms were being observed on a scale of metres in coal-bearing Carboniferous (Pennsylvanian) strata in Illinois and Kansas. In 1935, following further studies on Pleistocene glacio-eustatic changes, Wanless and Shepard proposed a control on the development of these Pennsylvanian ‘cyclothems’ by the accumulation and melting of Gondwana glaciers. This study and others like it resurrected the glacial-eustatic control on sedimentation developed by Croll many decades earlier.
The case for periodicity at a variety of scales in the stratigraphic record was thus becoming compelling. However, as with so many scientific bandwagons, it eventually ran out of steam. In a keynote address to the geological community in 1949, Gilluly argued for orogenesis as a continuous, rather than episodic process, and as a result the concept of rhythmicity of low order (tens of millions of years) gradually lost credibility. The Carboniferous cyclothems were then reinterpreted as autocyclic products, resulting from delta lobe switching and the internal re-organization of sedimentary systems. This latter point also emphasizes the ascendancy of process sedimentology in the early 1960s. Dott (1992a) amusingly points out that at that time many stratigraphers preferred to call themselves sedimentologists!
Ironically, Sloss, Krumbein and Dapples (1949) first outlined the concept of stratigraphic sequences at the same meeting that Gilluly proposed his ideas on orogenic continuum. Sloss, Krumbein and Dapples defined sequences as ‘assemblages of strata and formations’ bounded by prominent interregional unconformities. Despite the negative reaction to these ideas, Sloss (1963) published his major sequences correlateable across the North American Craton, the Indian Tribal names of which still appear as ‘super sequences’ on the Haq et al. (1987) chart. Sloss’s ideas were developed further by his graduate students at North-western University, one of whom was Peter Vail. Also published at this time was Harry Wheeler’s classic 1958 paper on time-stratigraphy which contains many of the concepts in use today, as well as an early attempt to introduce sequence stratigraphic terminology.
The next major breakthrough in sequence stratigraphy was in the 1960s and 1970s, when the development of digitally recorded and processed multichannel seismic data made large scale two-dimensional images through basins available. Vail et al. (1977a) in AAPG Memoir 26 is perhaps the most referenced work on sequence stratigraphy to date. It summarizes work carried out by Vail and his co-workers, first in the Carter Oil Company and subsequently at the Exxon Production Research Corporation, through the 1960s and early 1970s (Vail and Wilbur, 1966; Mitchum et al., 1976). This period of time marks a break where industry took the lead from academia in the development of sequence stratigraphy. Further papers on seismic sequence stratigraphy followed, and the ideas were gradually extended to incorporate both borehole and outcrop data (Vail el al., 1984). In this work, eustatic sea-level was emphasized as the controlling mechanism for sequence development. In 1985 AAPG Memoir 39 appeared, in which Hubbard et al. proposed a tectonic mechanism for the subdivision of basin fill into ‘megasequences’, driven by changes in tectonic process. The tectonic versus eustatic debate was beginning afresh, although for many at this time seismic stratigraphy was synonymous with eustatic sea-level change, possibly because of its appeal as a global predictive tool for hydrocarbon exploration. In 1987, the Haq et al. global sea-level cycle chart was published. This is possibly the single most contentious of all the ‘Exxon school’ publications, chiefly because the supporting evidence for the curves has not been released. It remains unclear whether local corrections for tectonic uplift or subsidence have been applied, and the dating of unconformities to the accuracy implied by the chart has been challenged (Miall, 1991).
Special Publication 42 of SEPM, Sea Level Changes — an Integrated Approach, was published in 1988 and introduced new concepts such as accommodation space and parasequences, and many of the concepts and principles described in Chapter 2 of this book. Special Publication 42 was important because it opened up the subject to a broader geological community beyond industrial seismic interpreters. In the late 1980s and in this decade, many sequence stratigraphic publications have appeared, some of which uncritically apply the tools and techniques, and some of which are strongly critical. Many question the validity of the interbasinal correlations upon which the Haq etal. (1987) curve is based, and others have questioned the validity of certain aspects of the sequence stratigraphic models presented in SEPM 42, such as Miall (1991) and Schlager (1992). Galloway (1989) presented an alternative model for the development of depositional units or ‘genetic stratigraphic units’ bounded by major flooding surfaces, rather than unconformities. Pitman (1978) has suggested that the origin of sequences and onlap patterns can be explained by variations in subsidence at continental margins, whereas Cloetingh (1988) and Kooi and Cloetingh (1991) proposed that relative sea-level changes and the formation of sequences of millions of years duration can be explained by intraplate stresses rather than eustatic sea-level changes.
The most recent developments in sequence stratigraphy have been in the area of high-resolution subseismic-scale sequence stratigraphy and computer modelling of sedimentary fill. Van Wagoner et al. (1990) led the way with the publication of a colourful text on high-resolution sequence stratigraphy from outcrops, logs and core. This stimulated excellent work in superbly exposed marine and marginal marine settings, such as the Jurassic of the Yorkshire Coast and the Cretaceous of the Western Interior Seaway, USA (see also Posamentier and Weimer, 1993, for review). High-resolution sequence stratigraphy also has been combined with work on metre-scale rhythmic successions, particularly bedded platform carbonates and mixed siliciclastic carbonate units (Hardie et al., 1986; Goldhammer et al., 1991). Milankovitch theory of orbital forcing has been revived by sequence stratigraphers in order to explain the origin of high-frequency subsequence-scale cycles. Computer modelling packages have been developed to analyse and replicate the sedimentary fill of basins, at scales from a few metres to entire basins. Basin-wide models include those developed by Royal Dutch/Shell, and published by Aigner et al. (1990), and the SEDPAK program developed at the University of South Carolina. Smaller scale cyclicity has been modelled by software such as Mr Sediment (Goldhammer et al., 1989) and by Bosence and Waltham (1990).
The future direction of sequence stratigraphy is difficult to predict, given the turbulent history of the sea-level change debate. At least in the short-term, carbonate systems require further case studies to demonstrate the importance (or otherwise) of controls other than sea-level change. Posamentier and Weimer (1993) have also emphasized the need for further work on the applicability of the concepts to non-marine and deep-marine settings, and further validation (or otherwise) of the sea-level cycle chart from outcrop and subsurface data. Schlager (1992) and others also argue for a more sedimentological approach to sequence stratigraphy, accounting for the autocyclicity of sedimentary processes within the sequence stratigraphic framework. At the very least we can expect considerable debate and further critiques of the subject. This level of activity and debate is all a far cry from the early 1960s when stratigraphy was unfashionable, before Peter Vail and others rescued the subject from its decline.
The stratigraphic signatures and stratal patterns in the sedimentary rock record are a result of the interaction of tectonics, eustasy and climate. Tectonics and eustasy control the amount of space available for sediment to accumulate (accommodation), and tectonics, eustasy and climate interact to control sediment supply and how much of the accommodation is filled. Autocyclic sedimentary processes control the detailed facies architecture as accommodation is filled. The purpose of this chapter is to introduce the principles that govern the creation, filling and destruction of accommodation. It then shows how these principles are used to divide the rock record into sequences and ‘systems tracts’, which describe the distribution of rocks in space and time.
The chapter uses siliciclastic systems to introduce the concepts and principles of sequence stratigraphy. Carbonate systems differ from clastic systems in their ability to produce sediment ‘in situ’, and they respond in a different manner to accommodation changes. Carbonates are therefore discussed separately in Chapter 10.
Tectonism represents the primary control on the creation and destruction of accommodation. Without tectonic subsidence there is no sedimentary basin. It also influences the rate of sediment supply to basins. Tectonic subsidence results from two principle mechanisms, either extension or flexural loading of the lithosphere. Figure 2.1 illustrates theoretical tectonic subsidence rates in extensional, foreland and strike-slip basins. These curves in effect govern how much sediment can accumulate in the basin, modified by the effects of sediment loading, compaction and eustasy.
Extensional basins form in a variety of plate tectonic settings, but are most common on constructive plate margins. In extensional basins, tectonic subsidence rates vary systematically through time, with an initial period of very rapid subsidence caused by isostatic adjustment to lithosphere stretching, followed by a gradual (60–100 million years) and decreasing thermal subsidence phase as the asthenosphere cools. This systematic change in tectonic subsidence rate has a strong influence on the geometry of the basin-fill, such that it may be possible to divide the stratigraphy into pre-, post- and syn-rift phases (these phases have been termed megasequences; Hubbard, 1988). In the simple syn-rift megasequence model the sediments are deposited in the active fault-controlled depocentres of the evolving rift and can show roll-over and growth into the active faults. Differential subsidence across the extensional faults may exert a strong control on facies distributions. In the post-rift megasequence, any remaining rift-related topography is gradually buried beneath sediments that fill the subsiding basin and onlap the basin margin, creating the typical ‘steers head’ geometry (McKenzie, 1978). The syn-rift and post-rift megasequences in a marine rift will contain sequences in which development is controlled by higher frequency changes in relative sea-level.
Foreland basins develop in response to loading of the lithosphere below thrust belts. The lithosphere bends in response to loading as the thrust sheets are emplaced, and creates a depression that is accentuated towards the load. The sedimentary fill to this foreland basin has a characteristic wedge shape, thickening towards the thrust front and forming a foreland basin megasequence. The width of the basin is proportional to the rigidity of the underlying lithosphere, and the depth is proportional to the size of the load. Foreland basins formed adjacent to growing mountain belts are characterized by large, and initially rapidly increasing, rates of sediment supply. Cessation of thrusting and continued erosion of the mountain belt leads to an eventual decrease in load, and many foreland basins become uplifted.
Strike-slip basins do not have a characteristic subsidence pattern, although in general, rates of subsidence (and uplift) are extremely rapid.
Tectonic subsidence curves provide a fundamental control on sediment accommodation, upon which higher frequency controls, such as eustasy, fault movement and diapirism, are superimposed. Figure 2.2 shows calculated tectonic subsidence curves for two real basins. In the Llanos Basin, Colombia, sediment supply has exceeded tectonic subsidence. The basin has remained full to base level, with excess sediment bypassed northwards to the sea. The subsidence curve shows slow subsidence through the late Cretaceous and early Tertiary, linked to thermal subsidence in a back-arc basin setting. Two distinct increases in subsidence rate occur in the mid –late Eocene and mid-Miocene, corresponding to two phases of mountain building in the Andes.
In the South Viking Graben example (Fig. 2.2), typical of a number of ritts, sedimentation has not always kept pace with true tectonic subsidence. This led to periods in the Cretaceous where water depths increased and sediment starvation occurred. In the Tertiary, uplift of the Scottish mainland and adjacent North Sea Basin resulted in increased sediment input to the basin (Milton et al., 1990), which locally filled to base level. The remainder of the basin subsequently filled with sediment, resulting in the present-day shallow sea. Separation of the syn-rift and post-rift in this basin is difficult, because the transition occurred during a period of sediment starvation (Milton, 1993).
During periods of rapid basin subsidence, sequence boundaries generated by higher frequency eustatic sea-level falls will be obscured. In times of slow tectonic subsidence or basin uplift, sequence boundaries will be enhanced.
Many of the concepts and principles of sequence stratigraphy are based on the observation from seismic data that prograding basin-margin systems often have a consistent depositional geometry (Fig. 2.3). Topset is a term used to describe the proximal portion of the basin-margin profile characterized by low gradients (< 0.1°). Topsets effectively appear flat on seismic data and generally contain alluvial, deltaic and shallow-marine depositional systems.
The shoreline can be located at any point within the topset. It can coincide with the offlap break or may occur hundreds of kilometres landward. The proximal termination of the topset is usually termed the point of coastal onlap, referring to the up-dip limit of coastal-plain or paralic facies. Clinoform is used to describe the more steeply dipping portion of the basin-margin profile (commonly > 1°) developed basinward of the topset. Clinoforms generally contain deeper water depositional systems characteristic of the slope. The slope of the clinoform generally can be resolved on seismic data. Bottomset is a term sometimes used to describe the portion of the basin-margin profile at the base of the clinoform characterized by low gradients and containing deep-water depositional systems.
The main break in slope in the depositional profile occurs between topset and clinoform and is called the offlap break (Vail et al., 1991). The offlap break previously has been termed the shelf edge (Vail and Todd 1981; Vail et al., 1984), leading to a confusion with the shelf break, i.e. the edge of the modern continental shelf, whtch is usually a relict feature rather than depositional feature. The term depositional shoreline break (Van Wagoner et al., 1988) also has been used, but this implies that the main break in slope in a depositional profile coincides with the shoreline. The term offlap break is preferred here as it does not imply coincidence of the main break in slope with the shoreline.
The topset–clinoform profile results from the interplay between sediment, supply and wave, storm and tidal energy in the basin. Sediment enters the proximal end of the profile through river systems and is distributed across the topset area by wave- and/or current-related processes. These may include fluvial currents, tidal currents, storm currents, etc. However, these topset transport processes are effective only at relatively shallow depths of up to a few tens of metres, and to move sediment into deeper water a slope must develop in order to allow sediment transportation by gravity processes. The clinoforms build to the angle needed to transport sediment at the required rate. Slope angle is strongly influenced by sediment calibre. Coarse-grained sediment, with a higher angle of rest, will build up steeper slopes than fine-grained sediment (Kenter, 1990). Also, carbonate systems generally can build steeper depositional slopes (up to 35°) than fine-grained clastic systems (0.5–3°) owing to their greater shear strength. Steeper slopes in clastic systems generally are either made of coarser grade material or are zones of erosion and sedimentary bypass.
The importance of the offlap break on the depositional systems is most apparent during relative sea-level fall (see 2.2.1). When relative sea-level fall exposes the offlap break, rivers commonly incise in order to re-equilibrate to lowered base level, with the result that the river becomes entrenched at its mouth (discussed in 2.4.3). The response of the depositional systems to this fall in relative sea-level depends on the nature of the basin margin (Fig. 2.4).
Shelf-break margins are those with well developed depositional clinoforms. Fluvial entrenchment during sea-level fall may result in focusing of the sediment load to discrete locations on the clinoform slope. Failure of the sediment mass has the capacity for forming large turbidity currents and submarine fan deposits. Shelf-break margins are typical of passive continental margins at times of slow rise of relative sea-level, when the delta systems can easily prograde to the shelf edge.
Ramp margins are characterized by relatively shallow water depths, where storms and current processes can operate over much of the area of deposition. Depositional angles are generally less than 1° and seismic clinoforms (if resolved) are shingled with a dip of around half a degree. The offlap break on a ramp margin is likely to be at the shoreline, where fluvial gradients pass into slightly steeper shelf or delta-front gradients. The response of the depositional systems in a ramp setting to relative sea-level change is therefore different from the shelf-break margin. In particular, deep-water turbidite deposition during lowstand may be absent, or of only minor significance. Depositional systems will, instead, be translated basinward without significant slope bypass or basinal deposition. Any turbidites found on a siliciclastic ramp margin are likely to be delta-front turbidites, rather than detached submarine fans (Van Wagoner et al., 1990).
Many modern delta systems can be considered to form ramp margins, as generally they are shelf deltas prograding on to the drowned topsets of a previous shelf-break margin (Fig. 2.4). Frazier (1974) has shown that deposition on the continental shelf of the Gulf of Mexico is confined to the Mississippi delta, which is prograding into about 100 m of water. The rest of the shelf is effectively an area of non-deposition. The Mississippi delta presently forms a ramp margin, although very little extra progradation is needed for the delta to reach the shelf edge, and for the margin to become a shelf-break margin.
Rift margins characterize basins undergoing active crustal extension. Extensional faults have a strong influence on both palaeogeography and sediment influx rates. The spatial distribution of sediment accommodation within the rift is controlled largely by tectonics. Subsidence rates generally will increase from the margins to the centre of the rift, although each individual fault block will have its own pattern of accommodation. The foot-wall crest will see the least subsidence and may experience uplift and erosion, whereas the hanging wall will experience progressively greater subsidence rates towards the controlling fault. The depositional systems that develop will depend on whether the rift is marine or continental. Transfer zones in the rift margin may control sediment entry points. Rift margins may be characterized by high topographic relief and relative sediment starvation, because sediment is bypassed towards the rift centre. Basin-margin systems may build out into deep water with long clinoform slopes and relatively minor topsets (Fig. 2.4). There is little potential for trapping coarse material in the topsets, and much may be bypassed to the basin.
Foreland-basin margins vary depending on whether sediment is being fed axially along the foreland basin or directly into the foreland basin from the thrust belt. In the latter case, the rate of tectonic subsidence increases towards the foreland thrust belt, i.e. the sediment source area. In other words, sediment accommodation may be relatively high in proximal areas compared with the basin centre. This has a marked affect on stratal geometries and may result in the aggradation of thick topset deposits, with little opportunity for seismic-scale clinoforms to develop (Posa-mentier and Allen, 1993).
Growth-fault margins are characterized by gravity driven syn-sedimentary extensional faults. The rate of subsidence is considerably greater on the hanging-wall side of the growth fault, resulting in an expanded sedimentary succession. The effect of the growth fault in the depositional systems developed will depend on whether the fault had a topographic expression on the sea-bed. At times when the hanging wall was a topographic low relative to the foot wall, facies differentiation occurs across the fault, with thick, deeper water clastic systems on the down-thrown side. Growth-fault margins are discussed further in section 9.3.3.
In order to understand the controls on sequence development, it is first necessary to define what is meant by eustasy, relative sea-level and water depth (Fig. 2.5, from Jervey, 1988).
Eustasy is measured between the sea-surface and a fixed datum, usually the centre of the Earth. Eustasy can vary by changing ocean-basin volume (e.g. by varying ocean-ridge volume) or by varying ocean-water volume (e.g. by glacio-eustasy). The interpretation of eustatic changes from the rock record is a complex and controversial topic, which will be discussed in detail in section 2.2.4. For the moment it is important only to emphasize that it can rise or fall, thus varying the base level for erosion on a global scale, where base level is defined as the level above which deposition is temporary and erosion occurs (see 2.2.2).
Relative sea-level is measured between the sea-surface and a local moving datum, such as basement or a surface within the sediment pile (Posamentier et al., 1988). Tectonic subsidence or uplift of a basement datum, sediment compaction involving subsidence of a datum within the sediment pile, and vertical eustatic movements of the sea-surface all contribute to relative sea-level change. Relative sea-level ‘rises’ due to subsidence, compaction and/or eustatic sea-level rise, and ‘falls’ due to tectonic uplift and/or eustatic sea-level fall. Relative sea-level should not be confused with water depth, which is measured between the sea-surface and the sea-bed in any given geographic location at a point in time. The term equilibrium point is sometimes used to distinguish the point on a depositional profile where the rate of relative sea-level change is zero. The equilibrium point will separate, at any given time, the zone at the basin margin where relative sea-level is falling from the zone where relative sea-level is rising.
Eustasy and subsidence rate together control the amount of space available for sediment accumulation – this is conventionally termed accommodation. Accommodation is defined as the space available for sediment to accumulate at any point in time (Jervey, 1988). Accommodation is controlled by base level because in order for sediments to accumulate, there must be space available below base level. The base-level datum varies according to depositional setting (Fig. 2.6). In alluvial environments base level is controlled by the graded stream profile, which is graded to sea-level or lake-level at its distal end (Mackin, 1948; and Chapter 7). In deltaic and shoreline systems base level is effectively equivalent to sea-level. In shallow marine environments base level is ultimately also sea-level, although fairweather wave base can form a temporary base level in the form of a ‘graded shelf profile’. Sediment supply fills the accommodation created and controls water depth:
Sediment supply fills available accommodation. If the rate of sediment supply exceeds the rate of creation of accommodation at a given point, water depths will decrease:
A series of cartoons in Fig. 2.7 illustrates the relationship between accommodation, relative sea-level and water depth in shoreline–shelf depositional systems. In these examples relative sea-level change and new sediment accommodation added are the same because base level is taken at the sea-surface.
In the discussion that follows we examine the relationship between relative sea-level and accommodation in shoreline–shelf depositional systems. Fluvial systems and the controls on the graded stream profile are discussed in Chapter 7, paralic systems are discussed in Chapter 8, submarine fans in Chapter 9 and carbonates in Chapter 10.
In order to understand how accommodation varies through time it is useful to consider how different rates of tectonic subsidence and, in this case, the same sinusoidal eustatic sea-level curve combine to give different rates of addition and destruction of accommodation (rise and fall of relative sea-level) (Fig. 2.8; from Jervey, 1988).
In Fig. 2.8 subsidence is represented as a straight line, the gradient of which indicates the rate of subsidence at each point. The different gradients can be thought of as representing positions in a basin with increasing subsidence rates or changes in subsidence rate through time. Eustasy is represented by the same smooth curve in each case. The change in relative sea-level through time is found simply by the addition of the two curves. Relative sea-level is, in this case, equivalent to accommodation because the curves begin at zero water depth.
Where slow subsidence occurs, maximum accommodation is developed near the eustatic maximum. When eustasy falls to its original position, accommodation falls to a value representing that created only by subsidence. With increased rates of subsidence, the time of maximum accommodation is progressively later. Points in the basin where subsidence rates are very high experience no decrease in accommodation even though eustatic fall may be occurring. Note that the same curves could be produced theoretically by adding variable rates of tectonic subsidence and uplift to a flat eustatic curve.
A depositional sequence represents a complete cycle of deposition bounded above and below by erosional unconformities. The sequence has a maximum duration, which is measured between the correlative conformities to the bounding unconformities. Thus, the duration of the sequence will be determined by the event controlling the creation and destruction of accommodation, i.e. tectonic subsidence and/or eustasy. Tectonic cycles of subsidence and uplift and eustatic cycles of rising and falling sea-level can operate over different time periods, and it is useful to classify sequences in terms of their order of duration, commonly termed first, second, third, fourth order, etc. (Fig. 2.9). A basin-fill can then be divided into a hierarchy of sequences, each representing the product of a particular order of tectonic or eustatic cycle.
In Fig. 2.9, from Duval et al. (1992), four orders of stratigraphie cycle are depicted. The continental encroachment cycle is defined by the very largest scale (> 50 million years) cycles of sedimentary onlap and offlap of the super-continents. There are only two such cycles in the Phanerozoic, according to the Haq et al. (1987) sea-level curve. First-order continental encroachment cycles are considered to be controlled by tectono-eustasy, i.e. changes in ocean basin volume related to plate tectonic cycles (Pitman, 1978).