Ancient Sedimentary Environments

And Their Sub-surface Diagnosis

Richard C. Selley

• Publisher: Routledge
• Year: 2013
• ISBN:
  9781135075798
• Edition: 4
• Topic: Scienze fisiche
• Subtopic: Geologia e scienze della terra

This edition retains the case history approach to emphasize the subsurface diagnosis of environments using seismic and geophysical well logs and their application to petroleum exploration and production. This book should be of interest to undergraduates in sedimentology and petroleum geology.

1 Concepts and methods

INTRODUCTION

This chapter introduces and discusses the various concepts that are employed in the interpretation of the depositional environments of ancient sedimentary rocks. It includes a review of how environments may be classified, a discussion of the concepts of facies and models, and an account of how facies analysis relates to stratigraphy in general., and to seismic sequence stratigraphy in particular.

SEDIMENTARY ENVIRONMENTS

A sedimentary environment may be defined as a part of the earth’s surface that is physically, chemically and biologically distinct from adjacent areas. Examples include deltas, reefs and submarine fans (Figure 1.1).

The physical variables that affect a sedimentary environment include temperature, water depth, current velocity, sunlight, wind speed and direction, and so forth. The chemical variables that affect a sedimentary environment include water composition (salinity), and the mineralogy of both the autochthonous and allochthonous sediment. The biological variables that affect a sedimentary environment include, on land, soil, grass or forest cover, and the life styles of land animals, especially the number and amount of herbivores. Under water the type and abundance of animals and plants exert a major influence on the sedimentary environment, particularly in generating carbonate material. These physical., chemical and biological variables form an intricately interconnected system. A small change in one of the parameters may have a significant effect on one or more of the others. The effect of mankind on modern environments is an obvious instance.

A cursory view of the globe shows that there are many different types of sedimentary environment, and that these occur repeatedly around the world. There are many schemes that attempt to classify sedimentary environments. Table 1.1 illustrates what may best be termed the ‘classic’ scheme.Though hallowed by time this classification has two main limitations for the purpose of environmental interpretation. The scheme contains several environments which are either erosional, equilibrial, or of only limited significance as depositors of sediments. More seriously, marine environments are differentiated by water depth.

Figure 1.1 Geophantasmogram to illustrate the various sedimentary environments. Note how, in addition to groupings into continental, shoreline and marine environments, it is also possible to differentiate environments of erosion, environments of non-deposition, or equilibrium, and environments of deposition.

Table 1.1 A typical classification of sedimentary environments. This has two limitations as a basis for interpreting ancient sedimentary environments. It contains several environments that are essentially erosional or equilibrial, and do not actually deposit sediment. Furthermore the division of marine environments by water depth is both difficult to do with any great accuracy, and is often of little significance as a controlling factor on sedimentation.

Water depth is a very obvious and easily measured parameter of modern environments. It is, however, not a major controlling factor on sedimentary processes. Furthermore, it is very difficult to determine the water depths in which ancient sediments were deposited, beyond such broad zones as intertidal, shelf and ‘pelagic’, meaning of the open sea. Interpretations of ancient water depths are largely based on palaeontology, particularly using foraminifera. It is widely recognized, however, that foraminiferal assemblages may correlate with temperature as much as depth. Thus changes in ancient faunal assemblages may indicate climatic fluctuations as much as depth changes.

Look at Figure 1.1 again. Note that in addition to the obvious groupings of the various environments in to continental shoreline and marine, there is another trinity of environmental types, namely environments of erosion, environments of non-deposition, or equilibrium, and environments of deposition. These are worthy of consideration.

Environments of erosion are areas that are predominantly continental and terrestrial, apart from cliffed coastlines. They are thus only of concern in so far as they are the sources for terrigenous detritus. The detrital mineralogy of a sand exerts a strong control on its diagenesis and porosity evolution, but is of little significance in terms of its environment of deposition.

Environments of non-deposition, or equilibrium, are areas where there is neither erosion nor net deposition taking place, though sediment may be transported across them. Such equilibrial environments may be found on land, on continental shelves, and even on ocean floors. Terrestrial equilibrial surfaces characterize many deserts. For example, travellers crossing the vast desert plains of the Sahara sometimes encounter the stone circles of old camp fires. Closer inspection may reveal the beer cans of a geological field party, the land mines of a World War II patrol, or the flint instruments of the Stone Age. The last of these indicate the equilibrial nature of the desert surface, where neither deposition nor erosion has occurred for tens of thousands of years.

Terrestrial equilibrial surfaces may be marked by palaeosols that may include cemented horizons of caliche, ferrocrete, or silcrete. As discussed in Chapter 9 many continental shelves are equilibrial surfaces, across which sand may be driven by tidal, wind or storm generated currents, leaving little trace of their passage. Shelf environments may thus leave nothing in the stratigraphic record beyond an unconformity, a lag gravel and, in carbonate sediments, a hard ground. Surfaces of non-deposition or equilibrium also occur in deep marine environments. Some modern ocean floors have sedimentation rates so negligible that the teeth of long extinct species of shark lie unburied on the ocean bed. Environments of equilibrium or non-deposition are represented in the stratigraphic record by unconformities. Furthermore, because they are commonly marked by a cemented layer overlain by less cemented sediment, they often show up on seismic sections as prominent reflectors and sequence boundaries.

Lastly, there is the third type of environment, the environment of deposition. This type occurs in continental, shoreline and open marine settings. It is the depositional environment that is of principal concern and interest to sedimentologists, because this is the environment that actually generates sedimentary sequences that are preserved in the stratigraphic column.

Table 1.2 A classification of depositional sedimentary environments. Equilibrial and erosional environments are omitted. Note that water depth is not considered. The marine environments may include both terrigenous and carbonate deposits. Like all classifications this one breaks down under close scrutiny. For example, lakes may contain deltas, linear coasts, reefs and subaqueous channel and fan systems. None the less this classification provides a useful framework for further study and discussion.

This recognition of environments of erosion, equilibrium and deposition may be used to modify the classic environmental classification shown in Table 1.1 and discussed earlier. Table 1.2 shows the result. This scheme retains the broad grouping into continental, shoreline and marine environments, but now omits the rare, obscure, erosional and equilibrial environments. This scheme is, therefore, not a classification of sedimentary environments. It is a classification of depositional sedimentary environments. Note also that the marine environmental grouping avoids reference to water depth. This scheme is much more appropriate for geologists to use when studying ancient sedimentary environments. It will therefore be adopted in this book, and forms the framework for the subsequent chapters.

When classifying sedimentary environments it is often hard to define their boundaries precisely. Rivers merge gradually in to deltas, and so forth. Furthermore most environments can be divided in to sub-environments. For instance fluvial environments contain active and abandoned channels, levees, crevasse splay and flood basin sub-environments. This hierarchical system of environments and sub-environments will be illustrated in subsequent chapters.

SEDIMENTARY FACIES

A second important aspect of environmental interpretation is appreciation of what is meant by the term sedimentary facies. The regio...