Introduction to Sedimentology

Supriya Sengupta

• Publisher: Routledge
• Year: 2017
• ISBN:
  9781351436571
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

A concise account of all major branches of sedimentary geology, highlighting the connecting links between them. Introduction; Processes of sedimentation; Sedimentary texture; Sedimentary petrology; Hydraulics, sediment transportation and structures of mechanical origin; Sedimentary environments and facies; Tectonics and sedimentation; Stratigraphy and sedimentation; Basin analysis: A synthesis; References; Index.

1

INTRODUCTION

SCOPE OF SEDIMENTOLOGY

Sedimentology deals with the processes and products of sedimentation. Sediments are produced either by disintegration and alteration of preexisting rocks or by precipitation from solution. The particles ejected out of volcanoes and the dust particles of cosmic origin also add to the sediment mass of the earth in a limited way. Sediments are transported by running water, wind or moving ice to various depositions environments. The process of sediment transportation is often accompanied by the production of rhythmic bedforms or more complex structures which, when well preserved, provide clues to the palaeoenvironment and palaeocurrent.

Sediments produced out of mechanical or chemical processes are consolidated into sedimentary rocks by the pressure of overburden, recrystallisation and cementation. Textures of sedimentary rocks bear the imprint of the nature of the changes (diagenesis) undergone during consolidation of sediments into sedimentary rocks. The whole process of rock decay, sediment transportation, deposition, precipitation and diagenesis takes place at or near the surface of the earth at normal pressure-temperature conditions (Fig. 1.1). This distinguishes the sedimentary processes from the igneous and metamorphic processes where a higher order of temperature and pressure is involved. Sedimentary rocks cover wide areas of the earth’s surface although they account for only about 5% of the crustal volume. Thus they form only a thin veneer on the outermost part of the earth’s surface. The average thickness of the sediment cover on the continental crust is about 2 km but in the ocean basin only about 1 km.

Sediments laid down in layers within depressions (basins) in the earth’s crust are preserved to constitute a stratigraphic record. The ultimate aim of a sedimentological study is to unravel the chain of events responsible for production of particular stratigraphic sequences.

Fig. 1.1: Modern version of the fundamental rock cycle conceived by James Hutton nearly 200 years ago (modified and redrawn after Siever 1983).

HISTORY OF DEVELOPMENT

The Early History

The pioneers responsible for laying the foundation of sedimentology were not geologists by profession. The basic principles of stratigraphy and sedimentation developed out of studies by naturalists belonging to different disciplines. In the mid-17th century, a Danish physician-cum-clergyman, Nicolaus Steno (1638–1687), noticed that layers of sediments are always laid down in water in a sequence in which the oldest one lies at the bottom and the youngest one on top. This observation led to the formulation of a very fundamental rule known as the Law of Superposition. Steno’s second law, called the Law of Original Horizontality, states that the primary bedding of the layers of sediments laid down in water always parallels the surface of the earth. His third law, the Law of Original Continuity, states that all water-laid strata, must continue laterally. A corollary to this law, which came to be recognised by the eighteenth century, states that a truncation of an original sedimentary layer implies removal of the original sediments, either by erosion or due to faulting.

These laws, which for centuries provided the basis for geological mapping and stratigraphic correlation, no doubt have their limitations. For example, ripple migration on a sediment bed may cause the original layers to be deposited at an angle to the horizontal plane. Similarly, variations in depositional condition at the time of sedimentation (facies change) can interrupt the lateral continuity of sediment layers.

Surprisingly, Steno’s fundamental laws did not find practical application for nearly a century. Johann Gottlob Lehmann is believed to be one of the first to have applied the Law of Superposition to large-scale geological mapping in parts of Germany in the mid-18th century. In 1815, a British civil engineer, William Smith (1769–1839), produced the first stratigraphic section from the records maintained at the construction site. His field studies also led to the production of the first geological map of Britain.

In 1785, James Hutton (1726–1797), a Scottish physician, recognised the cycle of weathering, erosion and transportation by running water (see Fig. 1.1). He also observed that after consolidation the sediments laid down in the sea produce stratified deposits. According to many, Hutton’s appreciation of the immensity of geological time and his rejection of supernaturalism in providing explanations for geological phenomena, marked the beginning of rational thinking in geology. Hutton’s ideas were publicised during his lifetime by his friend, John Playfair, and nearly half a century later, by Charles Lyell (1797–1875). Lyell, who expanded and modified the Huttonian thesis, was quick to recognise that the changes that have taken place in the geological past can be explained by the processes that are taking place today. In other words, the laws of nature guiding the earth processes have remained invariant, although the earth itself has changed with time. This concept of uniformity in the earth processes came to be known as the Principle of Uniformitarianism.

With time, many corollaries of uniformitarianism like ‘the present is a key to the past’ (a phrase coined by Geikie in 1882), developed from the original Huttonian-Lyellian percept. Some of these, such as ‘the rates of the earth processes have remained constant’, or that uniformitarianism should be called ‘actualism’ because it refers to actual or real events, have been challenged by later workers on the ground that they are fallacious. In spite of these criticisms it must be remembered that the Huttonian-Lyellian concept has been the basis for most of our geological thinking. Many of the so-called fallacious concepts developed from the beliefs of later workers and can hardly be traced to the original writings of James Hutton or Charles Lyell.

James Hutton was dedicated to the ‘volcanist’ idea that the internal heat of the earth is wholly responsible for the earth processes. This was in marked contrast to the ‘Neptunist’ concept, patronised by Abraham Gottlob Werner (1749–1817), that all rocks except recent volcanic lavas are produced from chemical or mechanical deposits in a universal ocean. So profound was the influence of the latter school on contemporary geologists that the idea of lateral change in depositional events (facies change), introduced by Gressly in 1833, did not find easy acceptance either in Europe or in North America for a long time. Geologists were still under the spell of the Neptunist belief that all rock layers initially laid down in a universal ocean, must also be continuous, without change.

An important event in mid-19th century was the observation by James Hall in the Appalachian mountains of North America that the thickest sediments accumulate in long, linear belts of subsidence in the earth’s crust. This observation eventually led to the formulation of the concept of geosyncline, a model that dominated the sedimentological world for nearly a century.

Developments Since Late 19th Century

The late nineteenth and early twentieth centuries witnessed rapid developments. Studies by pioneers such as Henry Clifton Sorby (1826–1908) in England led to the foundation of sedimentology as we know it today. Sorby was not only interested in the study of sedimentary structures and their implications, but was also one of the first to make extensive use of the petrographic microscope in the study of sedimentary rocks. He produced two classical volumes on sedimentary petrography and also a monograph on chertified limestones. A few years later, another important study on lithogenesis, by Johannes Walther, appeared in Germany. Walther also formulated the Law of Facies Succession which still bears his name (see Chapter 8).

The first half of the twentieth century witnessed the initiation of specialised studies in many different branches of sedimentology. Systematic studies on the processes of sedimentation and their bearing on stratigraphy were initiated in North America by Grabau, Barrell and Schuchert. Studies by W.M. Davis (1850–1934) and G.K. Gilbert (1843–1918) and later by Twenhofel (1875–1957), established a close link between surface processes of the earth, sedimentation and stratigraphy. This marked the beginning of the use of the present as a key to the past. In 1944 sedimentologists’ interests in modern sediments were revived with the publication of studies on the Mississippi delta by H.N. Fisk and his associates. Classical studies on modern eolian deposits (Bagnold 1941; McKee 1979), modern fluvial deposits (Hjulström 1935; Sundborg 1956; Leopold, Wolman and Miller 1964), and modern tidal sediments (Reineck since 1952, see Reineck and Singh, 1980; van Straaten 1954, 1954a; Ginsberg 1975) have been emulated by sedimentologists throughout the world for nearly half a century now (see Chapter 6).

Petrographic studies initiated by Sorby in England and Walther in Germany were continued, among others, by Grabau, Krynine and their students in North America (Chapter 4). The basic ideas on textural parameters of sediments were formulated during the first three decades of the present century (Udden 1914; Wentworth 1922; Wadell 1932, 1935; Zingg 1935; see Chapter 3). This was also the time when many of the procedures for sedimentological analyses were standardised.

The trend of laboratory simulation of sedimentary processes, initiated in France by Daubrée around 1870, was revived by G.K. Gilbert (1914) in North America. Since that time many of the hydraulic laboratories engaged in experimental studies on sedimentary processes have contributed greatly to our understanding of the basic processes involved in sedimentation. That such studies can be of importance to the perception of many fundamental geological problems was demonstrated in the Netherlands by Kuenen and Migliorini (1950). This was followed by many more contributions to experimental sedimentology throughout the world (see Chapter 5).

Vant Hoff’s experiments on marine carbonates in the early years of this century and Correns’ experimental studies around 1920 in Germany, marked the beginning of sedimentary geochemistry. This field of study is presently actively pursued, particularly in North America (see Krumbein and Garrels 1952; Garrels and MacKenzie 1971).

A major event of the present century was the revival of interest in palaeocurrent research, initiated in the nineteenth century in England by H.C. Sorby. Palaeocurrent studies by Brinkmann and also by Cloos in Germany around 1930 were rejuvenated in North America (see Pettijohn 1962), to be followed by development of many sophisticated techniques in palaeocurrent r...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• PREFACE
• 1. INTRODUCTION
• 2. PROCESSES OF SEDIMENTATION
• 3. SEDIMENTARY TEXTURE
• 4. SEDIMENTARY PETROLOGY
• 5. HYDRAULICS, SEDIMENT TRANSPORTATION AND STRUCTURES OF MECHANICAL ORIGIN
• 6. SEDIMENTARY ENVIRONMENTS AND FACIES
• 7. TECTONICS AND SEDIMENTATION
• 8. STRATIGRAPHY AND SEDIMENTATION
• 9. BASIN ANALYSIS : A SYNTHESIS
• REFERENCES
• SUBJECT INDEX