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1590 results in Sedimentology and stratigraphy

Page 30 of 80

  • 6 - Change in stress

  • David Muir Wood, University of Bristol
  • Book:
    Soil Mechanics
    Published online:
    28 May 2018
    Print publication:
    28 September 2009, pp 127-137

  • Summary

    Introduction

    In Chapter 4 we introduced the concept of the stiffness of soils under one-dimensional loading and were able to calculate the change in vertical strain, and hence the change in thickness of a soil layer that might occur as a result of a change in effective stress. In Chapter 5, we encountered the concept of permeability of soils and noted in particular the huge range of values of permeability for soils, broadly, of different particle sizes (but also influenced by the mineralogy and shape of the particles). The permeability of clays is many orders of magnitude lower than the permeability of sands and gravels. Change in effective stress implies change in vertical dimension of the soil layers, which implies the squeezing out or the sucking in of water (assuming that the soil is saturated). In a soil of very low permeability this cannot happen rapidly, and, in this chapter, we will make deductions about the short-term and long-term conditions that must apply. The analysis of the process that spans between the short term and the long term is called consolidation and is the subject of Chapter 7.

    Stress change and soil permeability

    Figure 6.1 provides an analogy for the behaviour of a soil with low permeability when it is subjected to a change in external stresses. The spring represents the soil, and the loads taken by the spring represent the effective stress carried by the soil particles.

  • Preface

  • David Muir Wood, University of Bristol
  • Book:
    Soil Mechanics
    Published online:
    28 May 2018
    Print publication:
    28 September 2009, pp ix-x

  • Summary

    This book has emerged from a number of stimuli.

    There is a view that soils are special: that their characteristics are so extraordinary that they can only be understood by a small band of specialists. Obviously, soils do have some special properties: the central importance of density and change of density merits particular attention. However, in the context of teaching principles of soil mechanics to undergraduates in the early years of their civil engineering degree programmes, I believe that there is advantage to be gained in trying to integrate this teaching with other teaching of properties of engineering materials to which the students are being exposed at the same time.

    It is a fundamental tenet of critical state soil mechanics – with which I grew up in my undergraduate days – that consideration of the mechanical behaviour of soils requires us to consider density alongside effective stresses, thus permitting the unification of deformation and strength characteristics. This can be seen as a broad interpretation of the phrase critical state soil mechanics. I believe that such a unification can aid the teaching and understanding of soil mechanics.

    There is an elegant book by A. J. Roberts which demonstrates in a unified way how a common mathematical framework can be applied to problems of solid mechanics, fluid mechanics, traffic flow and so on.

  • 7 - Consolidation

  • David Muir Wood, University of Bristol
  • Book:
    Soil Mechanics
    Published online:
    28 May 2018
    Print publication:
    28 September 2009, pp 138-168

  • Summary

    Introduction

    In previous chapters, we have noted that soils of low permeability – typically, clayey soils – will not be able to respond rapidly to changes in stresses which require deformation, which in turn implies, for our one-dimensional systems, change in volume. We suggested in Section 6.2 that there would be a transient process between time t = 0 and time t → ∞ during which the temporary disequilibrium of pore pressures would disappear as deformation of the soil spring permitted stresses to be transferred from pore pressure to effective stresses supported by the soil itself. The detail of this process was not considered: we merely looked at the two extremes of the immediate aftermath of the change in external stress (at t = 0) and the eventual equilibrium at infinite time (t → ∞). In this chapter, we will explore various aspects of the analysis of the transient process between these two extremes. The transient process is known as consolidation.

    We will first produce an approximate solution which forces us to concentrate on the important physical aspects of the problem. However, the governing differential equation – the diffusion equation – is one that is common to many problems which involve gradient driven flow: heat flows down a temperature gradient, pollution flows down a concentration gradient, pore water flows down a gradient of total head (Sections 5.2, 5.4).

  • 8 - Strength

  • David Muir Wood, University of Bristol
  • Book:
    Soil Mechanics
    Published online:
    28 May 2018
    Print publication:
    28 September 2009, pp 169-207

  • Summary

    Introduction

    In the context of our chosen one-dimensional approach to the mechanics of soils, we are somewhat limited in what we can say about the strength of soils but there are some ideas which can usefully be presented. Stiffness is concerned with the deformations of geotechnical systems – the serviceability limit states under operational or working loads. Strength is concerned with the collapse of geotechnical systems – the ultimate limit states for which failure of the geotechnical system will occur. Classically, it has always been easier to make statements about ultimate collapse conditions than about deformations, and geotechnical design often proceeds by starting with a collapse calculation and then factoring down the loading sufficiently that, from experience, the resulting reduced load would not be expected to produce excessive displacements. This is always a rather uncertain route by which to control those displacements, especially if the nature of the problem under consideration is more than somewhat different from those previously experienced – a proper understanding of stiffness is really more satisfactory. However, it does emphasise the traditional importance of understanding the strength of soils and the modes of failure of geotechnical systems.

    Failure mechanisms

    Figure 8.1 shows a schematic picture of a pile foundation. A pile is a long slender stiff structural member which is used to transfer loads from some surface structure through more or less soft soils to a certain depth in the ground (Fig. 1.9).

  • 10 - Envoi

  • David Muir Wood, University of Bristol
  • Book:
    Soil Mechanics
    Published online:
    28 May 2018
    Print publication:
    28 September 2009, pp 230-231

  • Summary

    Summary

    Each chapter has ended with a summary of the key points. Here it is appropriate to reiterate the primary intentions of this book and to point the way towards the next stages of soil mechanics education.

    There are two themes which have been developed in this book:

  • The model of soil stiffness and strength links effective stresses and density. Changes in mechanical behaviour of soils can only be properly understood when the stresses and volumetric packing are considered in parallel. The role that history plays in changes in density – which tend to be locked into soils as loads are applied and then removed – is key for the subsequent understanding of the strength of soils. There is then a link with permeability and the ease with which water (or other pore fluid) can move through the voids of the soil, and thus the ability of the soil to respond rapidly to changes in stress. The concepts of drained and undrained response again bring together the thoughts about effective stresses and volumetric packing. This is the essence of critical state soil mechanics, which at its simplest level (Fig. 10.1) tells us that we cannot hope to understand the behaviour of soils unless we think all the time in parallel of the changes in effective stress and of density.

  • […]

  • Chapter 5 - Conglomerates

  • from Part II - Siliciclastic sedimentary rocks
  • Sam Boggs, Jr, University of Oregon
  • Book:
    Petrology of Sedimentary Rocks
    Published online:
    05 June 2012
    Print publication:
    19 February 2009, pp 165-193

  • Summary

    Introduction

    Siliciclastic sedimentary rocks that consist predominantly of gravel-size (> 2 mm) clasts are called conglomerates. The Latin-derived term rudite is also sometimes used for these rocks. Conglomerates are common rocks in stratigraphic sequences of all ages, but make up less than about one percent by weight of the total sedimentary rock mass (Garrels and McKenzie, 1971, p. 40). Geologists have focused a level of attention on conglomerates, judged by the number of published papers, quite out of proportion to their relative abundance. This focus stems from their usefulness in tectonic and provenance analysis (Chapter 7) and the growing interest of sedimentologists in the rather specialized depositional environments of conglomerates. Also, some conglomerates serve as reservoir rocks for oil and gas.

    The framework grains of conglomerates are composed mainly of rock fragments (clasts) rather than individual mineral grains. These clasts may consist of any kind of rock. Some conglomerates are composed almost entirely of highly durable clasts of quartzite, chert, or vein quartz. Others are composed of a variety of clasts, some of which, limestone and shale clasts for example, may be unstable or weakly durable. Conglomerates may contain various amounts of matrix, which commonly consists of clay- or sand-size particles or a mixture of clay and sand.

    Owing to their coarse grain size, conglomerates do not lend themselves readily to study in the laboratory. They are studied primarily in the field, by a variety of techniques.

  • Chapter 8 - Diagenesis of sandstones and shales

  • from Part II - Siliciclastic sedimentary rocks
  • Sam Boggs, Jr, University of Oregon
  • Book:
    Petrology of Sedimentary Rocks
    Published online:
    05 June 2012
    Print publication:
    19 February 2009, pp 268-310

  • Summary

    Introduction

    Petrologic study of siliciclastic sedimentary rocks commonly focuses on interpretation of sediment provenance and depositional paleoenvironments. It may aim also at evaluating the economic significance of these sediments as source rocks or reservoir rocks for petroleum or as hosts for ore deposits. Discussion in previous chapters suggests that genetic interpretations can be compromised by postdepositional changes in composition or texture brought about by diagenetic processes. Provenance interpretation, in particular, can be severely affected by diagenetic processes that selectively remove feldspars, heavy minerals, or rock fragments by intrastratal solution. Diagenesis may also play an extremely important role in postdepositional modification of porosity, causing either decrease in porosity as a result of compaction and cementation or increase in porosity owing to solution processes. Thus, the economic importance of a particular sandstone body as a reservoir rock for petroleum, for example, may very well depend as much upon the diagenetic history of the sandstone as upon its original depositional characteristics. Therefore, to avoid erroneous interpretations of petrogenesis or costly errors in economic evaluations, it is necessary to understand as much as possible about the diagenetic processes that can affect siliciclastic sedimentary rocks. We are concerned not only with the nature of diagenetic processes but also with the types of changes that are produced in the rocks as a result of these processes. That is, we strive to recognize the diagenetic characteristics of siliciclastic rocks and to differentiate these diagenetic features from original depositional characteristics.

  • Chapter 1 - Origin, classification, and occurrence of sedimentary rocks

  • from Part I - Principles
  • Sam Boggs, Jr, University of Oregon
  • Book:
    Petrology of Sedimentary Rocks
    Published online:
    05 June 2012
    Print publication:
    19 February 2009, pp 3-18

  • Summary

    Introduction

    Sedimentary rocks form at low temperatures and pressures at the surface of Earth owing to deposition by water, wind, or ice. By contrast, igneous and metamorphic rocks form mainly below Earth's surface where temperatures and pressures may be orders of magnitude higher than those at the surface, although volcanic rocks eventually cool at the surface. These fundamental differences in the origin of rocks lead to differences in physical and chemical characteristics that distinguish one kind of rock from another. Sedimentary rocks are characterized particularly by the presence of layers, although layers are also present in some volcanic and metamorphic rocks, and by distinctive textures and structures. Many sedimentary rocks are also distinguished from igneous and metamorphic rocks by their mineral and chemical compositions and fossil content.

    Sedimentary rocks cover roughly three-fourths of Earth's surface. They have special genetic significance because their textures, structures, composition, and fossil content reveal the nature of past surface environments and life forms on Earth. Thus, they provide our only available clues to evolution of Earth's landscapes and life forms through time. These characteristics of sedimentary rocks are in themselves reason enough to study sedimentary rocks. In addition, many sedimentary rocks contain minerals and fossil fuels that have economic significance. Petroleum, natural gas, coal, salt, phosphorus, sulfur, iron and other metallic ores, and uranium are examples of some of the extremely important economic products that occur in sedimentary rocks.

  • Chapter 11 - Diagenesis of carbonate rocks

  • from Part III - Carbonate sedimentary rocks
  • Sam Boggs, Jr, University of Oregon
  • Book:
    Petrology of Sedimentary Rocks
    Published online:
    05 June 2012
    Print publication:
    19 February 2009, pp 408-458

  • Summary

    Introduction

    Chapter 8 discusses the processes that bring about diagenesis of siliciclastic sediments. In that chapter, the terms eodiagenesis, mesodiagenesis, and telodiagenesis are used to describe the stages of diagenesis that occur in siliciclastic sediments as they are progressively buried and subsequently uplifted. Significant differences exist between siliciclastic and carbonate sediments; therefore, the course of diagenesis differs in these two fundamentally different kinds of rocks. To illustrate, carbonate sediments are intrabasinal deposits, which are precipitated in some manner from the water in which they are deposited. Thus, initially, carbonate minerals are more or less in chemical equilibrium with the waters of their depositional environment. By contrast, siliciclastic sediments are brought into the depositional basin from outside. Further, carbonate sediments are composed of only a very few major minerals (aragonite, calcite, dolomite) in contrast to a much larger variety of minerals and rock fragments that may be present in siliciclastic sedimentary rocks. Carbonate minerals are more susceptible in general to diagenetic changes such as dissolution, recrystallization, and replacement than are most silicate minerals. Also, they are generally more easily broken down by physical processes, and they are much more susceptible to attack by organisms that may crush or shatter shells or that may bore into carbonate grains or shells.

    Nonetheless, carbonate sediments proceed in a general way through the same diagenetic regimes as siliciclastic sediments. That is, carbonate sediments go through early (shallow-burial), middle (deep-burial), and possibly late (uplift and unroofing) stages of diagenesis.

  • Chapter 12 - Evaporites, cherts, iron-rich sedimentary rocks, and phosphorites

  • from Part IV - Other chemical/biochemical sedimentary rocks and carbonaceous sedimentary rocks
  • Sam Boggs, Jr, University of Oregon
  • Book:
    Petrology of Sedimentary Rocks
    Published online:
    05 June 2012
    Print publication:
    19 February 2009, pp 461-526

  • Summary

    Introduction

    This chapter deals with the chemical/biochemical rocks other than carbonates. Thus, it discusses evaporites, the siliceous sedimentary rocks (cherts), iron-rich sedimentary rocks (iron-formations and ironstones), and phosphorites. Volumetrically, these rocks are far less abundant than siliciclastic sedimentary rocks and carbonate rocks. If we add the percentages of shale, sandstone, and carbonate rocks calculated or measured by various authors such as Ronov et al. (1980), the unaccounted volume of total sedimentary rocks that can be attributed to these chemical/biochemical rocks appears to be no more than about 2 percent. The small volume of these rocks is not, however, a measure of their importance or the interest that we have in them. All of the sedimentary rocks discussed in this chapter have considerable economic significance. Evaporite deposits such as gypsum, halite (rock salt), and trona are mined for a variety of industrial and agricultural purposes, iron-rich sedimentary rocks are the source of most of our iron ores, phosphorites are extremely important sources of fertilizers and other chemicals, and the siliceous sedimentary rocks have some economic value, e.g. in the semiconductor industry.

    In addition to their economic importance, all of the rocks discussed in this chapter are intrinsically interesting owing to their compositions and origins. The origin of many of these rocks is still enigmatic even after many years of study.

  • Chapter 9 - Limestones

  • from Part III - Carbonate sedimentary rocks
  • Sam Boggs, Jr, University of Oregon
  • Book:
    Petrology of Sedimentary Rocks
    Published online:
    05 June 2012
    Print publication:
    19 February 2009, pp 313-381

  • Summary

    Introduction

    As indicated in Chapter 1, carbonate rocks make up about one-fifth to one-quarter of all sedimentary rocks in the stratigraphic record. They occur in many Precambrian assemblages and in all geologic systems from the Cambrian to the Quaternary. Both limestone and dolomite are well represented in the stratigraphic record. Dolomite is the dominant carbonate rock in Precambrian and Paleozoic sequences, whereas limestone is dominant in carbonate units of Mesozoic and Cenozoic age (Ronov, 1983).

    On the basis of their abundance alone, about the same as that of sandstones, carbonate rocks are obviously an important group of rocks. They are important for other reasons as well. They contain much of the fossil record of past life forms, and they are replete with structures and textures that provide invaluable insight into environmental conditions of the past. Aside from their intrinsic value as indicators of Earth history, they also have considerable economic significance. They are used for a variety of agricultural and industrial purposes, they make good building stone, they serve as reservoir rocks for more than one-third of the world's petroleum reserves, and they are hosts to certain kinds of ore deposits such as epigenetic lead and zinc deposits.

    The microscopic study of carbonate rocks dates back to the beginning of petrographic analysis. The science of petrography was initiated by an English geologist named Henry Clifton Sorby, who began petrographic analysis in about 1851 with the study of limestones.

  • Chapter 6 - Mudstones and shales

  • from Part II - Siliciclastic sedimentary rocks
  • Sam Boggs, Jr, University of Oregon
  • Book:
    Petrology of Sedimentary Rocks
    Published online:
    05 June 2012
    Print publication:
    19 February 2009, pp 194-219

  • Summary

    Introduction

    Fine-grained, siliciclastic sedimentary rocks, composed mainly of particles smaller than ∼ 62 microns (coarse silt and finer), make up approximately 50 percent of all sedimentary rocks in the stratigraphic record (Chapter 1). Thus, they are about twice as abundant as sandstones and conglomerates combined. These fine-grained rocks are known by a variety of names, including lutites, siltstones, mudstones, mudrocks, claystones, and shales. Tourtelot (1960) reviews in detail the history of fine-sediment terminology. He points out (p. 342) that historically the term shale has been used in two ways: (1) in a restricted sense to mean a laminated clayey rock and (2) as a broad, group name for all fine-grained siliciclastic rocks. He concludes that it is acceptable practice to include both these meanings for shale, and, therefore, that shale is the appropriate class name for fine-grained rocks, of equal standing with sandstones and limestones as group names.

    Certainly our comprehension is broad enough to include two meanings of the word “shale”: First, the reasonably precise meaning of “laminated clayey rock” to which the origin of the word entitles it, and second, the meaning of the “general class of fine-grained rocks,” which our historical use of the word bequeaths to it.

    Although some geologists (e.g. Potter et al., 1980, p. 15) have agreed with Tourtelot's conclusion that shale is an acceptable class name for all fine-grained rocks, others (e.g. Lundegard and Samuels, 1980; Spears, 1980; Stow and Piper, 1984) consider the dual use of the term in this way to be confusing.

  • Chapter 3 - Sedimentary structures

  • from Part II - Siliciclastic sedimentary rocks
  • Sam Boggs, Jr, University of Oregon
  • Book:
    Petrology of Sedimentary Rocks
    Published online:
    05 June 2012
    Print publication:
    19 February 2009, pp 63-110

  • Summary

    Introduction

    Study of sedimentary structures has captured the interest of geologists for decades. Some sedimentary structures such as cross-bedding and ripple marks were recognized as early as the late eighteenth century, and perhaps well before. Progress in identification, description, classification, and interpretation of sedimentary structures has been especially rapid since the 1950s, and the fundamental origin of most sedimentary structures is now reasonably well understood. Nonetheless, empirical study of modern and ancient sediments and experimental investigation of the mechanisms that form sedimentary structures continue. Geologists are especially interested in understanding how specific sedimentary structures are related to such aspects of ancient depositional environments as relative water energy, water depth, and current flow directions. Investigation of the origin and significance of bedforms such as ripples and dunes has been a particularly active field of research.

    Many sedimentary structures originate by physical processes involving moving water or wind that operate at the time of deposition. Others are formed by physical processes such as gravity slumping or sediment loading that deform unconsolidated sediment after initial deposition (soft-sediment deformation). Still other structures are of biogenic origin, formed by the burrowing, boring, browsing, or sediment-binding activities of organisms. Some types of bedding, the laminated bedding of evaporites for example, are generated by primary chemical precipitation processes. A few other structures, such as concretions, form by chemical processes operating within sediment during burial and diagenesis; thus, they are regarded to be secondary in origin.

Page 30 of 80