Atlas Of Sedimentary Rocks A E Adams W S Mackenzie C Guilforde

Contents Preface vi Acknowledgements vii Part 1 Terrigenous clastic rocks 2 Part 2 Carbonate rocks 33 Part 3 Other sedimentary rocks 75 Appendix 1 Preparation of a thin section of a rock 97 Appendix 2 Staining a thin section of a limestone 99 Appendix 3 Preparation of a stained acetate peel of a limestone 100 References 101 Index 102 Preface The study of rocks using thin sections and a petrographic microscope was initiated by Henry Clifton Sorby in the middle of the nineteenth century and the first rocks he described were silicified limestones from the Harqate in Yorkshire. This work was published in 1851. His presidential address to the Geological Society of London in 1879 was entitled 'On the structure and origin of limestones', and Sorby had a series of plates made from camera lucida drawings, reproduced for private circulation with copies of the text of his address. These illustrated the microscopic characteristics of limestones from through out the British geological record and announced to the first petro graphic atlas. Despite the prominence of Sorby's work, much of which is still valid today, few books recognize its importance at the time. While many examples of, or specimens and illustrations of, thin section are extremely important, that of sedimentary rocks languished until well after Sorby's centenary. Since about 1960, with the much expanded teaching of geology, there has been a major development in microscopic disciplines of sedimentary rocks, sedimentary petrogaphy has described more and more important facts of geology and forms a key part of the term and thus teaching of geology in many institutions. The Atlas of homogeneous rocks is first of named bibliography board, a analogous introductory text to one you held by Director. We terrain on the one and the The aim has been to produce an atlas somewhat similar to that of the previously published miscroscopic illustrations. Whether or not it is necessary to include so many limited sections or carbonate rocks, and the photomicrograph is comprehensive, we have tried to include photographs of most of the components of sedimentary rocks encountered in thin sections during an undergraduate course in geology. The book is in three parts. Part 1 deals with the terrigenous clastic rocks and concentrates on sandstones, since the petrographic miner alogy is essentially employed with rocks of this grain size. We have attempted to show the common mineral components of sandstones and the range of rock types occurring, without becoming involved in details of the many classifications which exist. Part 2 deals with carbonate rocks and is the longest section in the book. This is because to the newcomer to carbonate petrology limestones contain a bewildering variety of shapes and types. The biologist in particular know scuba variation in shape and structure that it is often been difficult to know what to leave out. We have attempted to show Part 3 illustrates ironstones, cherts, evaporites, phosphorites and carbonateous rocks in this section. We hope the section on evaporites will be of particular interest, as published colour photomicrographs of some minerals are rare. Three appendices are included. Appendix 1 is a slightly modified form of the appendix in the Atlas of igneous rocks and their textures and illustrates how a thin section of a rock is made. Appendix 2 describes methods of staining thin sections of limestones, and Appendix 3 demonstrates how to make acetate peels. Throughout the book we have used those we feel best descriptive and those used in most geological works. New techniques have been illustrated. Extensive cross-referencing is given to hend our reader and to other photographic parallel phenomena. Help in many instan ces has helpful. We en the place expository much elemnts from the British Isles. we have however that it is representative of sedimentary rocks Acknowledgements Although this book is based on thin sections and acetate peels held in the teaching collections of the Department of Geology, University of Manchester, it could not have been possible without the generous loan of material from the research collections of many colleagues. We are particularly indebted to Professor Sir Frederick Stewart who loaned much material for the evaporites section. We are grateful to Dr. J. M. Aitkenell, P. Gutteridge, J. Kanttoute, J. E. Pollard, A. T. S. Ramsey, K. Schofield, Mr R. D. Vaughan and Professor R. Walton, all of whom loaned material and photographs for sections. In the appropriate captions we have also like thank Professor J. L. Dronchard for permission to include a photograph of one of Sorby's thin sections. Women would be test at Staffitres University and Rebeccar Hand with Patricia Crock for her patient typing of various versions of the text and Phil Stunley for drafting the originals of the diagrams. Finally we wish to acknowledge the help given to us by all of the working group government Group. We acknowledgeed permission from Springer Verlag and Professor F. T Pettijohn to reproduce Figs. A and D, and the American Association of Petroleum Geologists for Figs E and F and Tables 3 and 4. Atlas of sedimentary rocks under the microscope A. E. Adams, W. S. MacKenzie and C. Guilford Part 1 Terrigenous clastic rocks Introduction Terrigenous clastic sediments are made up of transported fragments derived from the weathering of pre-existing igneous, sedimentary or metamorphic rocks. These rocks are classified initially according to grain size, using the Udden-Wentworth scale (Table 1). It is those terrigenous sediments of intermediate grain size - the coarser siltstones, sandstone and finer conglomerates and breccia - that are most usefully studied using the petrographic microscope, since the grain types can be identified by this means. The principal component grain types are quartz, feldspar and rock fragments. The bulk of such sediments may be the fine-grained weathered products of the source rocks, such as clay minerals, or it may be a secondary cement. Clays and shales are too fine-grained for study using the petrographic microscope and must be examined by electron microscopy or X-ray diffraction. The components of coarser conglomerates and breccias can usually be identified with the naked eye or with a hand lens. The shape and roundness of the component grains of terrigenous clastic rocks are important in describing sedimentary textures. Categories of low sphericity and high roundness are shown in Fig. A. Sedimentary textures are discussed on p.34. Table 1. Grain-size classification of sediment Size in mm of class boundary Class term Grain size terms for rock 256 boulders -|- rudite |- coarse rock 64 cobbles conglomerate, breccia 4 pebbles |- arenite 2 granules erraceous rock |- sandstone 1 very coarse sand 0.5medium sand0.25fine sand0.1very fine sand0.0625coarse silt0.03125medium silt0.0156fine silt0.0078very fine silt0.0039clayclaystone Fig. A Categories of roundness for grains of low and high sphericity (after Pettijohn et al., 1973) 5. Well-rounded 4. Rounded 3. Subrounded 2. Subangular 1. Angular 0. Very angular Quartz The most abundant grain type in sandstones and conglomerates is quartz. In addition to the size and shape of individual quartz grains, the following features should be observed since they may provide clues to the provenance of he sediment: 1. Whether the quartz grains are single crystals (monocrystalline) or are made up of a number of crystals in different orientations (polycrystalline). 2. Whether extinction is uniform (the grain extinguishes in one position on rotation of the stage) or undulose (extinction straggle, over an area of at least 5°). 3. The presence or absence of inclusions. 4. In the case of polycrystalline grains, whether the crystal boundaries are straight or sutured. 1 and 2 show subrounded quartz grains with anserine crystals, taken with plane-polarized light (PPL) and cross polars (XPL). The matrix material between individual grains contains opaque iron oxide and some calcite. The latter shows high-order pink and green interference colours. (1 and 2: Red Mountain Formation. Silurian, Birmingham, Alabama, USA; magnification x 58, 1 PPL, 2 XPL) Terrigenous clastic rocks 6, 7, 8 Quartz (continued) The quartz grain in the centre of the field of view in 6 appears to be a single homogenous crystal. In 7 however, where the same field of view is seen under crossed polars, the quartz grain is clearly made up of parts of two crystals. One, comprising the upper left portion of the grain is showing entirely interference colours, whereas the rest of the grain comprises a crystal with areas showing slightly different interference colours. The colours become progressively darker towards the centre of the grain. Such a grain would show sweeping undulose extinction. It is the homogeneity of quartz when rotated, which is unusual, and is found in quartz grain from both igneous and metamorphic sources. Quartz grains containing prominent inclusions of a variety of minerals may yield information as to the provenance of the rock. Inclusions are identifiable only if they are fairly large or need-shaped inclusions, although they are too small for the mineral to be identified at the magnification shown. Inclusions of the fluid present at the time of crystallization are common in quartz crystals and are known as fluid inclusions or vacancies. 8 shows a quartz grain with abundant vacuoles. These appear as dark specks, and in the sample illustrated, many are concentrated in lines running at an angle to the length of the grain they are drilled with abundant vacuoles is usually derived from a source of low-temperature origin, such as a hydrothermal vein, and appears milky-white in a hand specimen. The photograph also shows a green mineral in the matrix around the quartz grain, which is chlorite. 6 and 7: locality and age unknown; magnification x 72. 6: PPL. 7: XPL. 8: Coal Measures, Upper Carboniferous, Lancashire, England; magnification x 72. PPL. Undulose extinction can also be seen in 5. Terrigenous clastic rocks 11, 12, 13 Feldspar (continued) 11 and 12 show a pebble-sized fragment composed almost entirely of microcline. Microcline can be identified only by the cross-hatched twinning which it invariably shows. Although the microcline shows little alternation, feldspar grains in the upper rock fragments, including microcline pla