Memoir 77 - Color Guide to Petrography of Carbonate Rocks

Carbonate petrography — the study of limestones, dolomites and associated deposits under optical or electron microscopes — greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies. Petrography is an especially powerful tool because it enables the identification of constituent grains, the detailed classification of sediments and rocks, the interpretation of environments of deposition, and the determination of the often complex history of post-depositional alteration (diagenesis). The last of these, the ability to determine the timing of diagenetic events such as cementation or secondary porosity development relative to the emplacement of hydrocarbons or metallic ores, makes petrography an important component of geochemical and sedimentologic studies in energy- and mineral-resource exploration applications as well as in academic research.

This volume expands and improves the AAPG 1978 classic, A Color Illustrated Guide to Carbonate Rock Constituents, Textures, Cements, and Porosities (AAPG Memoir 27). Carbonate petrography can be quite complicated. Changing assemblages of organisms through time, coupled with the randomness of thin-section cuts through complex shell forms, add to the difficulty of identifying skeletal grains. Furthermore, because many primary carbonate grains are composed of unstable minerals (especially aragonite and high-Mg calcite), diagenetic alteration commonly is quite extensive in carbonate rocks. The variability of inorganic and biogenic carbonate mineralogy through time, however, complicates prediction of patterns of diagenetic alteration.

This book is designed to help deal with such challenges. It includes a wide variety of examples of commonly encountered skeletal and nonskeletal grains, cements, fabrics, and porosity types. It includes extensive new tables of age distributions, mineralogy, morphologic characteristics, environmental implications and keys to grain identification. It also encompasses a number of noncarbonate grains, that occur as accessory minerals in carbonate rocks or that may provide important biostratigraphic or paleoenvironmental information in carbonate strata. With this guide, students and other workers with little formal petrographic training should be able to examine thin sections or acetate peels under the microscope and interpret the main rock constituents and their depositional and diagenetic history.

Carbonate petrography is primarily a qualitative skill. One must learn to recognize the distinguishing characteristics of skeletal grains of various ages, cut in various orientations, and preserved in various stages of alteration. There are no simple diagnostic tests (such as measuring birefringence or an optic figure) that can be used to identify a bryozoan, for example. It is simply a question of experience. Comparison of grains in thin sections with photographs of identified grains, in this and other books, allows geologists to readily identify the majority of the rock-forming grains in their samples.

Most pictures in this book were chosen to illustrate typical rather than spectacular, but unusual, examples of grains and fabrics. Introductory text in each chapter provides the reader with details about original grain mineralogies in order to help the reader anticipate such preservation problems. Examples also were specifically chosen from a variety of countries, basins, and units to provide a sense of the global consistency of carbonate fabrics. Furthermore, examples have been included from rocks of Precambrian to Holocene age because of the enormous evolutionary changes in organisms (and, therefore, carbonate deposits and their alteration) through time.

Petrography, when used in close conjunction with well-log analysis, seismic interpretation, regional geology, and other studies, can be an invaluable tool for applying these recently developed principles of carbonate sedimentology to ancient rocks. Furthermore, it is best applied by the explorationist who is deeply involved in techniques other than petrography, for that person is in the best position to ask the right questions — questions that petrography may be able to answer. That is the goal of this volume.