The petrographic study of carbonate rocks is particularly useful because carbonate grains, unlike clastic terrigenous ones, normally are produced in close proximity (from less than a meter to hundreds of meters) to the site of their ultimate deposition. In addition, carbonate grains are formed mainly by organisms, and thus the grains convey ecological information about the environment of formation as well as stratigraphical information on the age of the deposit.
In some ways, carbonate petrography is not a very complex undertaking, especially when compared to the petrography of clastic terrigenous deposits. Most carbonate rocks are dominated by just one or two common carbonate minerals (mainly calcite and dolomite) plus a limited number of accompanying minerals -- silica, detrital grains, phosphate, glauconite, and a few evaporite precipitates. The diagram below shows the general compositions of the full spectrum of carbonate minerals found in modern and ancient strata.
In other ways, however, carbonate petrography can be quite complicated. Many different organisms produce carbonate material and that requires learning how to recognize a wide variety of shell morphologies and wall structures. The 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 is by no means a complete treatise or textbook --that would be essentially impossible in a single volume. It does, however, include a wide variety of examples of commonly encountered skeletal and nonskeletal grains, cements, fabrics, and porosity types. 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. A selected bibliography is provided to permit the interested reader to pursue details that are only briefly covered in this book and to supplement the interpretive aspects of petrographic work.
Most pictures in this book were chosen to illustrate typical rather than spectacular, but unusual, examples of grains and fabrics. For example, grains that were originally composed of aragonite normally undergo wholesale diagenetic alteration and extensive destruction of primary structural features. Therefore, we show examples of these grains in their extensively altered state because that is the norm for what the user will encounter. 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.
In terms of the overall costs of energy exploration or academic geoscience today, the financial investment needed for petrographic work is relatively insignificant. A basic polarizing microscope can be purchased currently for $2000 to $25,000 depending on optical quality, accessories, and other factors. Thin sections can be purchased for $8 to $20 each from a number of commercial labs. Acetate peels (see technique section of the bibliography) can be made in any office in minutes from polished rock slabs, and can provide a remarkable amount of information. Outcrop samples, conventional cores, sidewall cores, and cuttings samples all can be examined microscopically, although the quality of textural information decreases with decreasing sample size. Even the investment of time involved in petrographic work need not be great relative to the potential for problem solving. Few other techniques are as valuable and accurate for the identification of preserved, destroyed, or created porosity, or the prediction of depositional and diagenetic trends.
Research conducted over the past several decades has outlined many principles of deposition and diagenesis in carbonate sediments. Facies models have been established for modern (as well as ancient) reefs and other bank-margin deposits, for tidal-flat and sabkha sedimentation, for basinal deposition, and for other environments. Diagenetic studies have pointed out the influence of syndepositional marine cementation, early freshwater diagenesis, and later subsurface compaction-dissolution phenomena. This work has clearly shown that, although carbonate depositional and diagenetic patterns may be complex, commonly there is a large volume of information recorded in the rocks, which can be used to decipher this record.
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.