Although petrography is an extremely valuable tool for the identification of minerals and their textural interrelations, it is best used (in many cases) in conjunction with other techniques.
Precise mineral determination commonly is aided by staining of thin sections or rock slabs, by X-ray diffraction analysis, or by microprobe examination. Minerals present in small amounts may best be analyzed after separation and concentration using heavy liquids, shaker tables, or other techniques. Likewise, noncarbonate minerals in a carbonate host rock are normally better analyzed in acid-insoluble residues than in thin section. Where detailed understanding of the trace element chemistry of the sediments is essential, X-ray fluorescence, microprobe, atomic absorption, or cathodoluminescence techniques may be applicable.
Commonly, sediments are too fine-grained for adequate examination with the light microscope. The practical limit of resolution of the best light microscopes is in the one to two micrometer (μm) range. Many detrital and authigenic grains such as clays, micritic carbonates, or organic matter fall within or below that size range. Furthermore, because most standard thin sections are about 30 μm thick, a researcher examines 10 to 20 of these small grains stacked on top of one another, with obvious loss of resolution. Smear mounts or grain mounts (slides with individual, disaggregated grains smeared or settled out onto the slide surface) are an aid in examining small grains where the material can be disaggregated into individual components. In most cases however, scanning and transmission electron microscopy are proved to be the most effective techniques for the detailed examination of fine-grained sediments.
Figures & Tables
A Color Illustrated Guide To Constituents, Textures, Cements, and Porosities of Sandstones and Associated Rocks
This book is designed as a companion volume to AAPG Memoir 27. As with its predecessor volume, the purpose of this book is to provide identified illustrations of important grains, textures, cements, and porosity types for geologists who may not be specialists in the petrography of sandstones and associated sedimentary rocks.
Sandstone petrography is of particular interest to the explorationist for several reasons. First, it can provide valuable information on the detailed composition of sedimentary rocks. From this, one can often draw conclusions about the lithology, climate, and tectonic history of the source area, as well as predicting the response of such units to a variety of subsurface diagenetic environments. Second, one can acquire significant data on the grain size, sorting, and rounding of sedimentary grains. For Iithified sediments this may be the only way to obtain such data, which may be useful in determinations of the transport mechanisms and depositional environment of the sediment. Third, information may be obtained on the postdepositional alteration history of sedimentary rocks. This may include data on compaction, cementation, leaching, fracturing, porosity types, and other factors. These are essential for a proper understanding of reservoir rocks and, commonly, petrography provides the only technique forgathering accurate data on such diagenetic factors.
This book is intended as an introduction for exploration geologists or students and is by no meansa complete textbook or treatise. However, it does include a wide variety of color photographs of terrigenous clastic grains, cements, and textures of sandstones and common accessory rock types. Although most of the illustrations are of features seen with the petrographic microscope, some scanning electron micrographs are included. The illustrations were made from samples having as wide a range of lithologies, geologic ages, and localities as possible to insure a fairly representative presentation. In addition, the photographs were generally selected to show the most common grain and textural types encountered by the geologist and to present typical, rather than spectacular, examples of most features. Thus, the book shouId have applicability to any sandstone petrographic study.
This volume focuses on the descriptive aspects of petrography and includes no text other than figure captions. Bibliographies are provided in each section of the book. For more detailed descriptive and interpretive information, the references listed in both the general and specific bibliographies should be consulted.
The major emphasis of th is book is on the fou... major fabric elements of sandstones: framework grains; detrital fine-grained matrix; cements; and pore space.