Cathodoluminescence can be an invaluable tool in petrographic studies. It provides information on the spatial distribution of trace elements in terrigenous clastic (as well as carbonate) grains and cements. Analysis can be done using polished rock chips, polished thin sections, or even unpolished and uncovered thin sections. The equipment requires costs about the same as a moderately priced polarizing microscope and can be installed on virtually any microscope.
This example shows a sandstone from the Devonian Hoing Sandstone Member of the Cedar Valley limestone in Illinois. The upper photo, taken with transmitted light, shows a quartz arenite with elongate, sutured intergranular boundaries which might be considered as indicative of compaction and pressure solution. The lower photo, taken with cathodoluminescence, shows the same field of view with dramatically different results. The detrital grain cores, which luminesce orange and blue, can be seen to be well rounded, and touch each other only at point contacts. Subsequent quartz overgrowths (generally nonluminescent with some luminescent zones) have obliterated most porosity and give the appearance of a compacted fabric when cathodoluminescence is not used. The differences in luminescence between detrital grain cores and authigenic overgrowths is a function of subtle differences in their trace element composition.
These differences are accentuated by long-exposure-time photography of small areas because of the inherent weakness of the luminescence. Photos by R. F. Sippel.
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.