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Abstract

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.

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.

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