Carbonate Sedimentary Rocks

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Carbonate Rocks and Dunham’s Classification

Palkowski, Daniel N.
245-002 General Geology
Dr. Wayne Schlipp
9 December 2007

The following contains a compendium of research on carbonate rocks. Displayed are a basic description of carbonate rocks, depositional environments, mineralogy, ancient and modern reefs, diagenesis, facies analysis, and classification. Most carbonate rock classification schemes take into account characteristics that may require intensive microscopic study. These classifications are comprehensive, but are only practical if a laboratory is available. Classification based on depositional texture gives a profound insight to the energy of ancient reefs. Dunham’s classification is more descriptive and is much easier to use. Included is a simple concise explanation of Dunham’s classification of carbonate rocks. With a basic understanding of internal structure and texture the Dunham classification can be employed as simply and with as much success as Pettijohn’s classification of clastic sedimentary rocks: saving time, money, and wasted laboratory effort.

At first glance, an outcrop of carbonate rocks may look very boring, for they are usually drab in color (grey or white) and do not display the typical points of reference that clastic sedimentary structures possess. They are equally unappealing in the scientific jargon given to them, with terms such as fascicular optic calcite and baroque dolomite, to name a few. The interesting aspect of carbonate rocks is not the glossy metallic shine of the sample but the importance of them and the clues they hold into Earth’s ancient past. Covering 7% of the Earth’s land surface carbonates play an important role in the planet’s history and in the future of human existence. Carbonates are vital in the manufacturing of cement, as building stone and aggregates, and they form the reservoirs for about 40% of the world’s oil reserves. In engineering terms they frequently underlie the most intractable geotechnical problems and often provide the foundations for coastal structures and offshore structures (Williams.edu2007). The understanding and efficient production of carbonate outcrops has become the priority of the oil and engineering industries. Present efforts in taking advantage of carbonate rocks focus on accurately targeting wells to optimize production from untouched reserves. With the abundance of classification schemes for carbonate rocks, which is the most concise and straight forward? With a fundamental understanding of texture and organic and inorganic grain type, ancient and modern reefs, diagenesis, the Dunham classification of carbonate rocks can indicate energy of deposition and sequence of deposition, resulting in accurate depictions of paleoenvironment (Akbar et al. 1995).

Carbonates for the Novice
Carbonates are a group of sedimentary rocks that are entirely different from the clastic sedimentary rocks (sandstones, shales, etc.) with which most people are familiar. The major defining factors that separate carbonates from clastic sedimentary rocks are composition, source, and durability. Clastic sedimentary rocks are composed of a variety of silica-based grains and carbonates consist mainly of three minerals--calcite, aragonite and dolomite. Carbonates are considered to be autochthonous (in situ), remaining near the point of origin where as clastic sediment may have traveled hundreds of miles from its original source, making it allochthonous. “Possibly, the ease with which carbonate grains are destroyed, or cemented, accounts for their relatively short transport (Dunham 1962).” The third separating feature of carbonate rocks is their propensity to undergo alteration. Clastics will generally stand up to the rigors of geologic time ( Akbar 1995). The main type of carbonate rock is limestone. Most carbonate sediments are formed in a special depositional...
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