October 25, 2020

Origin of limestones

by binhpm in News

Limestones originate mainly through the lithification of loose carbonate sediments. Modern carbonate sediments are generated in a variety of environments: continental, marine, and transitional, but most are marine. The present-day Bahama banks is the best known modern carbonate setting. It is a broad submarine shelf covered by shallow, warm seawater. The Bahama shelf, or carbonate platform, mimics the setting that repeatedly prevailed across the stable cratonic areas of the major continental blocks during late Precambrian, Paleozoic, and Mesozoic time and serves as a model for explaining the various limestone types that make up such ancient carbonate successions.

The edge of the shelf is marked by a topographically sharp escarpment flanked by coarse, angular limestone breccia. Submarine channels etched into the escarpment serve as waterways down which shallow-water carbonate sediment can be transported by turbidity currents capable of redistributing them as apronlike deposits on the oceanic abyssal plain. In many areas, the fringe of the Bahama banks is marked by wave-resistant reef rocks (sometimes classified as boundstone). Abrasion of these reefs by wave activity generates abundant skeletal debris. Variations in depth and current strength control the relative amounts of micrite and sparite, the prevalence of specific organisms and their productivity, and the likelihood of generating oöids, pellets, and carbonate rock fragments. Micrite and micritic allochemical sediments accumulate in deep-water, low-energy, protected areas like lagoons and tidal flats and on the leeward side of major islands. In high-energy, shallow-water locales such as beaches, coastal dunes, and tidal channels, currents winnow out any micrite, and these become the sites of sparry allochemical sediment deposition. Pinpointing the exact depositional setting for an ancient carbonate deposit requires detailed analysis of its texture, composition, sedimentary structures, geometry, fossil content, and stratigraphic relationships with modern carbonate depositional sites.

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