Abstract

Although carbonate rocks have not yet been successfully classified, they can still be used effectively for unravelling the paleogeography of a carbonate basin. Limestones are composed of lime mud (micrite), particles, cement, and porosity, which may be combined in varying amounts to form a myriad of different rock types. Cement and porosity are closely related to the diagenetic history of the rock, but mud and particles reveal the nature of the depositional site. Presence of the mud component signifies a depositional environment of very low turbulence with a lack of currents of removal. The particulate component consists of skeletal particles, pellets, coated grains (including ooliths), detrital particles, and composite grains, all of which are significant in determining environments at the site of accumulation.

Limestone deposits can be divided into clastic and biogenetic origins on the basis of rock texture and geometric configuration of the depositional features. Clastic sediments are those that were transported to the depositional site by wave action, tidal currents, or similar modes of water movement: they are usually distinguished by their degree of sorting along with such primary structures as cross-stratification. Biogenetic accumulations result from prolific in situ biologic activities. The deposits can be either reefs, which are composed of frame-building organisms held in place by an organic binding agent, or biohermal banks, which are accumulations of skeletal debris produced by biologic processes in quiescent waters.

Tidal shoals composed of carbonate sands are typified by the oolite of the Bahamian banks. These shoals are elongate sand complexes that parallel the shelf margin; however, their geometry is complicated by intricate bars and channels that trend at about right angles to the overall elongation. The oolite bars are built by flood tides but are modified to their present configuration by ebb tides.

The coral reefs of the south Florida shelf are good examples of the modern reef concept. They are elongate biotic jungles that flourish along the windward shelf margins. Modern reefs are composed of large, massive colonial corals that make the framework of the reef, and encrusting coralline algae that bind the reef into a rigid, wave-resistant body.

Biogenetic banks are mound-shaped shoals of muddy skeletal sediments that are built in place by intense biologic activity. They are typically represented in modern carbonate seas by Rodriquez, Tavernier, and Snake Creek Banks on the inner shelf of the upper Florida Keys. Bank-building organisms consist largely of red and green calcareous algae, but corals, molluscs, and foraminifera also are abundant: burrowing organisms continually stir the muddy bank sediments. Banks are constructed mainly through the trapping and binding functions of marine grasses and grow in quiet, well-protected marine waters of the back shelf, but their configuration and orientation are controlled by the effects of the prevailing winds on the biota; thus, they are good indicators of environment.

Limestones are particularly subject to alteration by diagenetic processes which may enhance or reduce their attractiveness to the petroleum geologist according to the nature of the resulting porosity. Careful analysis of carbonate rocks can aid significantly in the search for hydrocarbons, for sedimentary trends and configurations can be determined with a fair degree of accuracy. Consequently, the cost of exploration may be materially lowered.