Carbonate platforms or banks are buildups of locally-formed, laterally-restricted sediment accumulations which possess topographic relief and typically have a more or less horizontal top and abrupt shelf margins where “high energy” reefs or sand shoals occur (Wilson, 1975, p. 20–21). For purposes of this course, we will consider not only the platform but also: (1) carbonate ramps that build away from positive areas and extend down gentle paleoslopes with no striking break in slope; (2) atolls which are ring-like organic accumulations in oceanic environments that surround deep, central lagoons; and (3) barrier reefs which are offshore curvilinear belts of organic accumulation separated from the coast by a lagoon (Wilson, 1975).
Modern carbonate platforms assume many shapes and sizes, and occur in a variety of tectonic settings. However, most are climatologically restricted to within 30° N or S latitude (Fig. 3-1), which is essentially a response to temperature. Warm water environments, where most carbonate platforms develop, have a minimum near-surface temperature no lower than 14-15° C. These areas produce primarily skeletal components such as hermatypic corals, calcareous red and green algae, benthic foraminifera, molluscs, and bryozoans. They also produce non-skeletal grains such as ooids, peloids, and complex aggregate grains (Lees, 1975).
In contrast, temperate water carbonate accumulations consist of benthic foraminifera, molluscs, barnacles, bryozoans, and calcareous red algae; and few non-skeletal grains form in these environments (Lees, 1975). These colder-water, turbidity-free carbonate environments do not build the rapidly, vertically accreting, thick carbonate buildups seen in the lower latitudes.
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The increased need to find new energy resources in deep marine frontier environments has clearly intensified the importance and interest in deep water carbonate settings and how these settings interrelate to adjacent shoal water platform margins. Coarse-grained mass-flow deposits beyond the shelf break in terrigenous clastic environments have been known for many years to form major petroleum reservoirs, and it is likely that similar deep-water clastic facies will continue to be future exploration targets. The purpose of this short course is to improve approaches and ideas related to petroleum and mineral exploration in platform margin and deeper water carbonate environments. Emphasis is placed on understanding depositional environments, their contained facies and diagenetic patterns. Better geologic interpretation of these three elements in carbonate sedimentology and facies analysis is usually critical in petroleum exploration. These elements are also receiving wider importance in base metal exploration as many mineral deposits in carbonates are controlled by primary depositional patterns and not simply due to tectonics and/or proximity to igneous intrustions.