Bahamian platforms

The shelf seas of the Bahamas are among the best examples of contemporary limestone seas anywhere. Since the early Cretaceous pure carbonate deposits have been accumulating in the area to a depth of more than 14,500 feet. It may be inferred from this that the region has been long isolated from sources of terriginous sediments. There is no compelling evidence of folding or faulting in later geologic times, and there is little reason to think that interruptions in the general subsidence have been frequent during the past 130 million years or so. Probably this downwarping has resulted from isostatic adjustment to a steady accumulation of carbonates. The Bahamas include several more or less detached banks or platforms, each bounded by a steep submarine escarpment. The base of the escarpment is considerably below the minimum level reached by oscillations of the Pleistocene seas. For this and other reasons, it appears unlikely that the escarpment originated as a wave-cut cliff. Deep channels between the platforms are broad and flat-bottomed and in several respects are unlike most submarine canyons. It seems improbable that they were largely excavated by currents. It is more likely that the submarine profile is mainly the result of constructional processes. As with typical coral atolls, calcareous deposits originate and accumulate most rapidly near the margins of the Bahamian platforms, with the result that the central area, or lagoon, of each bank is somewhat depressed below the level of the rim. Loose sediments are concentrated along the rim by waves and wind. The Bahamian platforms resemble coral atolls of the Pacific and have been regarded as atolls by a number of investigators. However, unlike typical atolls, the exposed rocks are composed mainly of inorganic ooelite, rather than reef detritus. Modern coral reefs of the Bahamas are very new, having barely established themselves since the last glaciation, and the platforms evidently had assumed their characteristic form before the recent reefs were started. The marginal escarpment around each of the platforms resembles the steep profile of coral atolls, the result probably of vertical reef growth during Tertiary subsidence. It is highly improbable that deposition of unconsolidated sediments would produce such steep slopes. It is concluded that the submarine profile in the Bahamas is a late Tertiary coral-reef profile. The banks probably originated in the Cretaceous period as oceanic coral atolls, becoming gradually incorporated into the North American continent by the spread and coalescence of calcareous deposits. During the Pleistocene epoch the reef communities were nearly exterminated and blanketing ooelite deposits were laid down over the Tertiary reefs. Recently, renewed reef growth has been resumed around the bank margins.