Bahama carbonate platforms — The deep and the past

Abstract

Carbonate deposition prevailed for 150 Ma in the Bahama—Florida segment of the West Atlantic margin and provides an interesting test for actualism. Present-day facies patterns on the banks are controlled by the backward decrease of waves and tides and the concomitant increase of temperature and salinity variation. In the troughs, influx of sand and rubble from gravity flows varies with topography and distance from shallow-water sources and allows one to define facies belts: a rhythmic sequence of ooze and graded beds on the basin floors, subdivided into a basin-margin belt of coarse, thick turbidites and basin interior with fine turbidites; slope facies change with increase in height and declivity from accretionary to by-pass to erosional regimes. Stratigraphic history of the Bahamas is not simply a projection of the “Holo-Scene” back in time. Both long-term natural evolution (decrease in subsidence, upbuilding of the banks, submarine erosion) and outside factors (climate, eustacy) have caused significant changes. Since the Jurassic, the Bahamas seem to have evolved from a clastics-evaporite province to a single carbonate-evaporite platform and finally to an array of platforms and troughs. During the platform-trough stage, the rate of upbuilding of the platforms decreased, submarine canyon erosion increased. Platform flanks steepened as they grew higher and changed from accretionary to by-pass to erosional slopes. A change imposed by extraneous factors occurred in the Pliocene, when the Great Bahama Bank changed from a giant reef-rimmed atoll to a flat platform covered by oolites and peloid sands. The Bahamas share both long-term trends as well as random changes by extraneous factors with other platforms in the geologic record. Compared to ancient platforms, however, they are unusually long-lived and at a more advanced stage of growth, they are deeply dissected by erosion, their flanks are unusually high and steep and the troughs very narrow. The Neogene platform sequence is strongly controlled by eustatic sea-level fluctuations.