Evaluating Water-Depth Variation and Mapping Depositional Facies on the Great Bahama Bank – a “Flat-Topped” Isolated Carbonate Platform
Great Bahama Bank (GBB) stands behind many of the models used to illustrate depositional facies variation across flat-topped, isolated carbonate platforms. Such models have long served as subsurface analogs at a variety of scales. We have used Landsat TM and ETM+ imagery, and an extensive set of water depth measurements, to critically evaluate the magnitude and patterns of bathymetry across GBB. The refined bathymetric digital elevation model (DEM) and seafloor sample data were analyzed with eCognition to develop a map of depositional facies that is more robust than previous versions.
60%, or 61,400 km2, of GBB lies in 5 m or less of water. This includes carbonate sand accumulations where accommodation space is nearly filled, e.g., Tongue of the Ocean, Schooners, Exumas, Joulters, and the Cat Cay sand bodies, and also includes vast portions of the northern portion of GBB surrounding Andros Island and the New Providence Platform to the west of the Exuma Islands. The 40% of GBB lying in greater than 5 m of water occurs mainly in an east-west trending portion of the southern platform. These bathymetric patterns are important as a potential primary control over flooding history, filling (accommodation) history, and the resultant depositional facies patterns.
GBB is essentially a very grainy platform with muddier accumulations only in the lee of substantial island barriers; in this regard Andros Island, which is the largest island on GBB, exerts a direct control over the muddiest portion of the platform. Mudstones, wackestones, and mud-rich packstones cover 8%, 5%, and 14%, respectively, of the GBB platform top. By contrast, mud-poor packstones, grainstones, and rudstones account for 20%, 45%, and 3%, respectively. Of the 45% of the platform-top classified as grainstone, only 3% is composed of “high-energy” deposits characterized by the development of self-organized sandbar complexes that fill, or nearly fill, available accommodation. The most abrupt lateral facies changes are observed leeward of islands, areas which also hold the highest diversity in facies type. There is a clear trend that the widest portion of the platform, lying to the south of the Tongue of the Ocean and lacking islands, hosts the most continuous expanses of grainstone. The prevalence of rudstone increases from north to south in step with an increase in water depth.
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