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Great Bahama Bank – Part II: Mapping Depositional Facies on 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, which have long served as subsurface analogs at a variety of scales. We have used Landsat TM imagery, a refined bathymetric digital elevation model, and seafloor sample data compiled into ArcGIS and analyzed with eCognition to develop a depositional facies map that is more robust than previous versions. For the portion of the GBB lying west of Andros Island, the facies map was generated by pairing an extensive set of GPS-constrained field observations and samples (n=275; Reijmer et al. 2009) with computer and manual interpretation of the Landsat imagery. For the remainder of the platform, where we lacked such rigorous ground-control, the Landsat imagery was segmented into lithotopes – interpreted to be distinct bodies of uniform sediment – using a combination of edge detection, spectral and textural analysis, and manual editing. A map was then developed by assigning lithotopes to facies classes on the basis of lessons derived from the portion of the platform for which we had rigorous conditioning. Of particular importance is the detailed and generalized distributions of muddier lithologies and grainier lithologies shown by our mapping relative to that of previous maps. 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 GBB. 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 sandbar complexes. 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, which lies to the south of the TOTO and lacks 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.