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Great Bahama Bank – Part I: Evaluating Water-Depth Variation on a “Flat-Topped” Isolated Carbonate Platform


Great Bahama Bank (GBB) has long served as a frequently visited and well-studied example of a flat-topped, isolated carbonate platform. As defined by a 30m water depth contour, GBB extends over 103,000 sq. km. The maximum variation in depth-elevation over this vast platform extends from the 30 m contour to the highest Pleistocene eolianite ridge of 63m on Cat Island, but as islands occupy only 8%, or 8,000 sq. km. of GBB, we focus here on the bathymetric variation. We have used Landsat ETM+ imagery (a mosaic of 13 images) and an extensive set of water depth measurements (5,700 digitized soundings) to critically evaluate the magnitude and patterns of bathymetry across GBB. Shorelines of islands were derived from the shortwave-infrared band 5 of the Landsat TM mosaic, as water is black and land is shades of gray. The edge of the platform was visually interpreted and digitized (with reference to soundings) as a 30m contour from the blue light band 1 of the Landsat TM mosaic. The bathymetry depth model used the triangular irregular network (TIN) interpolation method to integrate the 5700 soundings, edge of platform contour, interpreted intermediate contours, and island shorelines. The TIN model was converted to a regular 150 m grid using the Interpolated Distance Weighting (IDW) method to facilitate visualization. The platform model supports 1:1,000,000-scale geologic interpretations and bathymetric profiling. 60%, or 61,400 sq. km. of GBB lies in 5m or less of water. This includes areas where accommodation space has been nearly filled by carbonate sand accumulations, e.g. TOTO, Schooners, Exumas, Joulters, and Cat Cay sand complexes, but 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 5m 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. Bathymetric profiles across GBB highlight the irregular filling of accommodation space and graphically emphasize the challenges that occur in correlating depositional cycles of variable thickness across a platform.