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Bahamian Facies: Abstract
The Bahama Banks have long been considered a classical area for the study of limestone genesis. The Great Bahama Bank, in particular has been the subject of a number of studies of carbonate deposition. Few of these studies, however, have considered the abundance and regional distribution of the various kinds of calcium carbonate grains which mantle the bank. The present study was initiated to delineate quantitatively calcium carbonate facies on the northwestern part of the Great Bahama Bank and to determine what biological and physical factors contribute to the origin and distribution of the various grain types.
Representative subsamples of 218 sediment samples were impregnated in a polyester resin and then thin-sectioned. A point count analysis of each thin section was made to determine the constituent composition of the fraction larger than mm. The weight percentage of the fraction smaller than mm was used as a measure of grain size. Statistical analysis of the accumulated data with a high speed digital computer resulted in the delineation of the following five facies: (1) coralgal facies—characterized by a relative abundance of corals and coralline algae; (2) oolitic facies—characterized by an abundance of oolitically coated grains; (3) grapestone facies—typified by an abundance of grapestone and cryptocrystalline grains; (4) pellet mud facies—characterized by an abundance of faecal pellets and particles smaller than mm; and (5) skeletal mud facies—typified by a relative abundance of skeletal debris and particles smaller than mm.
Differences between the last four facies are considered to be primarily the product of differential current strength, with current action decreasing progressively in intensity from the oolitic to the skeletal mud facies. In contrast, the coralgal facies appears to owe its distinctiveness to the relatively great depth and large areas of rock bottom which characterize this depositional environment.
Ideally, one might expect the Bahamian facies pattern to consist of a series of five concentric bands parallel to the bank's margin. This expected ideal pattern, however, is strongly modified by local environmental conditions created by the submerged karst surface of the Pleistocene basement rock.
Acknowledgments and Associated Footnotes
1 Professor of Geology, Rice University
Copyright © 2006 by the Tulsa Geological Society