Paleoecology and Stratigraphy of Holocene Carbonates, Frazers Hog Cay, Bahamas: ABSTRACT

Abstract

Vibro-coring and water-jet probing of the unconsolidated Holocene section in a 22-square-mile area of the southern Berry Islands region of the Great Bahama Bank have yielded data on the thickness and facies relations of this section, and the geometry of the underlying Pleistocene karst topography. Six sedimentary facies are recognized on the basis of petrographic examination of grain types and amount of fine carbonate material smaller than 1/8 mm. Bankward from the bank margin, the facies are, in order: (1) an facies, a thin veneer overlying the rock surface of the outer platform; (2) a much thicker (ca. 9 feet) oolite facies which apparently is separated in the subsurface from the algal-skeletal facies by knoll-like features of the karst surface; (3) an facies which both overlies and replaces bankward the grape-rich oolitic facies; (4) a transitional pellet facies; and (5) a facies which occupies the most bankward position in the study area. Beneath part of the grapestone facies, and evidently occupying an interior swale i the karst surface, is (6) a muddy-sand facies which has the highest percentage of fine carbonate (up to 40 per cent) observed. The anomalous position of the grape-rich oolitic facies is judged to be an effect of the knolls in the underlying karst surface. As sea-level rose after the Wisconsin low and began to cover the bank, these irregularities served as at least a partial barrier to strong currents--sufficient to allow the process of grapestone formation to proceed in an area much farther seaward than the modern site of formation. Similar irregularities (including nearby modern islands, or cays, which are exposed elements of the karst surface), and distance from the bank margin, are thought to account for the position of the muddy sand facies. The effect of these irregularities decreased as sea-level approached its present height. Preliminary examination of the foraminiferal faunas indicates that marine waters of essentially normal salinity reached interior positions over the inundated karst surface early in the transgressive history in spite of the irregularities of that surface. The study has revealed the existence of crusts or lenses of indurated sediment within the unconsolidated section which have cementation fabrics similar to modern beachrock at other Bahamian localities. End_of_Article - Last_Page 456------------