ABSTRACT

Gravity-displaced carbonate sediments on the lower slope and basin floor of a Bahamian intra-platform basin were studied using closely-spaced sediment cores up to 5 m in length, 1500 km of precision depth survey, and 3.5 kHz seismic surveys. Based on sedimentary structures and textures of these clastic carbonates, three main types were identified: 1) clean, graded sand and rubble (turbidity-current deposits); 2) massive, poorly sorted sand and rubble (deposits of grain flow or turbidity currents); and 3) muddy rubble and pebbly mud (debris flow and mudflow deposits). These deposits make up nearly 25 percent by volume of the upper 5-7 m of the basinal sediments, with the remainder comprised of peri-platform ooze. Compositionally, the clastic carbonates contain 50-70 percent shallow-wat r materials derived from the adjacent carbonate platform and 30-50 percent chalk lithoclasts and fauna derived from the slope and basin floor.

Three individual clastic layers were mapped in the northern part of the basin. Two are turbidites. The third, a composite of pebbly mud, muddy rubble, graded rubble and sand is interpreted as a debris flow deposit with a turbidite on top. The two turbidites have volumes on the order of 10⁷m³. Deposition of these turbidites occured on the basin floor within 10-15 km of basin entry, as lobe-shaped bodies that are thickest near the source area at basin entry and thin rapidly basinward. The third layer covers nearly the entire basin floor, 6400 km² and has a volume in excess of 10¹⁷m³. Unlike the turbidites, this layer cannot be traced to a point source at the mouth of a gully. Rather, it seems to have descended the entire northern ba in-slope in a broad front 20-30 km wide in sheet form extending well out onto the basin floor. Throughout the densely cored study area, the flow was wide enough that its two wings could maintain distinctly different composition. After traveling 100 km across the basin floor as a sheet flow, the denser, muddy rubble (debris flow) became "channeled" by the axial valley, while the closely following turbidity current covered the whole basin floor. Eventually, the entire flow became channeled as it traveled out Exuma Sound through the axial canyon connecting to the abyssal floor of the Atlantic Ocean. Nannofossil zonation and planktonic foraminiferal data suggest this layer was deposited 80,000-120,000 yrs. B.P. when sea level was lowering from the Sangamon high-stand.