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Abstract

Sequence stratigraphic interpretations of carbonate platform margins are based to a large degree on concepts of variable timing and nature of deposition relative to fluctuations in sea level. Quaternary platform margins, such as those found in the Bahamas, provide a unique opportunity to calibrate the sedimentary record because of the well-constrained nature of sea-level history during this period. Detailed observations and sampling from a research submersible combined with high-resolution radiocarbon dating in the Tongue of the Ocean, Bahamas, have enabled us to document variations in deposition along the upper parts of the marginal slope during the most recent rise in sea level.

We have found that the steep marginal slopes around the Tongue of the Ocean record deposition during the early rise of sea level following the last lowstand some 18–21 Ka. Coarse-grained skeletal sands, gravel, and boulders derived from reefs growing along the overlying escarpment were deposited on slopes of 35–45º and cemented in place within a few hundred years. Deposition by rockfall and grainflow resulted in a series of elongate lenses oriented parallel to the slope. These lenses are generally less than 0.5 m thick and pinch out downslope within tens of meters. Repeated deposition and cementation produced slope deposits that are both laterally discontinuous and internally heterogeneous. Radiocarbon dating of skeletal components and cements indicate that active deposition on the slopes ceased approximately 10,000 years ago as sea level rose above the escarpment and began to flood the top of the Great Bahama Bank. Fine-grained, nonskeletal sands and muds derived from the platform are presently bypassing these slopes and are deposited downslope as a wedge of sediment with slope declivities of 25–28º.

Cracks and slide scars are a common feature of the steep-cemented slopes. The cracks are a few centimeters wide and may extend for tens of meters across the slope with an arcuate, convex-up expression. The slide scars are generally a few meters wide by several meters long and cut back into the slope a few meters to less than 1 m, although one large example is 30 m wide, extends downslope for 75 m, and has exposed 10 m of the interior of the slope. Transects downslope from slide scars show that large blocks of the slope, some in excess of 10 m across, have been transported for tens or hundreds of meters downslope. The release and transport of such blocks may be one mechanism by which turbidity currents are initiated in deeper slope environments.

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