Intraclastic chalk, radiolarian mudstone, and carbonate silt (inferred sediment gravity-flow deposits) comprise the Miocene section in the Blake-Bahama basin at DSDP Site 391. Greenish-gray radiolarian mudstones, comparable to hemipelagic sediments that form the North American continental-rise prism elsewhere in the North Atlantic, occur as numerous clasts and a few thin intervals indicating that they formed the background sedimentation. The intraclastic chalk and carbonate silt generally lack sedimentary structures, but gravity-flow deposition is indicated by reworked fossils (forams,

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nannofossils, and radiolarians) as old as Cretaceous, mixed planktonic and shallow benthic forams, a few shallow-shelf limestone clasts, and preservation of carbonate materials deposited approximately 1 km below the Miocene calcite compensation depth.

The Great Abaco Member at Site 391 is 500 m thick. Intermittent coring, recovering 21% of the interval, indicates the following sequence, from top to bottom: (1) structureless white calcareous silt (57 m) with reworked fossils, including Eocene ones; (2) intraclastic marly chalk (123 m) with gray mud clasts of variable abundance and size, up to 2 cm long, attributed to debris-flow deposition; (3) dark, structureless radiolarian mudstone (28 m); probably background hemipelagic sediment, but contains a few reworked fossils; (4) intraclastic chalk (171 m) with a few intervals of dark mudstone and of shallow-water limestone lithoclasts showing lamination, scoured surfaces, and vague grading; debris-flow deposits; (5) heterogeneous interval (124 m) of intraclastic chalk and dark mudstone w th several graded beds of claystone intraclasts which show partial Bouma sequences, suggesting turbidity-current, as well as debris-flow and contour-current deposition.

The variety in types of clasts and in age and depth of reworked fossils, as well as the sheer volume of the unit, indicate several source areas including the continental rise and slope (hemipelagic clasts), the Blake Plateau (pelagic carbonate material) and the Great Bahama Banks (shallow-water limestones and benthic fossils).

Seismic profiling shows that the Great Abaco Member is about 500 m thick throughout the Blake-Bahama basin, an area of about 50,000 sq km, but pinches out abruptly at the basin margins, probably against hemipelagic muds. Several internal reflectors and a profound regional unconformity at the base (horizon A^u, Late Cretaceous to Miocene hiatus) provide good seismic mappability of the Great Abaco Member and several subunits. The internal acoustic character is a series of smooth, closely spaced reflectors, consistent with intermittent gravity-flow deposition, but local current sculpture is indicated by channels and long-wavelength bedforms.

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