Grains size and components distribution in turbidite layesr of ODP Leg 101

Textural and compositional differences were found between gravity-flow sheets in an open-ocean environment on the northern slope of Little Bahama Bank (Site 628, Pliocene turbidite sequence) and in a closed-basin depositional setting (Site 632, Quaternary turbidite sequence). Mud-supported debris-flow sheets were cored at Site 628. Average mean grain size of the turbidite samples was lower, mud content was higher, and sorting was poorer than in comparable samples from Site 632. This reflects the deposition of proximal, low-energy turbidity currents and debris flows on a base-ofslope carbonate apron. No mud-supported debris-flow sheets were deposited in the investigated sediment sequence of Hole 632A. Many larger turbidity currents from around the margins of Exuma Sound may have reached this central basin setting, depositing sediments that had been transported over longer distances. Planktonic components dominate in the grain-sized fraction (500-1000 µm) of turbidite samples from Hole 628A, while platform detritus is rare. We interpreted this as resulting from the erosion and reworking of a large area of open-ocean slope sediments by gravity flows. In contrast, large amounts of benthic and platform components were found in the turbidite samples of Hole 632A. This may be explained by the fact that the slopes of the enclosed Exuma Sound are steep, and turbidity currents bypassed much of these slopes through pronounced channels, delivering more shallow-water detritus to the deep basin. Erosion of slope sediments, a possible source area of planktonic detritus, is assumed to be low. The small slope area in relation to the larger surrounding platform areas and lower production of planktonic components in the enclosed waters of Exuma Sound may also explain the observed low number of planktonic components at Hole 632A. Turbidite material from both open-ocean and enclosed-basin environments was deposited at Site 635.