ABSTRACT

Sources and quantities of periplatform carbonate sediments have been determined in Northwest Providence Channel, Bahamas, by an end-member method employing geochemical, textural, petrographic, and SEM data. The Holocene sediment over most of this open seaway, 200 to 2,000 m deep, between Great Bahama Bank and Little Bahama Bank, is a drape of largely fine grained lime mud 50 cm thick. Shallow platform sources contribute 75-90 percent; the remainder is planktonic foraminifera, pteropods, and coccoliths. Mud (< 62 µm) from each bank and the planktonic source are each geochemically distinct and can be quantified. The two shallow platform sources dominate sedimentation in the deep basin. Trend-surface maps of mud derived from each bank reveal no windward-leeward asymmetry in cont ast to reported sand-transport patterns. This indicates that the fines are swept off both platforms by storm and tidal exchange. Concentric distribution of fines relative to source indicates no offset by currents, suggesting that fecal pelleting must rapidly remove fine sediment from the water column. Canyon valleys and intercanyon highs have equal thicknesses of Holocene sediment, which indicates that pelagic processes of deposition presently dominate over gravity-flow processes.

Confirmation of this depositional process is that the deposition rate increased from 2 cm/1,000 years to 10 cm/1,000 years when postglacial sea level flooded the bank tops. Thus 80 percent of the present sedimentation rate results from bank-top contribution. Furthermore, the calculated mass of mud traceable to Little Bahama Bank agrees excellently with the independent estimate of overproduction there. In cores, bank sediments resulting from the most recent sea-level highstand sharply overlie planktonic sediments, writing a sea-level history in alternating stable and unstable mineral suites.