Carbonate Sediment Drifts in Northern Straits of Florida

Abstract

High-resolution, air-gun seismic reflection profiles from the northern Straits of Florida (water depths < 800 m) have revealed the presence of large (up to 3,000 sq km) carbonate sediment drifts off the northwest corners of both Little Bahama and Great Bahama Banks. These drifts are hemiconical bodies of carbonate sands up to 600 m thick that have prograded north in response to an intensification of the Florida Current since the middle Miocene. Deposition of the sediment drifts occurred along the Bahama Banks on the lee side of the present oceanic circulation pattern, where there appears to be a convergence of bottom currents having velocities of up to 60 cm/sec. However, depositional rates may have been episodic, with the greatest rates correlating with intensified ci culation patterns during glacial events. Because of widespread submarine cementation, piston coring of these sediment drifts was difficult. Most cores contained less than 1 m of sand and angular rock fragments, suggesting that a thin veneer of sand overlays hardgrounds. In one 5-m-long core, granule-size material constitutes up to 40% of the total sediment (average of 3.9%), sands 50 to 80% (average of 60.4%), silts 5 to 35% (average of 25.0%), and clays 5 to 20% (average of 10.7%). Mean grain size ranges from 0.0 to 3.0 ^phgr (coarse to fine sand) and the sediments are moderately to poorly sorted. The granules are mostly submarine-lithified intraclasts; the fine sands are mostly planktonic Foraminifera; and the coarser sands consist of pteropods and shallow-water material such as Halimeda, peneropolid Foraminifera, fragment of coralline algae, ooids, or micritized mollusk debris. Off-bank sediment transport along the west side of the Bahama Banks and subsequent along-slope transport by the Florida Current appear responsible for the supply of bank-derived sediment to the drifts. Rocks recovered by dredging and in-situ sampling from submersibles are mostly grain-supported biomicrites, biomicrudites, and intramicrudites cemented by submicrocrystalline, amorphous to peloidal high-Mg calcite. Preliminary bulk-rock C14 dates suggest a late Pleistocene or older age. Porosities of up to 40% are common within these rocks, consisting of both primary intergranular and intragranular pores and secondary macroborings and microborings produced by endolithic organisms. In the rock record, sandy sediment drifts formed at relatively shallow depths (< ~ 1 km) by strong bottom currents associated with wind-driven surface currents may be attractive hydrocarbon reservoirs. In contrast, sediment drifts formed at abyssal depths (3 to 5 km) by relatively sluggish thermohaline bottom currents would probably contain too much impermeable muddy sediment to be potential reservoirs.