Carbonate Drifts and Contourites and the Underestimated Role of Current Erosion and Winnowing

Abstract

The Florida-Bahamas region is the home of the Earth's largest carbonate platforms and is the getaway of one of the world's largest ocean current, the Gulf Stream. Yet, off-bank transport, slope and basin sedimentation is generally thought to be dominated by gravity driven processes that in combination with pelagic sedimentation produce the slope apron and peri-platform ooze surrounding the platforms. In the last ten years geophysical and oceanographic data have produced a wealth of data that require considering both, surface and benthic, currents in depositional models for carbonate slope and basin deposition. These data also indicate that sediment transport by ocean currents is volumetrically more important than gravity driven transport. Equally important to deposition is erosion by currents that result, depending on the strength, in winnowing of fine-grained sediment and the submarine hardgrounds formation. Large drifts at the corners of Little and Great Bahama Bank and in the middle of the Straits of Florida and the Santaren Channel are the combined product of surface and benthic currents. The north flowing Florida Current and its tributaries are the dominant surface currents but benthic currents that consist of internal tides, south flowing counter currents are very influential on local sediment distribution. For example, the entire offbank transported Holocene highstand wedge is moved to form a plastered drift on the slope of Great Bahama Bank. Benthic currents can be strong enough to form extensive sand wave fields as observed along the Miami Terrace. They are also responsible for winnowing of sediments at the toe-of slope of Great Bahama Bank, producing a coarse-grained facies belt parallel to the margin. Along the West Florida Shelf currents driven by winds, tides and the transient intrusion of the Loop Current winnow all the fines to make this deep (350 – 400m) portion of the ramp a grainy environment. The facies in the deep-water carbonate environment show how erosion and winnowing are equally important to current deposition. Yet, drift and contourite models developed in siliciclastic environments concentrate on silt to sand-sized deposits from traction currents, ignoring the importance of winnowing of fines and the large-scale distribution of mud.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016