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Complex Patterns of Erosion and Sedimentation on an Open-Ocean Carbonate Foreslope: Holocene, Little Bahama Bank

Rankey, Gene *1; Doolittle, Daniel 1
(1) KICC, Department of Geology, University of Kansas, Lawrence, KS.

Foreslope deposits can form stratigraphically important portions of isolated carbonate platforms and can host important hydrocarbon reserves. In many cases, however, the stratigraphic architecture of ancient carbonate foreslopes is complex and heterogeneous, limiting predictive understanding of the means by which platforms evolve. To better understand geomorphic patterns and processes on carbonate foreslopes, this study describes high-resolution bathymetry (multibeam), geomorphic and sedimentologic (side scan sonar, optical images), and shallow Chirp subbottom profile data that document variability in geomorphic, sedimentologic, and stratigraphic character of platform-top to upper foreslope environments (<240 m depth) of the present north flank of the Little Bahama Bank, Bahamas.

In the focus area in the western Abacos, basinward of a discontinuous shelf-margin reef, a sandy to rocky shelf with ubiquitous but thin rippled sand and small patch reefs gently slopes to a prominent (20-25 m high), continuous, marginal escarpment at ~ 70 m depth. This escarpment exposes truncated, bedded rocky outcrops, and locally forms convex-platformward arcs of hundreds of m wavelength. Downdip from the 70-m escarpment, several complex digitate forms several hundred meters across and several meters high are interpreted to represent mass transport complexes that indicate downslope transport. A second, less pronounced escarpment at ~120-125 m depth is discontinuously exposed along strike. All geomorphic features of this margin are not erosional, however. Downdip (and partly draping the lower escarpment), a thin, seismically transparent sediment wedge up to 20 m thick contiguously onlaps the margin. The updip pinchout of the wedge is at ~180 m depth, and it continues downdip, below the depths of the survey.

Although at geologic timescales this margin is progradational, these results suggest that it is shaped by a complex mix of erosional and depositional processes. Data illustrate that this open-ocean margin is quite distinct from windward margins in the Bahamas, which typically include near-vertical walls of erosion or bypass, flanked basinward by downdip rubble and talus. Collectively, the results provide unique perspectives into the nature and controls on complex geomorphic patterns of erosion and deposition in Holocene upper foreslope systems, concepts that might be applicable to ancient analogs.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California