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New Insights for Carbonate Slope Anatomy from the Bahamas

Eberli, Gregor P.*1; McNeill, Donald F.1; Mulder, Thierry 2; Ducassou, Emmanuelle 2; Grammer, G. Michael 3
(1) Comparative Sedimentology Laboratory, University of Miami, Miami, FL.
(2) University of Bordeaux, Talence Cedex, France.
(3) Department of Geosciences, Western Michigan University, Kalamazoo, MI.

Recently acquired multibeam datasets over large slope areas of Little and Great Bahama Bank combined with submersible observations and seismic and core data comprehensively delineate the anatomy of these slopes from the platform edge to the basin. These combined datasets reveal several unexpected findings in regards to 1) the position and dimensions of slope failures, 2) the size and variability of slope canyons, and 3) the distribution of the fine and coarse sediment along the depositional profile.

Models place slope failures preferentially in the upper slope where declivity is relatively high. However, along the western slope of GBB several small and two large-scale slope failures occur in the middle and lower slope where declivity is less than 4 degrees. These failures are unusually long, one is approximately 25 km and another can be traced over 80 km. The scar of the latter was originally about 150 m high but is partly buried by late Pleistocene and Holocene sediment.

Slope canyons are typical for the middle and lower slope but the new data document that canyon formation can abruptly end along strike. In all three studied slopes the slopes are dissected by spectacular set of canyons (over 200 m relief, up to 1 kilometer width and 10 - 25 km length). They all abruptly change laterally to smoother slopes or muddy slopes with narrow equally spaced gullies. On the western margin of GBB, slope canyons are present north and west of Bimini and aligned with hundreds cold water coral mounds.

The near-vertical top of the slope releases talus blocks of various sizes but their deposition is restricted to a narrow band of ~1 km from the margin on the upper slope (50 - 300 m). The cemented upper slope itself is onlapped by mud wedges that cover the middle and lower slope and thin basinward. At the toe-of-slope and in the adjacent basin, coarse- grained bioclastic sediment is common. These deposits can form kilometer wide dune fields with similar morphology as shallow-water high-energy environments. These deposits are a product of strong bottom currents at the platform base.

Taken together these new data refine existing models of processes, morphology and facies distribution along carbonate slopes. In particular, they indicate the importance of slope instability on the lower slope, the lateral variability of slope canyons and the importance of bottom currents in the basinal areas.

 

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