--> Abstract: Slope Depositional Processes along Western Great Bahama Bank: A Rapidly Prograding Carbonate Platform, by R. J. Wilber, R. B. Halley, and J. D. Milliman; #91004 (1991)
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Slope Depositional Processes along Western Great Bahama Bank: A Rapidly Prograding Carbonate Platform

WILBER, R. JUDE, SEA, Previous HitWoodsNext Hit Hole, MA, ROBERT B. HALLEY, U.S. Geological Survey, St. Petersburg, FL, and Previous HitJOHNNext Hit D. MILLIMAN, WHOI, Previous HitWoodsTop Hole, MA

The western margin of Great Bahama Bank has prograded rapidly into the Santaren Channel throughout Quaternary time. Platform growth is accomplished mainly by vigorous outbuilding of the slope--up to 500 m in the last 6-10 ka.

Seismic stratigraphy and sediment and rock analysis reveal sharp contrasts in depositional facies and mechanisms of growth between the upper slope (50-160 m) and lower slope (160-600 m).

The upper slope is steep (30-60 degrees) and rocky with a thin (<1 m) layer of coarse Halimeda sand. In situ sediment production, in combination with cementation by magnesium calcite, allows this interval to grow as a slowly accreting (meters/1000 yr), lithified surface. Because of cementation, slope failure is rare but, in some areas, large blocks calved off the upper slope are found on the lower slope (mega-breccia).

The lower slope is gentle (10-30 degrees) and consists of thick (up to 90 m) accumulations of fine bank-top sediment. Deposition occurs episodically via settling of sediment plumes swept off the bank during large storms. Lower slope deposits are absent where off-bank transport is barred by shelf-edge islands. In addition, hyperpycnal flows which cascade over the margin both sweep the upper slope and erode lower slope deposits near the depth of onlap. Slope failure in the uncemented lower slope deposits is primarily via slumping and possibly gravity flows.

Lower slope deposition occurs only during sea level high stands when the rates of growth may be an order of magnitude higher than those of the upper slope which apparently experiences slow growth throughout most of a sea level cycle.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)