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ABSTRACT: Quaternary Stratigraphy of Western Great Bahama Bank: Eustatic Signatures in Progradational Slope Units Along a Leeward Margin

R. J. Wilber, J. D. Milliman, R. B. Halley

Higher resolution seismic profiles, submersible observations, and cores taken along the 350-km-long leeward slope of western Great Bahama Bank (GBB) demonstrate large-scale export of banktop sediment and rapid progradation of the slope during late Holocene time. A wedge-shaped periplatform sequence, up to 90 m thick, is present along most of the slope and consists primarily of aragonite-mud derived from GBB since the flooding of the platform 6-8 Ka. The sedimentary volume of the Holocene slope sequence is 40-80% that of the Holocene sediment presently retained on the platform; it contains up to three times the amount of mud found in banktop facies. Maximum rates of vertical accumulation and lateral progradation are 11-15 m/Ka and 80-110 m/Ka respectively--two orders of ma nitude greater than previously known for periplatform deposits.

A group of geometrically and seismically analogous units underlie the Holocene, 12 of which can be traced for hundreds of km along the strike. By analogy to the Holocene, we interpret these older units as rapidly deposited muddy sequences formed during Pleistocene sea level highstands (bank-flooding events). Each sequence is bounded by strong reflections which represent erosional surfaces and/or lithified horizons formed during the relatively nondepositional intervals of sea level lows (banktop exposure). The number of individual sequences found in the Quaternary group indicates a periodicity of banktop flooding of 105 yr (4th order cycles). The timing of deposition of these units is approximately a half-wavelength out of phase (with regard to sea level curves) when compared to slope eposits from terrigenoclastic systems.

These deposits represent a highly expanded record of Quaternary sea level changes and may provide a critical modern analog for both the mechanisms and timing of rapid lateral platform growth in ancient carbonates.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990