ABSTRACT: Sediments of Western Great Bahama Bank: Facies Patterns and Depositional Processes Along a Rapidly Prograding Leeward Margin

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

The 350-km-long western side of Great Bahama Bank (GBB) is a rapidly prograding leeward margin that has grown laterally as much as 700 m during the late Holocene (6-8 Ka). The processes of leeward transport, sediment bypassing, in-situ sediment production, and cementation act in combination to control the rate and style of progradation along this margin.

Three major surface sediment facies are present. At the shallow bank edge (10-55 m) a 4-km-wide facies band composed of nonskeletal sand (ooids, pelletoids, and composite grains) caps an 11-20-m-thick unit. On the lower slope (>140 m), a 5-10-km-wide facies band composed primarily of needle aragonite mud typifies a 10-90-m-thick onlapped wedge. The interval between these two depocenters (55-150 m) is a steep (35°), rocky slope veneered with coarse skeletal sand dominated by Halimeda.

Sediments at bank edge and on lower slope are the result of the sustained leeward transport of banktop sediments across and off the GBB platform. At present, the bank-edge depocenter continues to build vertically and is an effective trap for most transported sand. The sedimentary volume of this unit is increased within the facies band via inorganic precipitation of aragonite as oolitic coatings and/or interstitial cements. Mud and very fine sand bypass both the bank edge and rocky slope and accumulate rapidly on the lower slope. There is relatively little increase in sedimentary volume in this depocenter related to precipitation. The sediments found on the rocky slope are primarily of local origin, formed in situ in the uppermost portion (photic zone) of this interval. Precipitation o fine-grained carbonate is very active in this facies, which apparently acts as a slowly growing front of rapidly cementing skeletal sand.

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