Vertical and Lateral Facies Patterns in a Holocene Ooid Shoal Complex, Fish Cays, Crooked-Acklins Platform, Southern Bahamas

Ooid shoals are ubiquitous in the stratigraphic record, where they can form prolific hydrocarbon reservoirs with considerable depositional heterogeneity. One approach to understanding their variability is through analysis of surface sediments and processes active on modern analogs; these types of data are more powerful if they are integrated with core data, to constrain changes through time. To better understand the nature and scale of lateral and vertical sedimentologic and stratigraphic variability that can occur in ooid shoals, this study integrates observations of spatial patterns in surface sediments and stratigraphic (vertical) patterns from 15 shallow cores from the Holocene Fish Cays ooid shoal complex (Crooked-Acklins Platform, southern Bahamas).

The Fish Cays shoal complex occurs on the arcuate southwestern, leeward margin of the platform, where they form a shoal system 35 km long that reaches up to 9 km onto the platform. The shoal complex includes a series of shallow subtidal to intertidal bars, separated by several channels that extend normal to the margin. Both surface sediment and cores from the oceanward flank of the complex reveals that these areas include up to 3.5 m of medium to coarse sand with abundant ooids and skeletal fragments. Up to 1 km inboard of this flank, shallow subtidal red algal-coral-sponge flats, up to 0.9 km wide by 1.7 km long, that form a thin (~0.4 m) veneer on top of 2 m of peloid-ooid medium sand. These flats pass laterally and platformward into a broad stabilized sand flat (up to 3 km wide) that include ~3.7 m of peloid-ooid medium sand. Within ~2 km of the platformward flank of this shoal complex, surface ooid-rich sediment caps a ~3 m thick package of moderately well-sorted ooid sand.

The data thus reveal several trends: 1) numerous cores include Pleistocene lithoclasts and coarse skeletal debris near their base, and generally fine up above this interval. 2) coarser ooids are ubiquitous on the surface and in cores on both the oceanward and platformward flanks of the shoal complex; 3) between these ooid-rich areas, finer, well-sorted peloid sand dominates; 4) in surface samples and core, skeletal fragments decrease in abundance away from the open-ocean margin. In general, but with important exceptions, the patterns of surface sediments generally mimic patterns evident in the cores, suggesting that the shoal complex as a whole is aggradational, and has not migrated markedly through the Holocene.

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