Morphology and Dynamics of Carbonate Tidal Sand Ridges: Schooner Cays, Bahamas

Eugene C. Rankey¹, Stacy L. Reeder², Scott Ritter³, and Paul M. Harris^4
1RSMAS/University of Miami, Miami, FL
²Schlumberger-Doll Research, Cambridge, MA
³Brigham Young University, Provo, UT
⁴Chevron Energy Technology Company, San Ramon, CA

Understanding and modeling many carbonate reservoir systems requires knowledge of depositional trends, including links between geomorphology and granulometry. To further explore such trends in carbonate systems, this study explores Holocene ooid shoals of Schooner Cays, Bahamas.

In this area, sands occur in geomorphic forms including both parabolic bars and flow-parallel tidal sand ridges and channels between a rocky to skeletal-sand rich outer shelf (5-8 m deep) and the muddy peloidal platform interior (~4-6 m deep). Within this bar-and-channel belt, individual flow-parallel sand ridges are up to 13 km long and 1.5 km wide, and generally radiate outward. Sand ridge crests include bare, rippled sands, with superimposed sand waves of various orientations. The crest sediments are clean, moderately well-sorted oolitic sands, with mean grain size of ~ 600 μm and no mud or silt. In contrast, most bar flank and channel areas are burrowed and partly seagrass-stabilized sediments with moderately- to poorly-sorted peloid-skeletal-ooid sands and silts, with up to ~30% mud and silt. Within the channels, there is a trend of decreasing grain sizes, from oceanward to bankward. Measured depth-averaged current velocities reach a maximum of ~80 cm/sec in channels, whereas velocities decrease to < 40 cm/sec on shallow crests; current velocity generally decreases platformward within channels. Significant wave height is < 1 m, and waves are not powerful enough to initiate large-scale sand transport, although winds may drive some westward transport on the platform.

Collectively, the results illustrate geomorphic-sedimentologic links, driven by physical oceanographic controls. The quantitative information provides metrics and trends for developing more realistic geologic or simulation models.

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