Approximately 162 km of high-resolution subbottom seismic reflection profiles, collected in the Quicksands area west of the Marquesas Keys off south Florida, indicate extensive westward transport of Halimeda sand. The east-west-oriented, carbonate-sand accumulation is up to 12 m thick and encompasses an area 13 by 29 km. The Quicksands area is ornamented by east-west-trending submarine sand dunes 2 to 3 m high, which are shaped by strong, reversing north-south tidal currents. Many dunes break the surface at low tide. Submarine dunes lie directly on Pleistocene bedrock at the eastern end of the study area, but at the western end, dunes lie on 7 to 10 m of Holocene carbonate sand. Near the western terminus, the sands have accreted over carbonate muds.

Westward drift, probably caused by prevailing east and southeast winds superimposed on the tidal currents, is indicated by (1) thickening of the Holocene accumulation to the west and (2) large-scale, westward-dipping, accretionary bedding. Seismic reflection profiles show spitlike accretionary bedding in a package up to 1 km long at the western end, where carbonate sands spill onto deeper water muddy carbonates.

The submarine sand body is surrounded on the south, west, and north by equivalent-age, topographically lower lime muds

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and silts up to 7 m thick. The configuration and pattern of deposition suggest that this area could be used as a petroleum exploration model. The model consists basically of a reservoir-size porous carbonate-sand ridge surrounded downdip by organic-rich carbonate muds, which could serve as source beds. Reversing tidal currents and bed forms are identical to those of oolitic areas in the Bahamas, however, the Quicksands area does not contain ooids.

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