An 80 km² 100 kHz side- scan mosaic of a patch of seafloor off the mouth of Tampa Bay, Florida reveals seabed features that mark a transition from estuarine influence to open shelf influence. Tampa Bay is a large estuary on the west coast of Florida that possibly formed as result of regional limestone dissolution from below in the Florida Platform creating a surficial depression which filled with seawater during sea-level highstands. At the mouth of the estuary is the largest ebb-tidal delta in the Gulf of Mexico having a total volume calculated to be 1.4 x 10⁸ m³ of sediment.

The sidescan mosaic coupled with seabed observations and sediment sampling, clearly shows the transition from the distal portions of this enormous sand body to shelf features that seem unrelated to the presence of the estuary. The distal ebb-tidal delta is acoustically featureless, but grades seaward into a NW/SE oriented linear pattern of high and low backscatter returns produced by a distinct partitioning of coarse molluscan gravel and fine quartz sand (<1 m relief, ~150 m spacing). Further seaward is a topographically lower, acoustically featureless lobe that we interpret to be a relict ebb-tidal delta perhaps active when sea level was slightly lower. Finally, the outermost area is dominated by 10-100 m wide circular patches (bedforms?) of finer quartz sand largely surrounded by coarse, molluscan shell concentrations. We propose that this bed configuration responds to open-shelf physical conditions.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah