Bed Form Patterns and Sedimentary Processes Along a Mixed Energy Inner Shelf: Northwestern Florida

K. T. Logue, L. J. Doyle

GECO Geophysical, Inc. funded a study that included the collection of continuous side-scan sonographs, depth records, and bottom samples in order to determine the regional sedimentary processes and surficial geology along the inner 25 km of continental shelf between Pensacola and St. Joseph Point, Florida. The region is a transition zone between the terrigenous clastics of the Mississippi River delta to the west and the carbonates of the Florida platform to the east. Coastal rivers are a negligible sediment source to this reworked Pleistocene quartz sand sheet, the surface morphology of which reflects the dual influence of high-energy and low-energy events.

Sonographs indicate limited areas of hard substrate nearshore off Choctawhatchee Bay. Further seaward, in 30-35 m water depths, large high-energy sand waves up to 12 m high were recorded. In the western third of the study area, topography ranges from flat or low relief swells to sand waves 1-4 m in height. Sand-wave crests display distinct northwest-southeast orientation west of Choctawhatchee Bay and shift to a northeast-southeast orientation east of the bay. Apparent counterclockwise circulation peaking at Choctawhatchee Bay accounts for this shift in sand wave orientation and is supported by earlier current studies.

Grain size analysis of 681 bottom sediment samples revealed only 41 samples containing greater than 5% mud. Predictably, all 41 sites were located just west of St. Joseph Point, where currents are weak and topography is relatively flat. Extensive sand ribbons along the inner shoreface here are an exception in that they are formed by storm-generated currents. Sonographs indicate possible epibenthic faunal activity along these sand ribbons. This study provides an example of the complex bedform morphology of a mixed-energy inner shelf.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.