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Coastal and Marine Geology Program
Morphology and Evolution of a Holocene Carbonate/Siliclastic Sand Ridge Field, West-Central Florida Inner Shelf

West-Central Florida Coastal Studies Home
Open File Report: Second West-Central Florida Coastal Studies Workshop
Chief Scientist
Scott E. Harrison, Department of Marine Science, University of South Florida, St. Petersburg, FL

To date, there have been no studies which have which have sought to investigate the morphology and evolution of a series of small-scale sand ridges identified along the low to moderate energy coastline of west Florida. A high-resolution side-scan sonar mosaic collected in the fall of 1994 provided the first detailed view of these shallow water features ever seen. This imagery, in conjunction with seismic profiles, has revealed that the ridges commonly range from 1-4 m in height, strike oblique angles with the coastline, and stretch from the toe of the shoreface to more than 25 km seaward. Morphologically, these ridges appear similar to the storm-built ridges along the US Atlantic margin inner shelf. This study was conducted to determine the timing for the formation of the Pinellas County ridges, the possible modes of evolution, and how are these ridges are influenced by modern hydraulic processes.

AMS radiocarbon dates from forams at the base of a sand ridge off Sand Key, Florida, suggest that the initial formation of the sand ridges began approximately 3400 yrs BP. The presence of the basal gravel near the bottom of the sand ridges support the possibility that the ridges were initially deposited in a higher-energy environment when sea level was lower. Analysis of vibracores also reveal that the sand ridges are fining upward sequences of sand and shell which appear to have been deposited gradually over time as the sea level has risen to its present position.

Current meter data indicate that storms and tides induce a prevailing bi-directional (N-S), shore-parallel bottom flow in the nearshore region off Sand Key during the recording period. Highly detailed shallow water side-scan sonar mosaics collected in time series have revealed short-term localized southward shifts in ridge morphology up to 5 m over a 6 month period. Asymmetric surficial grain size sediment profiles across a ridge also reveal the imprint of a dominant southward directed bottom flow.

This study has revealed that the Sand Key sand ridges are not relict features, but more likely have gradually developed during the late Holocene, and are in relative equilibrium with the modern hydraulic environment. The present hydraulic regime appears to be actively modifying the morphology of these sand ridges. Underlying bedrock does not appear to influence the position or orientation of the ridges.

Coastal & Marine Geology Program > St. Petersburg Coastal and Marine Science Center > West-Central Florida Coastal Studies Project > Second West-Central Florida Coastal Studies Workshop > Framework > Morphology and Evolution of a Holocene Carbonate/Siliclastic Sand Ridge Field, West-Central Florida Inner Shelf U. S. Department of the Interior | U.S. Geological Survey
Center for Coastal & Regional Marine Studies

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Updated May 06, 2013 @ 09:24 AM  (JSS)