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Second West-Central Florida Coastal Studies Workshop

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Open File Report: Second West-Central Florida Coastal Studies Workshop
Introduction
Agenda
Processes
Framework
Morphodynamics
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Chief Scientist

West Florida Inner Shelf Provinces--Links to Present Coastal System, to Modern Shelf Processes, and to the Geologic Past

A.C. Hine, Department of Marine Science, University of South Florida, St. Petersburg, FL
J. Edwards, Department of Marine Science, University of South Florida, St. Petersburg, FL
S.D. Locker, Department of Marine Science, University of South Florida, St. Petersburg, FL
S. Harrison, Department of Marine Science, University of South Florida, St. Petersburg, FL
D. Twichell, U.S. Geological Survey, Woods Hole, MA

Our collective acoustic surveys have demonstrated that the west Florida inner continental shelf is dominated by a Cenozoic limestone bedrock unconformity supporting a thin, mixed siliciclastic/carbonate sedimentary veneer. The unconformity has various spatial scales of antecedent relief: (1) pits, depressions, ledges from cm's to several m's of relief and cm's to 100's m in width/length, to (2) broad rise, flat bedrock plain, and shelf valleys from m's of relief to km's in width/length. The sedimentary cover is commonly arranged in: (1) linear ridges ranging (.5 to 4 m's of relief, 10's m's in width, 100's m's of spacing, and km's in length), (2) broad, very thin sheets, or (3) active ebb-tidal deltas located just off tidal inlets.

Ongoing mapping has allowed us to define distinct areas or shelf provinces that transition from one to another both alongshore and onshore/offshore. In addition, shelf provinces can be distinguished by either by their surface and subsurface characteristics or both. For example, subsurface shelf valley may support a relatively featureless sandy plain or a sediment ridge complex.

We have defined the following provinces:

  1. Bedrock Rise/Linear Sand Ridges (Indian Rocks Headland)
  2. Estuarine Retreat Path (Tampa Bay)
  3. Shelf Valley (off Manatee County and Venice)
  4. Sand Ridge Plain (off Sarasota County
  5. Sediment Barren Bedrock Terrace (off Venice)

Some of these provinces have significant onshore/offshore trends as well the north to south trends seen above. For example, the shelf valley systems have smaller relief going offshore. In contrast, the shelf sand ridges off Indian Rocks and Sarasota increase in relief going seaward. However, close to the nearshore, the sand ridges seem to disappear all together suggesting that they do not provide sediment to the beach.

The link between coastal sectors and adjacent shelf provinces ranges from a strong direct link in the Indian Rocks Beach area and the Tampa Bay mouth area to no apparent link at all in the Sarasota/Venice area. For example, the bedrock rise supporting the linear ridges off Indian Rocks Beach is the direct seaward extent of the coastal headland. Antecedent rock topography controls both coastal headland and inner shelf geology. Similarly, the estuarine retreat path of Tampa Bay has left a featureless sediment plain that transitions into a swash-bar dominated, relatively new barrier island system covering open estuarine deposits. The coastal system south of Tampa Bay seems to have no large-scale morphologic relationship to the adjacent inner shelf provinces. However, most likely there are local direct links between barrier island/inlet morphology and underlying antecedent rock topography.

At the moment, we see little linkage between modern shelf processes and shelf provinces. This suggests that the shelf provinces are more a product of the geologic past having inherited large-scale properties such as regional bedrock topography, valley infill, and uneven sediment cover from long-term processes such as subterranean and surface dissolution, paleofluvial activity, climate change, and sea-level fluctuations.

Finally, there are a number of questions that remain elusive. For example, we do not understand the relationship between nearby exposed (or in shallow subsurface) bedrock and sediment cover. Is the bedrock controlling sediment type? Does the bedrock control sediment production either biotic (calcareous communities, bioerosion) or physical (breakage)? More importantly, we do not understand the detailed relationship between antecedent rock topography and the overlying stratigraphy. Just how does a seemingly random bedrock topographic pattern affect sedimentary facies distributions? What fragments or patches of stratigraphy lie preserved within broad, shallow depressions or beneath sand ridges? Do these stratigraphic fragments from the geologic past provide sources of beach sand? Can unique sedimentary constituents from these stratigraphic units such as phosphate be used as a natural tracer to study modern shelf sediment transport patterns? Will we be able to predict what sedimentary products lie on this vast inner shelf without having to sample every m 2?

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 > West Florida Inner Shelf Provinces--Links to Present Coastal System, to Modern Shelf Processes, and to the Geologic Past


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