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Stratigraphy of West-Central Florida's Inner Shelf Sand Ridges

West-Central Florida Coastal Studies Home
Open File Report: Second West-Central Florida Coastal Studies Workshop
Introduction
Agenda
Processes
Framework
Morphodynamics
Attendees
Contact:
Chief Scientist
J.H. Edwards, Department of Marine Science, University of South Florida, St. Petersburg, FL
A.C. Hine, 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.E. Harrison, Department of Marine Science, University of South Florida, St. Petersburg, FL
G. Brooks, Department of Marine Science, Eckerd College, St. Petersburg, FL
D. Twichell, U.S. Geological Survey, Woods Hole, MA

The West-Central Florida inner shelf has been the focus of extensive seismic and side scan sonar surveys. Within this region are sand ridges oriented obliquely to the shoreline 1-4 meters in height and 100-300 meters in width. Previous studies (Harrison and Twichell, 1995) have made valuable insights to the understanding of the morphology and evolution of these ridges but little is known about the stratigraphy with exception to what can be inferred from acoustic imaging. The sand ridges of this area appear to be very complex and difficult to produce a stratigraphic model for. In addition to being very complex, the ridges also differ from the north to the south of Tampa Bay. A few preliminary stratigraphic models have been developed based on a few cores that have been taken on the ridges in conjunction with the existing high frequency seismic profiles that goes for both north and south provinces. These models are very similar to one another with the major differences being the origin of the clays and the topography of the limestone bedrock.

  1. The first model is a sand ridge that is composed of an open marine, fining upward sand made up of a mixture of siliciclastic and carbonate facies which overlies a muddy back barrier, commonly organic rich sediment deposited during the most recent transgression of sea level. Beneath this open marine and back barrier package exists a detrital blue-gray clay believed to be a part of the Hawthorne Formation. The detrital clays are deposited in various pits and depressions the antecedent limestone bedrock provides where they are sheltered from erosional processes and can be preserved.
  2. A relatively flat Cenozoic bedrock topography that as undergone diagenetic alterations producing some clays and lime mud. Overlying the clay exists the same package of back barrier mud underneath an open marine facies as described in model one.
  3. A third model developed for the ridges involves a flat bedrock surface under shelf sands from the latest transgression of sea level. This model has no detrital clays and relatively no diagenetic alteration.
  4. The fourth model essentially is identical to the second with one exception. In this model the bedrock consists of a depression or pit that has been diagenetically altered. This produces the clays and lime muds described at the base of the cores. Above the authigenic sediments are again the same modern transgressional sediments.
  5. The last model is an irregular limestone feature exhibiting positive relief in contrast to a depression. The limestone is possibly diagenetically altered to produce minimal clays and lime muds. This underlies the mud and sand packages seen in all four models of the ridges.

In the future additional Chirp data of the ridges will be obtained in anticipation of resolution that will detect impedances within the ridges. Also numerous vibracores will be acquired along the length and across the width of the ridges. With future acoustic imaging, future vibracores and use of pre-existing acoustical studies of the ridge system a detailed stratigraphic analysis can be completed and compared to these 5 preliminary models developed above. Subsequently, we hope to assess the role antecedent bedrock topography plays in the stratigraphy of these inner shelf sand ridges. With exception to the second model's relatively flat bedrock, this abstract suggests that the underlying host limestone bedrock does in fact play a role in the stratigraphy and facies types deposited within and below these ridges. Also with completion of this study questions such as the mobility and the stability of the ridges can be addressed. Are the ridges actively migrating or are they in a state of stability with present day sea level and oceanographic conditions? Why and how the ridges differ in the different provinces? In addition to these obvious questions of stability and mobility being addressed with a detailed stratigraphic model, other acoustic imaging problems still reside within the ridges. A common trend displayed in the seismic profile is the inconsistency of the limestone bedrock reflector. At the periphery of the ridge many profiles exhibit a reflector of high amplitude, but within the ridge the reflector tends to become intermittent. With high resolution chirp data and strategically placed vibracores, quantification of a reflection coefficients may be possible. This will correlate the amplitude of the reflection to lithologic changes in the subsurface.

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 > Stratigraphy of West-Central Florida's Inner Shelf Sand Ridges


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