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Regional to Local Trends in Bedrock Characteristics and Sediment Thickness from Seismic and Side Scan Data

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Open File Report: Second West-Central Florida Coastal Studies Workshop
Introduction
Agenda
Processes
Framework
Morphodynamics
Attendees
Contact:
Chief Scientist
S.D. Locker, 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. Harrison, Department of Marine Science, University of South Florida, St. Petersburg, FL

A regional overview of geophysical data from the west Florida shelf have led to the recognition of shelf provinces based on a combination of bedrock characteristics and sediment distribution patterns. Major shelf-valley systems are identified of Tampa Bay and of Venice. These valley systems are attributed to large-scale pre-Quaternary warping and infill of the carbonate bedrock. Estuarine retreat paths, shallow channel systems and thicker surficial sediment deposits tend to associate with these valley systems. A secondary offshore trend in sediment thickness is characterized by thinning away form the coast to sediment thickness < 2 m. Further seaward, sediment thickness increases to over 2 m, but occurs in localized sand bodies of more limited extent. This offshore trend suggest patterns controlled by the character of recent sea-level rise. A smaller-scale bedrock warping style is most common south of Tampa Bay, but also occurs off Anclote Key in the North. These bedrock zones tend to exhibit a flat erosional unconformity forming a terrace overlain by sand ridge plains or sand flats. Bedrock terraces may also exhibit relatively flat strata with little acoustic penetration.

The bedrock characteristics are important in several regards: 1) the structural antecedent topography may influence barrier island location and evolution, 2) exposed bedrock can provide a relict sediment source or provide substate for production of biogenic carbonate grains. Attempts to map this surface at the base of the modern sediments becomes difficult close to the coastline and draws attention to the need to better understand what "bedrock" really is. Generally, a flat reflector is observed offshore beneath sediment bodies. This reflector is not observed to project beneath the coastal barriers, nor to have an obvious counterpart below the inland Bays. Offshore this reflector may reflect a ravinement surface, but not necessarily the base of the Holocene. Thus this reflector would not yet exist landward of the coastline. Probing results have indicated penetration depths that appear to penetrate into soft bedrock sediments associated with shelf-valley systems (Pre-Holocene). This all points to the need to carefully document the age of sedimentary facies pre-dating island development in order to access the role of "hard" of "soft" bedrock controls on coastal evolution.

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 > Regional to Local Trends in Bedrock Characteristics and Sediment Thickness from Seismic and Side Scan Data


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