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The Florida Middle Ground: A Complex Product of Geological, Physical, and Biological Interactions

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
David J. Mallinson, Department of Marine Science, University of South Florida, St. Petersburg, FL
Albert C. Hine, Department of Marine Science, University of South Florida, St. Petersburg, FL
Stanley D. Locker, Department of Marine Science, University of South Florida, St. Petersburg, FL
Mark R. Hafen, Department of Marine Science, University of South Florida, St. Petersburg, FL

The Florida Middle Ground (FMG) was visited by scientists from the University of South Florida in October of 1995. Greater than 350 nautical miles of seismic and side scan sonar data were acquired, and bottom reconnaissance by a remotely operated vehicle was performed. Seismic surveys extend west to the 200 meter isobath. Seismic data reveal useful stratigraphic information to a penetration depth of approximately 40 ms in some areas. These data reveal the FMG reef complex and underlying geology to be more complex than previously recognized. At least five seismic units are recognized which dip to the southwest and are truncated by a subbottom reflector beneath the FMG reefs. The deepest continuous reflector exhibits a high amplitude return and shows 4 ms of karstic relief. This reflector was recognized by Brooks et al. (1991) and is believed to be of mid-Miocene age. Overlying, seaward-dipping seismic units are interpreted to be of Plio-Pleistocene age. Seaward dipping units are truncated beneath the FMG reefs by a high-amplitude reflector also showing karstic features. Two units above the angular unconformity are interpreted as Late Pleistocene and Holocene age. The reefs were found to extend upward from a shallow subsurface (Late Pleistocene?) reflector to a minimum water depth of -30 meters, suggesting that the complex may be Stage 5a or perhaps Stage 3 in origin. ROV reconnaissance reveals a fauna consisting predominantly of sponges, Gorgonian corals, and hydrozoans, with few head corals. The presence of significant reef rubble on slopes, and the lack of abundant carbonate producers indicates a degradational environment.

At present, the FMG area is affected by periodic upwelling (Austin and Jones, 1974), and seasonal high-chlorophyll plumes indicating high productivity and eutrophic water conditions (Gilbes et al., in press). These plumes have been recognized by Coastal Zone Color Scanner imagery every Spring between 1979 and 1986. Plumes begin north of the FMG area near Apalachicola Bay and migrate south-southeast directly over the FMG. The origin of the plume is not known, but several mechanisms have been proposed including Loop Current interactions with the platform margin producing upwelling, and entrainment of high nutrient water masses from fluvial discharge. The productivity plume certainly has a significant influence on biogenic sediment production directly, and indirectly by contributing resources to higher trophic levels. The resulting organic, carbonate, and siliceous materials become part of the shelf sediment budget. These eutrophic water conditions inhibit modern coral production and have significant bearing on the development of the FMG reef complex and implications for paleocirculation patterns. The massive FMG carbonate buildups indicate that water column conditions, existing at the time of FMG formation, were significantly different from those which exist today, suggesting more oligotrophic conditions. The change in trophic conditions indicates that different physical processes operated during the lower sea-level stands of Stage 5a or 3. Paleocirculation models should reproduce changes in physical processes and trophic conditions on the West Florida Shelf which are indicated by the geology, particularly the presence of the FMG reefs.

References

  • Austin, H., and Jones, J., 1974, Seasonal variation of physical oceanographic parameters on the Florida Middle Ground and their relation to zooplankton biomass on theWest Florida Shelf: Florida Scientist, v. 37, p. 16-32.
  • Brooks, G., and Doyle, L., 1991, Geologic development and depositional history of the Florida Middle Ground: A mid-shelf, temperate-zone reef system in the northeastern Gulf of Mexico: in, From Shoreline to Abyss, SEPM Special publication No. 46, p. 189-203.
  • Gilbes, F., Tomas, C., Walsh, J., and Muller-Karger, F., in press, An episodic chlorophyll plume on the West Florida Shelf: Continental Shelf Research.

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 > The Florida Middle Ground: A Complex Product of Geological, Physical, and Biological Interactions


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