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Abstract

The Florida Middle Ground (FMG), the northernmost living coral reef in the Gulf of Mexico, was investigated by high-resolution seismic-reflection methodology, surface-sediment analyses, and direct observation by submersible to identify controls and processes responsible for recent geologic development, and to determine how it relates to development of the surrounding west Florida continental margin, a non-reef-rimmed carbonate platform. Modern reef growth is developed on a foundation consisting of a dead reef that probably was formed during one or more sea-level highstands of the Quaternary. Initial reef growth probably was controlled by bathymetry and regional circulation patterns that served to recruit Caribbean fauna to the FMG.

Surface sediments are distributed in indistinct zones exhibiting a patchy distribution controlled by physical processes and the rugged bathymetry. Sediments throughout the FMG consist dominantly of molluscan shell hash derived from both the reef ridges and surrounding shelf environment. Barnacle fragments are the only major constituent found associated with reef ridges and not the surrounding shelf sediments. Barnacles probably are prevalent on the FMG because of the suitable sites for attachment provided by the dead-reef foundation. Thus, barnacles provide the only major sedimentological indicator of a transition from an open-shelf to reefal environment. The lack of a clear distinction between sediment types of the two environments may have implications for those investigating ancient carbonate environments.

Recent geologic development of the FMG parallels that of the west Florida margin. Driven by high-frequency sea-level fluctuations, similar carbonate sediments are produced in both settings during periods when the shelf surface is flooded. Thick surface sediments on the FMG, relative to the surrounding shelf, indicate reef ridges act to trap sediments much like shelf-edge reefs trap sediments on a reef-rimmed carbonate platform surface. Reef growth and sediment accumulation cannot keep pace with sea-level rise, which results in the drowning of FMG reefs along with the remainder of the west Florida carbonate margin.

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