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.

Abstract The southwest Florida continental slope is part of the vast, non-rimmed west Florida carbonate platform. Bordering the southern Florida Straits, it has been the site of a thick accumulation of seaward-prograding sediments throughout at least the Late Quaternary. High-resolution seismic-reflection data and sediment cores were collected in order to determine: (1) provenance and dominant depositional processes; (2) how slope development has related to high-frequency sea-level fluctuations; and (3) how deposits compare with other carbonate slope environments. Nine seismic sequences have been identified, each bounded by an erosional unconformity. All nine sequences consist of seaward-trending prograding clinoforms. Sediments of sequences penetrated by cores (sequences 1, 3, and 5) consist of a mixture of shallow- and deep-water biogenic carbonate sands and muds deposited rapidly (averaging greater than 2.5 m/ka) on the upper slope. Shallow-water sediments are transported to the depositional site by oceanic currents, intermittent storms, and tidal currents sweeping the southern shelf and upper slope. Based upon biostratigraphy, radiocarbon age determinations, and seismic-sequence and facies analysis, depositional patterns are interpreted to be a function of high-frequency sea-level fluctuations operating during the Late Quarternary. Most vigorous offshelf transport occurred during periods in the sea-level cycle when the shelf surface was flooded but shallow, as would occur during early transgressions and late regressions. During sea-level highstands, offshelf transport was less vigorous. During sea-level lowstands, no offshelf transport took place. Erosion of the previously deposited sequence resulted from an increase in erosive capacity of the Florida Current during glacially induced sea-level lowstands. Four such sequences, collectively attaining a thickness of 330 m, have been deposited during approximately the last 100 ka. This would correspond to a maximum of several tens of meters on most carbonate shelves and is represented on the adjacent west Florida shelf by only a few meters of carbonate sediment accumulation. Controls on recent margin development are dominated by high-frequency sea-level fluctuations coupled with the physical processes that act to concentrate sediments in the study area, and the lack of a shelf edge rim of reefs, enabling offshelf transport of shallow-water carbonate sediments.