LOW-RELIEF, SPATIALLY RESTRICTED, LATE CENOZOIC SILICICLASTIC BASINS EMBEDDED WITHIN A LARGE CARBONATE PLATFORM: TAMPA BAY AND CHARLOTTE HARBOR, WEST-CENTRAL PENINSULAR FLORIDA

Prior studies have revealed an extensive north-south transport pathway of quartz-rich sand and gravel sediments extending from the SE US across the Florida Platform. This multi-phased dispersal of siliciclastics occurred in the late Miocene and early Pliocene covering carbonates that dominated platform development since the early Jurassic. Sediment movement on the platform occurred both in: (1) sea-level highstand fluvio-deltaic systems with much greater bedload capacity than modern-day Floridian streams and (2) coastal longshore and shallow-shelf transport systems. These siliciclastics filled in small basins on the carbonate platform surface, including Tampa Bay and Charlotte Harbor.

Recent seismic and borehole data show that Tampa Bay is underlain by two low-relief (~40-60 m deep) subsurface basins separated by a seismic “basement” high. Modern Tampa Bay has an average depth of only 4 m and is a surface manifestation of these underlying geologic basins. Seismic “basement” is limestone at the top of the lower Miocene Arcadia Formation, which has been deformed into complex sags and domes presumably due to deep-seated dissolution of older carbonates. Dissolution at depth caused karst collapse of the overlying strata, including the Arcadia, creating the low-relief surface basins. This collapse occurred during extended sea-level lowstand in the late Miocene, or alternatively, when Kohout convection stimulated carbonate dissolution at depth.

Sediments filling these basins can be subdivided into at least 6 mappable seismic sequences, many containing pronounced prograding clinoforms that point to a south/southeast source area. The lowermost sequence is deformed indicating syn-depositional collapse. Borehole descriptions demonstrate that the basin fill consists of undifferentiated quartz sands, clays, and shells.

The siliciclastic-filled basins underlying Tampa Bay and Charlotte Harbor occur in a mid-carbonate platform setting. Rather than incised valley fills or reef-margin, backfilled basins, they represent spatially-restricted, filled-in karst features. The “dimpling” of a carbonate platform by coalescing karst basins provides a previously unrecognized mechanism for the creation of accommodation that can result in the “drowning” of a carbonate platform by siliciclastics.