Karst sub-basins and their relationship to the transport of Tertiary siliciclastic sediments on the Florida Platform

Multiple, spatially restricted, partly enclosed karst sub-basins with as much as 100 m of relief occur on a mid-carbonate platform setting beneath the modern estuaries of Tampa Bay and Charlotte Harbor located along the west-central Florida coastline. A relatively high-amplitude seismic basement consists of the mostly carbonate, upper Oligocene to middle Miocene Arcadia Formation, which has been significantly deformed into folds, sags, warps and sinkholes. Presumably, this deformation was caused during a mid-to-late Miocene sea-level lowstand by deep-seated dissolution of carbonates, evaporites or both, resulting in collapse of the overlying stratigraphy, thus creating palaeotopographic depressions. Seismic sequences containing prograding clinoforms filled approximately 90% of the accommodation space of these western Florida sub-basins. Borehole data indicate that sediment fill is mostly siliciclastic deposited within deltaic depositional systems. The sedimentary fill in the Tampa Bay and Charlotte Harbor sub-basins is mostly assigned to the upper Peace River Formation of late Miocene to early Pliocene age. This fill is part of a > 1000 km long, Tertiary siliciclastic deposit that stretches north-to-south down peninsular Florida. Sediment fill of these two sub-basins is linked to erosion and remobilization of pre-existing, middle Miocene quartz-rich sediments via enhanced sediment transport by local, short-length rivers and discharge into coastal-marine depositional environments. Increased sediment discharge possibly resulted from amplified thunderstorm activity and enhanced runoff during a warm period of the Pliocene. Rather than incised valley fills or reef-margin, backfilled basins, Tampa Bay and Charlotte Harbor represent spatially restricted, sediment-filled karst palaeotopographic lows. The 'dimpling' of a carbonate platform by karst sub-basins provides a previously unrecognized mechanism for the creation of accommodation that can result in the 'drowning' of a carbonate platform by siliciclastics.