The inner west-central Florida continental shelf is an extensive sediment-starved system with ledges and hardbottoms cropping out through a thin veneer of mixed siliciclastic/carbonate sediments. Recent work funded by the U.S.G.S. and based on lithologic and sedimentologic data indicates that these ledges and hardbottoms may provide a source for quartz and phosphatic ‘black' sands that make up the siliciclastic portion of the sediments blanketing the shelf.

Two different ledge lithologies have been identified. In the northwest portion of the study area, massively-bedded, low-relief (<2 m), dolomitic ledges have been correlated with the early to middle Miocene Arcadia Formation of the Hawthorn Group. Throughout the remainder of the study area calcitic ledges of variable relief (up to 4 m) are more common. These ledges are less linear than the dolomitic ledges and occasionally overlie a layer of blue-gray clay, believed to represent the weathered upper surface of the Hawthorn Group. ⁸⁷Sr/⁸⁶Sr ratios of the calcitic ledges indicate Quaternary ages while the blue-gray clay layer ratios indicates a late Miocene age.

Both the calcitic and dolomitic ledges are distinguished by a landward facing scarp and overhang. Active bioerosion has produced undercuts (some in excess of I m) on their leeward side and a proximal rubble ramp/field created by failure of the overhanging wall. The siliciclastic portion of the ledge framework is nearly identical to the siliciclastic portion of the modern sediment cover, suggesting that the ledges may supply sediments to the inner shelf. However, quantification of bioerosion rates is needed to ascertain the extent to which these ledges add to the sediment budget of this mixed siliciclastic/carbonate continental shelf.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah