Abstract: Hardbottoms of the Inner West-Central Florida Continental Shelf

OBROCHTA, STEPHEN, DAVID DUNCAN, AND GREGG BROOKS, Eckerd College, St. Petersburg, FL

The inner west-central Florida continental shelf consists of a Quaternary sediment veneer overlying a mid-Miocene, carbonate platform, that correlates with the dolomite-rich Arcadia Formation of the Hawthorn Group. Extensive systems of shoreparallel hardbottoms crop out through the veneer and exhibit two distinct lithologies that are typically rich in quartz and phosphorite. Active bioerosion has produced undercuts on their shoreward (lee) side and a proximal rubble ramp that is created by failure of the overhanging wall, resulting in the addition of phosphatic and siliciclastic sediments to the inner shelf system.

Massively-bedded, low-relief (<2m), dolomitic hardbottoms, that are comprised of zoned rhombs, appear to be extensions of the underlying platform. Based on association with phosphorite and correlation with similar Hawthorn dolomite, this dolomite appears to have formed by sulfate reduction in marine pore waters from sediments that were deposited in a low-enery setting.

Quaternary calcitic hardbottoms of variable relief (up to 4m) are located to the southeast of the dolomitic hardbottoms. The calcitic hardbottoms are also comprised of sediments that were deposited in a low-energy setting, but the tops of the hardbottoms contain higher amounts of quartz and are relatively unaltered, while underlying beds show signs of extensive meteoric phreatic diagenesis and contain lesser amounts of quartz. They appear to be perched upon the lithified platform because they occasionally overlie a layer of blue-gray clay, which is believed to represent the weathered, upper surface of the Arcadia Formation.

AAPG Search and Discovery Article #90932©1998 GCAGS/GCS-SEPM Meeting, Corpus Christi, Texas