Abstract: Pliocene-Pleistocene Paleogeography of Florida Gulf Coast Interpreted from Relict Shorelines

Charles D. Winker, James D. Howard

Studies of the Atlantic coastal plain have demonstrated the value of mapping paleogeography from ancient shoreline morphology. By use of large-scale topographic maps, relict coastal features can be established and the compiled shoreline record may be differentiated into relative age groups. Each shoreline sequence is mapped on the basis of lateral continuity and alignment, state of topographic preservation, and elevation, taking into account the possibility of regional warping. This method indicates, for each period of high sea level, not only paleogeography but regional variations in erosion and deposition as well.

In Florida, three sequences of relict Gulf shorelines, none of which has previously been mapped, are recognized as either a terrace or a stratigraphic unit. The Escambia sequence is well preserved from Mobile Bay to south of Tampa Bay, at elevations below 10 m, primarily as erosional scarps, but in Escambia and Gulf Counties as beach-ridge plains. These shorelines have been described as Sangamon (late Pleistocene) in age. The Wakulla sequence consists primarily of a discontinuous scarp, between 20 and 30 m above present sea level, which forms a broad arc between the Apalachicola River and Tampa Bay. Beach ridges in Wakulla and Leon Counties and some scarps around Tampa Bay may be part of the sequence. The Gadsden sequence consists of an extensive beach-ridge plain occupying a warped, issected upland surface between Tallahassee and the Escambia River. This surface reaches a maximum elevation of 100 m. Original ridge-and-swale trends are preserved as, and accentuated by, a deeply incised network of trellis drainage. Because surficial sediments of this region, mapped by the Florida Bureau of Geology as Citronelle and Miccosukee, unconformably overlie the Pliocene Jackson Bluff Formation, the Gadsden sequence is post-Jackson Bluff. On the Florida Peninsula, the Brooksville, Lakeland, and Bartow Ridges probably represent contemporaneous deposition.

During formation of the Gadsden sequence, the Apalachicola River built a large cuspate delta into the Gulf of Mexico. At the same time, beach ridges were built on the peninsula by sand brought from the delta by longshore transport. Later, during formation of the Wakulla and Escambia sequences, clastic deposition was localized near the mouths of the Apalachicola and Escambia Rivers. Little sand was introduced to the Gulf side of the peninsula, in striking contrast to the Atlantic side during the same period.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas