COASTAL LITHOSTRATIGRAPHY AND GEOMORPHOLOGY OF THE FLORIDA PANHANDLE

The lithostratigraphic mapping of Quaternary coastal units in the Florida Panhandle is complicated by the general lack of dateable material and the sedimentologic similarity of most coastal, siliciclastic facies. Consequently, the Florida Geological Survey (FGS) has mapped coastal units, in part, based on geomorphology. Holocene sediments (Qh) have been identified based upon association with recent coastal environments. Improvements in lidar-derived digital elevation models (DEMs) have made possible the differentiation of Holocene coastal ridges, which have a crisp appearance, from older Quaternary beach ridges and dune (Qbd) deposits, which have a more muted, degraded appearance.

Previous work has identified significant areas of MIS-5 and older coastal ridges in the Florida Panhandle. Some relict coastal deposits might be recognized by trellis fluvial drainage patterns, following former inter-dune/barrier swales. Exposures along the Gulf Intracoastal Waterway between Choctawhatchee Bay and Panama City show a sequence of Late Pleistocene, Ophiomorpha-rich, humatic quartz sands and peat overlain by Holocene dune sands with subfossil wood. Fieldwork at these exposures and lidar-derived DEMs, have allowed a draft re-mapping of the Qh and Qbd deposits in the region.

Coastal terraces must be identified based on coastal processes, depositional and erosional, in combination with elevation. Previous, elevation-based terrace classifications have proven problematic in Florida because of lack of differentiation of depositional and erosional coastal landforms with respect to terrace elevations. Additionally, both subsidence and isostatic adjustment due to karst-related dissolution, have impacted preservation and elevation of ancient coastal landforms in Florida. Coastal terraces must also not be extended into river valleys where they might be confused with fluvial terraces. A combination of lithology, lidar-derived DEMs, and re-evaluation of coastal terraces will be required to identify and name these undifferentiated, surficial coastal sediments, both in Florida and in adjacent states along the Gulf of Mexico and Atlantic coasts.