OVERWASH DEPOSITS ON FLORIDA BARRIER ISLANDS: A STUDY OF CONTRASTING DEPOSITIONAL STYLES

The overwash generated by the storm surge of hurricanes is an important process for the landward redistribution of sediment on barrier islands. Several factors have been previously identified as controls on the morphological changes to barrier islands from overwash generated by hurricanes, including sediment type and size, shape and topography of the coastline, elevation of the water, and the presence of dunes. A sedimentological study of the overwash was undertaken using trenches and cores of overwash from Hurricane Ivan (2004) and Hurricane Dennis (2005) on the Florida Panhandle Coast. The Panhandle Coast consists of low-lying barrier islands with some dune development. A similar study of overwash from Hurricane Frances (2004) and Hurricane Jeanne (2004) was conducted on the Florida Atlantic Coast. There the depositional environment is a barrier island with a mangrove forest. Different sedimentary structures were found associated with the overwash deposits studied. A well-defined bottom layer with vegetation was present at the base of Hurricane Ivan deposits. Overwash from Hurricane Dennis occurred ten months after Hurricane Ivan. Therefore, the boundary between the two was difficult to distinguish as there was little intervening time for dune and vegetation re-growth. Fine-grained laminar beds with occasional layers of dark, heavy minerals were present. On the Florida Atlantic Coast, graded bedding was observed instead of laminar bedding possibly indicating that the mangrove trees inhibited lateral sediment deposition. The recognition of this graded bedding helps define different depositional events. Another factor recognized is the apparent control of previous episodes of overwash on subsequent overwash deposition. Although different coastal environments with differing depositional styles were studied, both environments revealed that repetitive overwash events tend to result in sediment deposition in the same area within each environment.