Abstract: Physical Characteristics of Washover Fans of Georgia Coast

J. R. Deery, J. D. Howard

Washover fans are present along the Georgia coast on Little Tybee, Ossabaw, St. Catherines, Cabretta, Egg, and Wolf Islands. Major concentrations of washovers are on Ossabaw, St. Catherines, and Cabretta Islands where field studies indicate two types of primary physical sedimentary structures: (1) washover structures which consist of subhorizontal stratification, ripple lamination, foresets (planar cross-bedding), trough cross-bedding, convolute bedding, and climbing ripple lamination; (2) wind-generated structures which form between episodes of washover and consist of dunes and their associated structures, ripple marks, blowouts, and sand lenses.

Surface morphology and internal structures delineate three facies: beach, fan, and marginal. Each facies has a distinctive suite of physical and biogenic structures.

Washover fans are wedge-shaped in cross section, thinning in a landward direction and fan-shaped in plan. Fans are thin deposits which extend onto the marsh from the beach.

Shore-parallel dimensions of washovers range from 95 to 1,800 m and shore-normal dimensions from 120 to 250 m. Areas and volumes of the fans are from 20,160 to 450,000 sq m and 7,980 to 157,500 cu m, respectively.

Washovers on the Georgia coast generally form in response to storm surges; all show the same internal structures and occur along areas of coast undergoing rapid erosion. St. Catherines Island has the highest rate of erosion of all of the islands, and has been the focal point of washover activity since the 1930s. Cabretta Island has shown intermittent activity that may be cyclic. Ossabaw Island washovers were not established until the early 1960s.

In addition to washover fans, the washover environment includes other characteristic features: extensive shoaling, eroding beaches, and spit-building.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC