Abstract: Origin and Character of Washover Fans on Georgia Coast

John R. Deery, James D. Howard

Washover fans on the Georgia coast initially form in response to storms, but continue to develop after the storm has passed. For the most part, the storm-induced fans flood only during spring tides but some are continuously active on a day-to-day basis. All of the washovers have essentially the same surface morphology and internal physical and biogenic sedimentary structures and, on the basis of these features, the washover area can be subdivided into beach, fan, and marginal facies.

Our field studies, centered primarily on Ossabaw, St. Catherines, and Sapelo Islands, indicate that Georgia washover fans have an "active" and a "passive" phase of fan development and each leaves a characteristic record in fan stratigraphy and sedimentary structures. The "active" phase occurs when foreshore waves top the beach crest and water and foreshore sediments flow landward into the back-barrier salt marsh. At such times, subhorizontal stratification, ripple lamination, planar foreset cross-bedding, and trough cross-bedding are formed. The "passive" phase occurs when the fan is not undergoing overwash from the beach. During this time eolian and biogenic processes and rain runoff are the principal agents in the formation of sedimentary structures. "Passive" phase structures include dunes, ripple marks, blowouts, wind-generated levees, bioturbation, and climbing ripples. Although seemingly less dramatic in nature, the "passive" phase structures represent the greatest period of time in washover-fan development.

Georgia coast washover-fan deposits are wedge shaped in cross section, thin in a landward direction, and are fan shaped in plan. In most places, the fans build across salt-marsh deposits. Shore-parallel dimensions of washovers range from 95 to 1,800 m and shore-normal dimensions range from 120 to 250 m. Areas and volumes of individual fans are from 20,000 to 450,000 sq m and represent sand accumulations of up to nearly 160,000 cu m. These deposits, composed predominantly of fine sand, are derived from adjacent and updrift beaches, tidal-creek shoals, and shoreface-shelf deposits.

Washover fans occur along parts of the Georgia coast undergoing rapid erosion. St. Catherines Island has the highest rate of erosion of all the islands and has been the focus of washover activity since the early 1930s. Sapelo Island, adjacent on the south, has shown intermittent, possibly cyclic activity for the last 30 years. Ossabaw Island, directly north, has had washover-fan development only since the early 1960s.

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