HOLOCENE COASTAL PROGRADATION OF THE ST. HELENA MARSHES, BEAUFORT CO., S.C.: EPISODIC BARRIER ISLAND DEPOSITION RESULTING FROM BOTH EBB-TIDAL DELTA AND IMMEDIATE OFFSHORE SAND SOURCES

The St. Helena Marshes record 8 km of coastal progradation along 22 km of Atlantic shoreline between St. Helena and Port Royal Sounds. This region contains three shore-parallel bands of beach-ridge barriers separated by clay-rich salt marsh deposits. The youngest band is the present-day barriers of Hunting, Fripp, Pritchards, and Capers whose beach ridges have patterns indicative of significant onshore sand transport. Hunting and Pritchards are experiencing pronounced erosion and the beach-ridge portion of Capers has all but disappeared. The middle band, 1-2 km inland from the present shore, consists of Old and St. Phillips Islands, both containing beach ridges that reach elevations equal to or slightly lower than those of the youngest band. The ridges of Old Island indicate significant SW longshore transport as well as onshore transport. The St. Phillips ridges indicate onshore transport. Storey and Machete Islands comprise most of the oldest band, 0.5 km inland from the middle band, and contain low, isolated ridges and/or ridges largely buried by marsh. These ridge patterns also indicate significant onshore transport. Salt marsh and inter- and subtidal clay-rich deposits of Harbor River separate this oldest band from the adjacent Pleistocene St. Helena Island. The chronologic framework of this 8 km-wide progradation is poorly known. Archaeological data indicates that the oldest band was in existence 4000-3500 years BP. Radiocarbon ages of midden shells show that Old Island had been deposited some time prior to 2700 years BP. The younger third of Hunting Island is no older than 1300 years BP and Pritchards Island is no older than 1700 years BP, both based on radiocarbon ages of shell deposits. Hunting Island is adjacent to the St. Helena Sound ebb-tidal delta and St. Phillips was most likely adjacent to an older ebb-tidal delta of what now is Trenchards Inlet. The youngest part of Fripp Island is related to the Fripp Inlet delta. The rest of the Holocene barriers are not located nearby any ebb-tidal delta and thus probably received their sand directly from the immediate offshore. The division of these barriers into three, shore-parallel bands separated by clay-rich salt marsh strongly suggests some region-wide mechanism to episodically pulse sand landward from an offshore source, such as a sea-level fluctuation.