LATE QUATERNARY AMINOSTRATIGRAPHY AND GEOCHRONOLOGY OF THE ST. HELENA ISLAND AREA, SOUTH CAROLINA COASTAL PLAIN

The availability of fresh mollusk samples from a borrow pit on Edisto Island, SC provides an opportunity for a detailed aminostratigraphic analysis of the SC Coastal Plain using improved analytical methods. Gas chromatographic (GC) analyses of D/L values for six amino acids in 13 mollusks from a superposed section in the Edisto pit are compared with new and previously published analyses of Mercenaria and Mulinia from vibracores and carefully sampled outcrops between Charleston SC and Savannah GA. D/L results from two sites with U-series coral dates of ca. 80 kyr, Scanawah Island SC (Szabo, 1985) and Skidaway Island GA (York et al., 1999) are particularly important for this comparison. Scanawah Island is 5 km inland from the Edisto Site in an older geomorphic setting. Outcrops at Sease Pit (32.7112N, 79.9471W) have no associated U-series dates but two superposed aminozones provide an important reference section.

Results for Mulinia at outcrop positions from 8' below land surface to 15' below land surface in the Edisto section show no trends of D/L with depth, hence no age difference. The mean Mulinia D/L values (leucine D/L=0.30) from this section are identical with those from the upper unit at Sease Pit and from the Skidaway site; the mean Mulinia D/L values from Scanawah Island are significantly greater (leucine D/L=0.43), similar to those seen in the lower unit at Sease Pit. Data for Mercenaria from the Edisto and Scanawah sites are consistent with the Mulinia results. Mercenaria D/L values from the Edisto pit match those seen in beach specimens (>40 kyr by C-14) (Wehmiller et al., 1995), indicating the regional significance of this aminozone.

Using Skidaway Island U-series data as calibration, the Edisto unit is interpreted to be approximately 80 kyr in age (late Stage 5). However, this age estimate conflicts with the Scanawah Island "calibration", so previous questions about the regional geochronology and sea level history remain unresolved.