RESOLVING THE AGE CONTROVERSY OF PLEISTOCENE COASTAL DEPOSITS IN THE LOWER COASTAL PLAIN IN NORTHEASTERN SOUTH CAROLINA

The Lower US Atlantic Coastal Plain consists of multiple packages of Pleistocene interglacial coastal deposits that are valuable for investigating climate change, sea-level variability, and regional tectonic movements. Although the deposits were recognized about 80 years ago and have been subject to many studies, their ages are uncertain in many locations. One of the controversies is the age of the deposits in the northeastern South Carolina between Myrtle Beach and Charleston for which contradicting uranium-series ages (~200 ka) and amino acid racemization (AAR) ages (80-125 ka) exist. Here we present new lithological and GPR stratigraphy and 18 OSL ages for deposits from three sites, North Santee and Hobcaw Barony near Georgetown and the airport at Myrtle Beach, to resolve this controversy.

The shallow lithostratigraphy at the three sites is characterized by backbarrier deposits overlain by barrier foreshore and dune deposits, which is transgressive in nature. GPR profiles show progradational clinoforms and a few large scale erosional discontinuities within the foreshore deposits at North Santee and Hobcaw Barony. The backbarrier and the overlying foreshore deposits are contemporary at each site based on OSL dating, in agreement with the transgression interpretation. However, ages of the deposits were distinctly different at these sites. OSL ages near the Myrtle Beach airport cluster around 200 ka except for one 80 ka age for a shallow unit at the Intracoastal Waterway while those at North Santee and Hobcaw Barony cluster together around 80 ka. The dune deposits at the latter two sites are dated to about 23 ka.

Our data suggests that the youngest Pleistocene coastal deposits are not contemporary along the northeastern South Carolina coast, despite their stratigraphic and geomorphic similarity, and cautions against regional stratigraphic correlation without numerical age control. In Myrtle Beach, the Pleistocene barrier is formed at ~200 ka (MIS 7) while the transgression at ~80 ka (MIS 5a) probably flooded some area landward of the MIS 7 barrier. The MIS 5a shoreline is preserved between Georgetown and Charleston. Elevations of the dated backbarrier and foreshore deposits suggest that relative sea level probably peak at 1 to 6 m during MIS 5a and -2 to 2 m during MIS 7 along the northeastern South Carolina coast.