RADIOMETRIC DATING SUPPORTS CONTENTION THAT BLACKBEARD ISLAND IS THE TIP OF ST. CATHERINES SPIT ON THE GEORGIA COAST, USA

Radiometric dating supports the hypothesis that Blackbeard Island originated as a spit severed from the southern end of St Catherines Island. 14C dates from Cracker Tom Hammock at the north of St. Catherines spit show a range of ages from 6020 ± 50 to 2850 ± 30 BP, while the southern part of the spit is no older than 2.6 ± 0.4 ka (OSL). This confirms the interpretation that the island was breached along the line of Zapala scarp around 2700 BP. It is also in agreement with archaeological evidence indicating St. Simons age sites (>2950 BP) north of the scarp and Early Refuge sites (2950-2500 BP) to the south. Blackbeard Island is the next island south of St. Catherines, down the longshore transport system. Beach ridges at the northwest end of Blackbeard Island give radiometric ages between 3700 ± 40 and 2720 ± 30, coeval with terrains on Cracker Tom Hammock. Beach ridges on Blackbeard and Cracker Tom have similar morphology and are concave to the Atlantic, while the younger part of St. Catherines spit is convex to the Atlantic. Since the breach occurred, the south end of St. Catherines Island has migrated about 5.5 km south through accretion. Likewise, Blackbeard has eroded by an equivalent amount and attached to Sapelo Island. Breaching was probably a consequence of rising sea level combined with erosion and storm avulsion along the S. Newport River that formerly debouched between Blackbeard and Sapelo islands (modern Blackbeard Marsh). Vibracores indicate an active inlet beneath Blackbeard Marsh up to at least 3090 ± 40 BP. Like Sapelo-Blackbeard, the northeast end of St Catherines Island is interpreted to have included a doublet (Guale Island; Bishop et al., 2007) that may have originated similarly, as a spit at the south end of Ossabaw Island.