A NEW HYPOTHESIS FOR THE FORMATION OF THE GEORGIA SEA ISLANDS THROUGH THE BREACHING OF THE SILVER BLUFF BARRIER AND DISSECTION OF THE ANCESTRAL ALTAMAHA-OGEECHEE DRAINAGE

Georgia’s Sea Islands have formed by the breaching of the Silver Bluff (Pleistocene) strandline as a consequence of the Holocene transgression. Breaching occurred during two intervals beginning around 4500 and 2500 BP., related to rising sea levels and separated by an apparent lowstand (~2800 BP). Three inlets (St. Catherines, Sapelo, Altamaha) date to the early period and others formed later (Wassaw, Ossabaw, St. Simons, St. Marys, Nassau). During the 2800 BP. lowstand, St Catherines and Sapelo sounds were partly closed by spits that were breached when transgression resumed. Trim lines cut into the Silver Bluff barrier (~ 4500 BP) project across the inlets and imply a relatively continuous wave-dominated morphology similar to the Grand Strand of S. Carolina. With most modern inlets closed coastal drainage occupied the low-country behind the Silver Bluff barrier. It is hypothesized that the ancestral Altamaha originally entered the Atlantic at St. Andrews Sound, forming a delta in front of Cumberland, Amelia and Talbot islands. The Ogeechee, Satilla and St. Marys are interpreted to be original tributaries of the ancestral Altamaha. Supporting evidence is provided from the dating of Holocene beach ridges (Tybee, Wassaw, Ossabaw, St. Catherines and Blackbeard islands) and vibracores from inlet-fill (Ossabaw, St Catherines, Sapelo, St. Simons and St. Andrews sounds). Numerous ‘fossil’ tree stumps from beneath tidal marsh (dated 3900-2100 BP) support the conclusion that the low-country was formerly a freshwater habitat, especially during the 2800 BP. lowstand. Indigenous native American populations intensively exploited marshland and terrestrial resources during transgressive periods but virtually abandoned the islands during the 2800 BP. lowstand.