CHANGE IN THE BALANCE BETWEEN WAVE AND TIDE-DOMINATED STRANDS AS A CONSEQUENCE OF RISING SEA LEVEL IN THE GEORGIA BIGHT, USA

During marine transgression backbarrier marshes and estuaries are flooded, leading to an increase in the volume of the tidal prism and sediment trapping in the estuaries. I expect such conditions to favor tidal over wave processes, because tidal currents are strengthened and the shoreface starved of sand. While wave processes encourage longshore transport, spit-building and the diversion of inlets, tidal processes interrupt longshore transport, encourage the straightening of inlets and the breaching of spits. At present the tide-dominated section of the Georgia Bight is characterized by relatively short islands with inlets perpendicular to the coast. By contrast wave-dominated strands in the Carolinas and Florida are characterized by long, continuous barrier islands with inlets that are blocked or deflected. Many estuaries within the Sea Island sector of the Georgia Bight reveal abandoned inlets and dissected spits indicating that they have undergone recent straightening, probably related to the Holocene transgression. Evidence is presented from facies analyses, accompanied by radiometric dating of more than 50 vibracores. In particular, Ossabaw, St. Catherines, Sapelo and St. Simons sounds appear to be recent breaches accompanied by abandoned inlets and dissected spits. Blackbeard Island is interpreted to have originated as a spit at the southern end of St. Catherines Island, formed when S. Newport River debouched through a channel beneath Blackbeard Marsh. Similar abandoned channels underlie Seaside Marsh on St. Catherines Island and Clam Creek Marsh on Jekyll Island. An earlier episode of spit building and diversion may date to a period of stillstand or minor regression between 3.6-4.3 ka. It is probable that during this interval wave-dominated shorelines predominated throughout the Georgia Bight. I suggest that the balance between wave and tide-dominated systems has shifted as a consequence of the Holocene transgression. At present the transition from wave to tide-dominated morphologies occurs near Cape Romain, SC, in the north and Jacksonville, FL, in the south. It is proposed that the tide-dominated segment is currently expanding to the north and south as a consequence of transgression.