CHANGES IN INLET LOCATION AT JEKYLL ISLAND, GEORGIA, RELATED TO HOLOCENE TRANSGRESSION AND STILLSTAND

Vibracoring on Jekyll Island has helped prove the extent of Pleistocene substrate and established the size of the Holocene spit that comprises about half the island. Prior to growth of the modern spit, Brunswick River emptied south of Jekyll Island, via St. Andrews Sound; St. Simons Sound at the north end of the island was occupied by drainage solely from Mackay and Frederica Rivers. Rerouting of Brunswick River through St. Simons Sound reduced outflow through St. Andrews Sound and allowed the extension of Jekyll spit. It also increased and redirected outflow at St. Simons Sound, destroyed the spit at the southern end of St. Simons Island and caused abandonment of an old channel beneath Clam Creek at the northern end of Jekyll Island. Radiometric dating of shells of Mulinia from muds within the fill of this channel shows that capture occurred sometime before 1480 B.P. The present location of Brunswick River is favored by Holocene transgression, which has increased the size of the tidal prism and trapped sediment upstream in the estuary. It is argued that a stillstand (or minor regression) between 3100-2400 B.P. would have allowed more sediment to be released at the shore and reduced the size of the tidal prism thus favoring wave over tidal processes. Such conditions would favor the construction of spits and displacement of inlets south of their modern positions. An inspection of other islands in the Georgia Bight indicates a similar pattern, suggesting that the rearrangement at St. Simons Sound is part of a regional, rather than local phenomenon. Transgression favors tidal over wave processes, breaks the longshore transport system into cells and encourages straightening of inlets. Stillstand favors wave over tidal processes, strengthens longshore transport, aids spit building and the diversion of inlets.