LATE HOLOCENE GEOLOGIC DEVELOPMENT OF THE GEORGIA COAST

Deposits associated with the geologic development of the Southeastern United States during the Late Holocene preserve the record of coastal response under the influence of rising sea levels and changing climates. Documenting the age, geomorphic character and stratigraphic signature of these deposits provides insight into both the sedimentary processes that formed the landforms and the timing associated with these processes. Although these issues have been investigated in the SE US over the past several decades, the Georgia coast has received relatively little study when compared to other states. Back-barrier islands (BBIs) along the Georgia coast provide a depositional record of Late Holocene coastal development, and are the focus of this study.

BBIs found along the southeastern coast of the United States are poorly studied, yet common topographic highs in the back-barrier environment. These islands are presently surrounded by salt marsh, are generally of limited area and elevation (less than 10 hectares and less than 2 m above mean sea level), can be either Pleistocene, Holocene or Recent in age, and can have natural or anthropogenic origins. The characteristically long, linear BBIs are Holocene in age and preserve the record of coastal change. Stratigraphically, Holocene BBIs consist of unconsolidated sediments: fine sand beds that coarsen downcore, typically resting unconformably on older marsh muds, which grade into interbedded tidal channel deposits. Grain size and sedimentary structures in cores, document that these BBIs formed as discrete beach ridges as the coastline prograded seaward, either under the influence of a short-term sea-level fall, or an increase in sediment supply to the coast. Although far from a comprehensive dataset, existing OSL dates for Holocene BBIs associated with major barrier islands in Georgia document that the major period of coastal progradation occurred between ~1700 and ~500 years ago, during which time many sections of the Georgia coast prograded 2-5 kilometers seaward. Since ~500 year ago, rapid progradation has ceased, and the shoreline has been comparatively stable, with a coast-wide, mean shoreline change rate of +0.14 m/y. Archaeological estimates of BBI age from the 1970s, based on shell-midden ages, support these OSL results.