INVESTIGATION OF HOLOCENE ACCRETIONARY TERRAIN ON ST. CATHERINES ISLAND, GEORGIA

St. Catherines Island Georgia is a barrier island located between the mouths of the Savannah and Altamaha Rivers at the head of the Georgia Bight. The 20 km by 2 to 4 km island consists of a Pleistocene core flanked by Holocene terrains on the north, northeast and southeast. The Holocene terrains are characterized by ridge and swale topography with older ridges heavily vegetated with by saw palmetto, cabbage palm, pine, hickory and live oak and young ones with dune grasses. The northern Holocene accretional terrain bounded by Engineers Scarp to the south and St. Catherines Scarp on the east was investigated by vibracoring and Ground Penetrating Radar (GPR) profiling. Four shallow (<2.8 m) cores were obtained including one at the base of Engineers Scarp (Pleistocene-Holocene surface boundary), two in a broad swale ~ 370 m north of Engineers Scarp, and one in a broad swale ~ 150 m south of the present northern shore. Cores from the interior swale exhibit fine, well-sorted sands to a depth of .6 to 1 m. At 1 to 1.5 m the appearance of poorly sorted coarse sand to granules (including feldspars) alternating with finer sands mark high energy events, with some beds showing crude normal grading. Sands from Pleistocene and Holocene terrain cores have been examined with petrographic microscopes. The suite of heavy minerals and associates identified in the Pleistocene core and the Holocene sediments include the “immature” components microcline, plagioclase and amphibole. The presence of these relatively unstable minerals requires contributions from the Piedmont-Blue Ridge source from the Pleistocene through the Holocene and/or very rapid recycling of Pleistocene sediment. GPR profiles were conducted with a MALA system by towing 100 MHz and 250 MHz shielded antennae behind an ATV. GPR profiles allow delineation of the Pleistocene-Holocene contact beyond the scarp and below the Holocene strata. Profiles reveal the internal structure of the ridge and swale systems and allow the determination of the extent of swale basins and the ridge sediments associated with island progradation.