GEOMORPHIC EXPRESSION OF SUBSURFACE STRUCTURE AND STRATIGRAPHY ON ST. CATHERINES ISLAND, GEORGIA

St. Catherines Island (20 km by 2 to 4 km) has a Pleistocene core associated with the Silver Bluff paleoshoreline. The island core is covered with a veneer of Holocene deposits, and exhibits some distinctive geomorphic features. The highest elevations (4.3 – 7.9 m) occur along the northern and eastern portions of the core, with a low (2.5 – 5.0 m) region dominating the western portion. Geoprobe cores reveal < 7.6 m of eolian and back beach sands above muddy strata in the western portion, but > 10.7 m in the eastern portion. The 1954 USGS Topographic map and aerial photography (1951) reveal a series of elliptical ponds concentrated in a linear manner in the western region. The linear concentration of ponds is interpreted as originating via dissolution focused along fault zones in Upper Floridan carbonates, followed by cavern collapse and subsidence of surficial sediments. Those faults facilitated artesian spring flow that fed the fresh water wetlands of the western lowlands prior to the industrial development of the Coastal Plain, and consequent lowering of the artesian pressure surface. The extent and freshwater nature of these former wetlands coincides with the distribution of Mandarin and Rutledge soils (Thomas, 2008) on USDA Natural Resources Conservation Service soil maps of St. Catherines Island. A Horizons in black hydric Rutledge soils locally exceed a meter thickness. Vibracoring in ephemeral remnants of these wetlands, coupled with palynological analyses and radiocarbon dating, indicate < 2 m of Holocene freshwater wetland deposits overly Pleistocene marginal marine sands. A thicker cover of eolian sand to the east and north explains much of the elevation contrast across the island. Faults, serve as vertical conduits across confining layers of the Upper Floridan aquifer, explaining former artesian springs and mixing models that indicate the saline water now contaminating the Upper Floridan below St. Catherines Island is derived from saline Lower Floridan water. Faults may also play a role in generating a Central Depression within the western lowlands. Offsets in radar elements on GPR profiles suggest fault splays characteristic of near surface, weakly consolidated sediments. The sharp eastern margin of the western lowlands in the central and southern portion of the island also suggests fault influence.