SAG STRUCTURES ON ST. CATHERINES ISLAND, GEORGIA: IMPLICATIONS FOR BARRIER ISLAND HYDROLOGY AND COASTAL WATER RESOURCES

St. Catherines Island, Georgia is a barrier island situated at the head of the Georgia Bight, between the mouths of the Savannah and Altamaha Rivers. The Island is ~ 20 km long and 2 to 4 km wide with a Pleistocene core flanked by Holocene ridge and swale terrains on the north, northeast and southeast. LIDAR topographic terrain models show a distinct elevation subdivision of the Pleistocene core into a higher (4.3 – 7.9 meters above sea) eastern portion and lower (2.4- 5.0 meters elevation) western portion. Artesian springs and fresh water marshes that characterized the western core in the past have given way to ephemeral ponds and wetlands. Ground Penetrating Radar profiling using a MALA system with 100 MHz and 250 MHz shielded antennae indicates 6 to 8 meters of sandy surficial strata in the eastern Island core thinning westward to 2 to 5 meters. Sag structures identified in GPR profiles of the western portion of the Island appear to have been produced by 2 to 5 meters of subsidence concomitant with filling of the sag basins. These sag structures are tentatively interpreted as the uppermost manifestation of solution collapse structures that may originate in the Floridan Aquifer. Solution features concentrate and grow along primary hydrologic conduits such as joints and faults; consequently, the sag structures and the linear concentration of former ponds at the surface may mark the conduits that once carried artesian waters to the surface. Joint systems and faults penetrating Coastal Plain strata are well documented (Bartholomew et al., 2007) and the potential communication (via joints and faults) between the shallow unconfined aquifer(s) of the Island and the deep confined Floridan Aquifer becomes a critical question as sea level rises. Will the joints, faults and associated solution conduits accelerate salt water intrusion into the Floridan Aquifer? This is a critical question with respect to the sustainability of a major coastal fresh water resource. Our research group is working to define the internal structure of the Island and model the hydrologic response to eroding shorelines and rising sea level to evaluate this potential threat.