SEDIMENT SIZE DISTRIBUTIONS AND THE LIMITS OF RECENT SEDIMENT IN THE GEORGIA BIGHT

Reconnaissance studies of Georgia continental shelf sediments in the 1960s and 1970s have shown a zone of influence approximately 5-16 km in width that defines the maximum seaward extent of Recent, river-derived sediments. Seaward of this narrow zone of influence lie relict sediments from the last lower stand of sea level, which are characterized by coarser grain sizes and iron-stained quartz. The purpose of this study was to identify sediment distribution patterns within the Georgia Bight to more precisely determine the present location of the recent-relic sediment boundary. Sediment samples were collected at 103 sites, wet sieved at 4 phi and dry sieved at 0.25-phi intervals from -2 to 4 phi to determine mass per phi interval, and observed under a microscope to collect point count data to determine percentages of quartz, opaques, carbonate, and rock fragments. Grain sizes were calculated using the method of moments, classified using the Udden-Wentworth size scale, and plotted in ArcGIS for spatial data analysis. Grain size analysis showed that the sediments were dominated by 4 grain sizes – coarse, medium, fine, and very fine sands. Coarse sands (mean size = 0.8±0.2 phi) and medium sands (mean size = 1.5±0.3 phi) were located seaward of 13 km offshore and in depths greater than 14 m. Fine sands (mean size = 2.5±0.3 phi) and very fine sands (mean size = 3.1±0.1 phi) were located closer to shore (within 13 km) and in depths less than 13 m. These findings are consistent with previous research (Pilkey and Frankenberg, 1964; Bigham, 1973) and validate the concept of a zone of modern sediment influence. The location of the boundary between the Recent and relict sediments does not appear to have changed, within the limits of our analyses, since these previous studies were conducted. However, the boundary’s location is now better constrained between the widely spaced, 1960s-1970s survey transects.