POSSIBLE IMPACT-RELATED CONGLOMERATIC SEDIMENTS IN THE UPPER COASTAL PLAIN OF SOUTHWESTERN GEORGIA

Geologic mapping in the Upper Coastal Plain of Southwestern Georgia from the Flint River on the east to the Chattahoochee River on the west has revealed a number of exposures of exotic multilithic conglomerates or conglomeratic sandstones of uncertain origin. Field mapping during the past seven years was supported by the U.S. Geological Survey's STATEMAP program. These multilithic conglomerates are found at two stratigraphic horizons. One horizon lies between sandstone of the youngest Upper Cretaceous-age Providence Formation and clay of the Lower Paleocene-age Clayton Formation. The second horizon lies at a major unconformity between middle Eocene Claiborne Group sandstones and Miocene-age Altamaha Formation sandstones. Common factors include clast heterogeneity, variable clast angularity and size, and variable clast to matrix ratio. Clasts may include shale or clay, kaolin, kaolinitic sand-stone, sandstones, chert, quartz, Fe-oxide cemented sandstone, and iron oxides. Some clasts can be recognized as being derived from older sediments in relative stratigraphic proximity, while the source stratigraphic units of other of clasts are presently unknown. Clast sizes range from pebbles to several feet across. Clasts may be well rounded to highly angular. Clast to matrix ratio may range from about 90:1 to 1:10. Matrix material may range from clay to sand. Clast size, widely variable degrees of angularity, and clast populations suggest rather high energy environments of transportation and deposition that differ significantly from the adjacent stratigraphic units. Stratigraphic positions of these conglomerates are permissive for being derived from high-energy water surges related to major impact events. As Georgia's Coastal Plain is uniquely situated to reflect events in both the Gulf of Mexico and Atlantic basins, sediments related to major impacts at Chicxulub, Mexico and Chesapeake Bay would be expected to occur in Georgia. Recent studies indicate numerous tektites (Albin, 1995) and shocked quartz (Harris et al., 2004) within sediments on the Upper Coastal Plain in east-central Georgia are the same age as the Chesapeake Bay impact and are attributed to that impact. Mineralogic and geochemical studies of these exotic conglomerates are needed to determine their likely origin.