Abstract

St. Catherines Island is a barrier island on the Georgia coast, lying within Liberty County about 65 kilometers south of Savannah (31 degrees 37'N latitude, 81 degrees 09'W longitude). The sediments of St. Catherines Island are of Late Pleistocene and Holocene age; understanding their relationship to other inland and marine sediments is critical to assessing geological and biological changes that have occurred in the region. Extensive palynological studies along Georgia's current coastline are completely lacking, though sediments from Georgia's barrier islands provide a potentially unique opportunity to investigate the Quaternary history of the region. The accumulation (and erosion) of these sediments was directly related to sea level changes brought about by global climate shifts, and these changes in topography and sedimentation patterns are probably reflected in pollen assemblages. A transect of three sediment cores from the mid-southern portion of the island was analyzed to determine ages, environments of deposition, and relationships to other coastal and inland localities. Core localities. Core localities are known as Cracker Tom Bridge, Cracker Tom Hammock, and Cracker Tom Rosetta. Sediment consists of peat, shells, sand, and clay. Peat from 5.02-5.12m at Cracker Tom Bridge was radiocarbon dated at 47,620 B.P. Above this peat lies an irregular erosional surface, which is in turn overlain by marine mollusk shells and charcoal. The charcoal was radiocarbon dated at 6020 B.P. and an Americardia shell was radiocarbon dated at 4060 B.P., indicating a depositional hiatus of at least 40,000 years following deposition of the peat. The subsequent marine transgression resulted in the accumulation of the sediments which make up the remainder of the strata in the three cores. Palynologically these are characterized by abundant Pinus pollen, high percentages of broken Pinus pollen grains, and the presence of dinoflagellates and microforams. Pleistocene sediments deposited during low sea level were derived from inland plant communities. The 47,620 year old Cracker Tom Bridge peat consists dominantly of Woodwardia spores and other hydrophytic taxa. Regional vegetation represented in the Pleistocene portion of the cores is dominated by Pinus, Quercus, Carya, and Poaceae. Pollen of boreal taxa is infrequent, though an indication of the presence of northern temperate forest elements in the region is suggested by very low percentages of Picea, Tilia, Fagus, and Tsuga. Holocene portions of three cores are derived from near shore marine and salt marsh/tidal flat environments. Composites, chenopods, and grasses are present as well as Pinus and Quercus. Consistently present in small percentages are Carya, Myrica, Taxodium, Liquidambar, and Nyssa. Results of this study are consistent with other studies indicating that the Silver Bluff terrace, which comprises the northern portion of St. Catherines Island, formed over 40,000 years ago following a period of exposure during the latest Pleistocene lowstand. St. Catherines Island subsequently became isolated from the mainland due to eustatic sea level rise approximately 4060 B.P. Subsequent accretion of the Holocene portion of the island southward resulted in the establishment of the modern hammock and marsh communities at the Cracker Tom locality after 3200 B.P. Pleistocene and Holocene sediments are separated by a disconformity in St. Catherines Island sediment cores and in other studied inland localities on the Georgia coastal plain, indicating a regional distribution of this unconformity. Palynological analysis of St. Catherines Island sediment cores attests to the stability of south-eastern floral elements throughout portions of the Late Pleistocene and Holocene in coastal Georgia.

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