AGES AND STRATIGRAPHY OF “MEGA-MEANDER” PALEOCHANNELS OF THE OCONEE AND PEE DEE RIVERS, ATLANTIC COASTAL PLAIN, GEORGIA AND SOUTH CAROLINA

Exceptionally large, terminal Pleistocene to early Holocene meandering paleochannels occur along many rivers in the southeastern Coastal Plain. Previous research suggests that these features may represent bankfull flood magnitudes two to four times larger than those of modern rivers and (at least seasonally) wetter paleoclimate. Here we report radiocarbon age estimates for one mega-meander and one small meandering paleochannel of the lower Oconee River (Georgia), along with results from stratigraphic investigations of these paleomeanders and a previously dated 13.2-13.8 ka mega-meander of the Pee Dee River (South Carolina). These efforts are part of an ongoing project intended to improve understanding of paleohydrologic conditions in the Southeast by modeling the channel-forming (bankfull) discharge represented by mega-meanders along six rivers in Georgia and the Carolinas using hydraulic modeling techniques and field-surveyed paleochannel dimensions. Cross-sectional coring transects located near the apex of meander bends indicate that both Oconee paleochannels are filled with 1.0-2.0 m of clay, while the Pee Dee paleochannel contains a 4.0-5.0 m thick fill comprised of peat, muck, and stratified sand and clay. In all cases, paleochannel fill abruptly overlies coarse channel-bed sand, indicating paleochannel boundaries can be accurately delineated by high-resolution coring and stratigraphy. Radiocarbon dating of organics (seeds, leaves) situated immediately above the contact with channel-bed sands in the thalweg of each Oconee paleochannel indicates that the mega-meander was active at or shortly before 12.4 ka, while the small meander was active later, at or immediately prior to 4.5 ka. These results agree with dates reported from other rivers in the region, which place the most recent phase of mega-meanders in the 16-5 ka range. Efforts are underway to model mega-meander paleodischarge using cross-sectional dimensions, field-surveyed and LiDAR-based estimates of channel slope, and flow resistance formulae. Results will quantitatively characterize the natural range of variability in bankfull flood regime experienced by these rivers over terminal Pleistocene to Holocene timescales and improve our understanding of late Quaternary fluvial behavior in the Southeast.