AGES AND STRATIGRAPHY OF “MEGA-MEANDER” PALEOCHANNELS OF THE ATLANTIC COASTAL PLAIN, SOUTHEASTERN USA

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 wetter paleoclimate (at least seasonally). Here we report new radiocarbon and optically-stimulated luminescence (OSL) age estimates for mega-meander paleochannels from the Oconee and Ogeechee Rivers (GA), the Black and Congaree Rivers (SC), and the Neuse River (NC) along with results from stratigraphic investigations of these paleomeanders and one previously dated 13.2-13.8 ka mega-meander of the Pee Dee River (SC). 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 of mega-meanders using hydraulic modeling techniques and field-surveyed paleochannel dimensions. Coring transects near the apex of meander bends indicate that mega-meander paleochannels commonly contain fills of clay, peat and muck that vary in thickness from 1.0-5.0 m. In most cases, paleochannel fill abruptly overlies channel-bed sand, indicating paleochannel boundaries can be accurately delineated by high-resolution coring and stratigraphy. Radiocarbon dating of plant macrofossils collected from basal channel fill returned ages of 11.5-14.1 ka, while OSL dating of upper point bar sediment yielded generally correlative but slightly older age estimates in the 17.3-18.9 ka range. Older OSL ages are attributable to the fact that point bar sediments slightly pre-date post-abandonment channel fill, but may also reflect a degree of “inherited” age resulting from partial bleaching during transport. Nonetheless, results reinforce previous studies that indicate that many Coastal Plain rivers had mega-meander morphologies during the terminal Pleistocene. This research suggests that the tandem application of AMS radiocarbon and OSL dating is effective for paleochannels in the region. However, care must be taken to radiocarbon date uncarbonized organic materials that appear fresh (seeds, nuts, leaves) and not reworked by transport, and the possibility of incomplete bleaching must be recognized when interpreting OSL ages.