QUATERNARY ALLOSTRATIGRAPHY OF THE LOWER OCONEE RIVER VALLEY, UVALDA QUADRANGLE, GEORGIA, USA

Allostratigraphic mapping indicates that the Quaternary valley of the lower Oconee River contains eight informal alloformations within the Uvalda 7.5-minute quadrangle, in the Atlantic Coastal Plain of Georgia. Oconee River-derived alloformations were delineated based on geomorphic surfaces, topography, lithofacies, soils, and bounding discontinuities. Radiocarbon and optically stimulated luminescence (OSL) dating provide numerical ages for the youngest five map units.

Alloformations Qh2 and Qh1 respectively date to the late and early Holocene and have geomorphic surfaces characterized by modern-size meandering paleochannels. These units consist of fine-grained overbank and abandoned channel fill vertical accretion facies that abruptly overlie sandy lateral accretion and bedload deposits. Large, meandering paleochannels characterize alloformation Qp6, which dates to the terminal Pleistocene (~11-17 ka) and contains meandering channel deposits with lithofacies like those of Qh1 and Qh2. Alloformation Qp5 is of late Wisconsin age (17-34 ka), has braided topography, and consists of fine-grained vertical accretion and sandy upper braid bar sediments that overlie a gravelly sand lithofacies interpreted as braided channel bedload. Poorly resolved braided and meandering channel deposits comprise alloformation Qp4, which returned an OSL age of 52 ka. Alloformations Qp3, Qp2, and Qp1 pre-date 52 ka, lack distinct fluvial topography, and exhibit extensive alteration by erosion, weathering, and pedogenesis. Cross-cutting geometries and surface elevations indicate relative ages of Qp3<Qp2<Qp1 for these units, which are assigned to the Pleistocene because they postdate the nearby late Pliocene-Pleistocene Hazlehurst marine terrace.

Allostratigraphy effectively delineates Quaternary sediments in this setting, but degradation of fluvial topography and lithofacies by weathering reduces the resolution at which Qp4 and older alloformations can be mapped. The Oconee River had a braided pattern during late Wisconsin time, a large meandering morphology in the terminal Pleistocene, and modern-like meander dimensions in the early to late Holocene. These findings are generally consistent with the major morphological phases exhibited by other rivers in the region during the late Quaternary.