LITHOFACIES CHARACTERISTICS OF LATE QUATERNARY ALLOFORMATIONS OF THE LOWER OCONEE RIVER VALLEY, UVALDA QUADRANGLE, GEORGIA, USA

Textural characteristics and depositional environments were evaluated for lithofacies of four late Wisconsin and younger (~34-0 ka) alloformations of the lower Oconee River valley in the Georgia Atlantic Coastal Plain. Alloformations were mapped based on bounding discontinuities, and lithofacies were characterized by field morphological descriptions and particle size analysis. Alloformation ages were previously established by radiocarbon and optically-stimulated luminescence (OSL) dating.

Alloformations Qh2, Qh1, and Qp6 respectively date to the late and early Holocene and terminal Pleistocene (~17-11 ka); have geomorphic surfaces that contain modern-sized (Qh2, Qh1) or large (Qp6) paleomeanders; and consist of normally graded alluvium composed of sand overlain by fine-grained sediments. The sandy lithofacies contains coarse to medium sand with infrequent gravels and represents lateral accretion and bedload sediments deposited by meandering channels. Overlying fine-grained sediments are mainly composed of loamy to clayey vertical accretion and clayey abandoned channel fill lithofacies that lack gravel and respectively reflect sedimentation in overbank and meander scar/oxbow environments. Alloformation Qp5 has a late Wisconsin age (~34-17 ka), braided topography, and basal sediments composed of a gravelly sand lithofacies representative of braided channel bedload that averages more gravel than Qh2, Qh1, and Qp6 bed sediment (11.8 vs. 8.2% by weight). In many areas, this gravelly sand bedload is overlain by a coarse loamy to clayey vertical accretion lithofacies interpreted as an overbank flood drape that accumulated during and after Qp5 time. In braided paleochannels and on braid bar ridges, however, braided channel fill and sandy upper braid bar lithofacies respectively overlie bedload. On Qp5, Qp6, and Qh1, a similar, texturally gradational lithofacies sometimes occurs between bed sediments and finer overlying vertical accretion deposits representative of sandy overbank floods.

Results illustrate the variety of sediments in the Oconee valley and should help guide lithofacies interpretations along other Piedmont-draining Coastal Plain rivers. Efforts are underway to identify particle size variables best suited for discriminating among lithofacies types in these settings.