CHARACTERIZATION AND AGES OF QUATERNARY RIVER TERRACES IN THE BUFFALO NATIONAL RIVER, ARKANSAS

To characterize and identify the spatial and temporal distribution of terraces in the Buffalo National River watershed, we conducted extensive field mapping, measured sediment grain size and provenance, and dated terrace sediments using optically stimulated luminescence (OSL). The unpaired terraces tend to be beveled bedrock straths overlain by alluvial sediment composed of sandy sediments interbedded with a gravel, cobble, and boulders, sometimes expressed at the surface as a lag deposit. Terraces are preserved throughout the entire 214 km of the river and occur in both major lithologies: the Boone Formation (chert-bearing limestone) and Everton Formation (dolomitic sandstone). The top surface of the terrace units vary in elevation above the modern channel from 4 to 90 m, with distinct groups occurring at ~ 6 m, 25 m, 50 m and 70 m above the modern channel. The roundedness and mixed geologic provenance of coarse sediments (5-128 mm) on the two higher (50 and 70 m) terraces confirms that they are not locally derived and have been transported downstream by the river rather than by hillslope processes. OSL results show there is sufficient quartz that is datable using fast component quartz OSL. Samples show a strong likelihood of partial bleaching, which is expected for fluvially transported sediments. The OSL results successfully differentiate between terrace units in an expected sequence; stratigraphically lower samples are older than upper samples in the same terrace and in cross section; higher terraces are older than lower ones. Preliminary OSL results show that terraces of the same unit in different parts of the long profile are older where meander amplitude is high. Although it is generally accepted that incised valley meanders of the Buffalo National River are antecedent to the modern gravel mantled, bedrock channel of the BNR, no absolute dating or other quantitative evidence supports this hypothesis.