CHARACTERIZATION OF QUATERNARY ALLUVIAL TERRACES IN THE BUFFALO NATIONAL RIVER VALLEY, ARKANSAS

Despite its topographic prominence in the mid-continent of North America, there is no definitive geomorphic history of the Ozark Plateaus region. The Buffalo National River (BNR) in northern Arkansas has the highest topographic relief in the Ozark Plateaus, is un-dammed over its 214 km (134 mi) length, and has little urban development; thus its catchment best exemplifies the erosion rate of the Ozarks. The BNR valley is underlain by Ordovician, Mississippian, and Pennsylvanian carbonate and clastic sedimentary strata that are subject to both physical and chemical (karst) erosion.

It is generally accepted that incised meanders of the BNR are antecedent to the incision of the valley, but no absolute dating or other quantitative evidence supports this hypothesis. Newly discovered alluvial terraces preserved at ~43, 25, and 8 m above the modern channel along the length of the BNR archive its incision history. A mixed geologic provenance of coarse sediments (5-128 mm) on the two higher (25 and 43 m) terraces confirms that they are not locally derived and have been transported downstream by the river. The grain size distributions of the modern point bars and the two higher terraces are uniform. Slopes of former channel long profiles reconstructed from terrace elevations are parallel with the modern channel slope. Together, these lines of evidence suggest that there has been no significant change in sediment composition or transport capacity over the period of alluvial terrace formation. We plan to extend this work in the future by using a variety of geochronologic techniques to define the timing and rate of incision of the Buffalo National River and test hypotheses about the processes and rates of channel incision in this ingrown meandering, gravel mantled, bedrock channel.

In contrast to tectonically active, mountainous landscapes; low topography, mid-continent, and karst landscapes like the Ozark Plateaus are under-represented in modern landscape evolution studies. The incision history of the BNR typifies the erosion rate of the Ozark Dome and will enable estimation of the minimum age of uplift of the Ozarks. Quantifying the geomorphic history of the Ozark Plateaus will extend understanding of landscape evolution processes in mixed carbonate-clastic and post-tectonic settings.