New constraints on the late Cenozoic incision history of the New River, Virginia

Abstract

The New River crosses the core of the ancient, tectonically quiescent Appalachian orogen as it follows its course through North Carolina, Virginia, and West Virginia. It is ideally situated to record the changes in geomorphic process rates that occur in the Appalachians as a response to late Cenozoic climate variations. Active erosion features on resistant bedrock that floors the river at prominent knickpoints demonstrate that the river is currently incising toward base level. However, large packages of alluvial fill and fluvial terraces cut into this fill record an incision history for the river that includes several periods of stalled downcutting and aggradation. Cosmogenic 10-Be exposure dating, aided by mapping and sedimentological examination of terrace deposits, is used to constrain the timing of events in this history. Fill-cut and strath terraces at elevations 10, 20, and 50 m above the modern river yield cosmogenic exposure ages of approximately 130, 610, and 955 ka, respectively, but uncertainties on these ages are not well-constrained. This translates to a long-term average incision rate of 43 m/my, which is comparable to rates measured elsewhere in the Appalachians. During specific intervals over the last 1 Ma, however, the New River's incision rate reached 97 m/my. Fluctuations between aggradation and rapid incision appear to be related to late Cenozoic climate variations, though uncertainties in modeled ages preclude direct correlation of these fluctuations to specific climate change events. Erosion rates on higher alluvial deposits adjacent to the river are estimated from 10-Be concentrations; these rates are very low, about 2 m/my or less. This demonstrates a disequilibrium in the modern landscape, with river incision greatly outpacing erosion from nearby landforms.