NEW CONSTRAINTS ON THE LATE CENOZOIC INCISION HISTORY OF THE NEW RIVER, VIRGINIA

The New River crosses three physiogeologic provinces of the ancient, tectonically quiescent Appalachian orogen and is ideally situated to record the fluvial 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, thick sequences 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. We used cosmogenic ¹⁰Be exposure dating, aided by mapping and sedimentological examination of terrace deposits, to constrain the timing of events in this history. Terrace surfaces were excavated and sampled for quartz sand at multiple depths, allowing construction of ¹⁰Be concentration depth profiles to help account for variables such as cosmogenic inheritance and terrace bioturbation. ¹⁰Be concentrations increase with terrace elevation above the river and decrease with depth in each deposit. Fill-cut and strath terraces at elevations 10, 20, and 50 m above the modern river yield cosmogenic exposure ages of 130, ~600, and ~950 ka, respectively, but uncertainties on these ages are not well-constrained. The new results provide the first direct constraint on the history of alluvial aggradation and incision events recorded by New River terrace deposits, and suggest that the river responds to external stimuli operating at timescales of 10⁴-10⁵ yr. The exposure ages yield 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 ~100 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 ¹⁰Be 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.