COSMOGENIC ISOTOPE DATING OF RAPID, DISEQUILIBRIUM INCISION OF THE JAMES RIVER IN CENTRAL VIRGINIA

Using cosmogenic isotopes to estimate ages of fluvial terraces, we demonstrate rapid incision of the middle James River in central Virginia. Near Howardsville, Virginia, the highest two of three extensive and well-defined terraces are preserved at ~60 m and ~75 m above the modern bedrock-floored James River. Alluvium atop the underlying strath is ~10-15 m on both terraces, suggesting the James River has undergone long-term incision into bedrock interrupted by at least two extensive periods of equilibrium and/or minor aggradation. Numeric ages on these previously undatable terrace surfaces are obtained by measuring concentration of the in situ--produced cosmogenic radionuclide 10Be in quartz as a function of depth within the deposits. The profile technique is utilized to allow correction for inherited 10Be accumulated through exposure during exhumation and transport prior to deposition on the terrace. Analysis of 4-5 samples from ~0.5 m to ~2.5 m deep obtained from a single pit on each terrace reveals that 10Be concentrations are nearly uniform to depths of ~2 m. Deviation from the expected exponential decay in concentration implies efficient homogenization of the soil profile to that depth, presumably through bioturbation. Assuming a 10Be production rate of 5.55 atoms/g qtz-yr, integration of the homogenized portion of the profiles yields ages of ~1.1 Ma and ~1.3 Ma for the lower and higher terraces, respectively. Average rock incision rates between the upper and lower strath surfaces and between the lower strath and the modern river are ~50 m/Ma and ~45 m/Ma, respectively. These incision rates are high, up to an order of magnitude higher than Piedmont interfluve lowering rates determined elsewhere in Virginia, and twice estimated incision rates into carbonate rocks determined in the adjacent the New and Shenandoah River basins. These high rates suggest that the Piedmont here is not in dynamic steady state, but has been out of equilibrium as the James River incises rapidly below a low-relief Piedmont upland.