10BE DATING OF RIVER TERRACES REVEALS PIEDMONT LANDSCAPE DISEQUILIBRIUM IN THE CENTRAL JAMES RIVER BASIN, VIRGINIA

Cosmogenic radionuclide dating of terraces along the James River in the Virginia Piedmont suggests a recent transition to disequilibrium landscape conditions. We have collected samples from five well-preserved terrace surfaces along a ~100 km reach in the western Piedmont beginning ~100 km upstream from the Fall Zone. All appear to be strath terraces, with alluvium thicknesses comparable to those on the modern James River floodplain. Three of the sample locations can be correlated, as the terrace in each location caps the highest topography adjacent to the river. These high terraces have treads ~80-90 m above present river level (PRL), and may to record the position of the river prior to the onset of disequilibrium conditions, as they are at or above the highest elevations of the surrounding Piedmont interfluves. Two other dated terraces have treads ~60 m above PRL. Depth profiles of ¹⁰Be suggest the alluvium is well mixed through bioturbation, as ¹⁰Be concentrations are nearly uniform to depths of ~1.5 – 2 meters. Given this mixing, we obtain ages by depth integration of the ¹⁰Be inventory. Ages for the three high terraces range from ~1.10 – 1.19 Myr, while ages on the two lower terraces are ~1 Myr. These ages yield average river incision rates into rock of ~35-60 m/Myr. As the terrace ages include undetermined periods of river stability and minor aggradation, these average rates must be less than the actual rates of rock incision. These river incision rates are greater than interfluve and basin-averaged lowering rates over similar time scales presented elsewhere, implying landscape disequilibrium and the growth of Piedmont relief. The high terrace ages indicate a transition to disequilibrium conditions in the mid-Pleistocene at this location. Possible explanations for the onset of disequilibrium include, among other possibilities: 1) passage of knickpoints related to Pleistocene base-level changes; 2) landscape destabilization associated with rapid Pleistocene climate fluctuations; and 3) enhanced Piedmont uplift associated with flexural-isostatic response to drainage basin capture. In any case, rapid river incision rates and the recent transition to disequilibrium are not compatible with either a dynamic steady state or a dissected ancient peneplain model of landscape evolution in this portion of the Piedmont.