DELAYED AND EPISODIC EROSIONAL DISSECTION OF AN UPLIFTED APPALACHIAN MARGIN

Streams of the James River basin are like many in the Appalachians in that they are far from dynamic equilibrium where processes and slopes adjust to rock resistance during slow, post-orogenic topographic decay. Instead they rise in low-relief uplands and drop into valleys through knickzones and gorges that signal ongoing transient incision. The uplands have been shown to be slowly eroding elsewhere and locally contain deep weathering profiles consistent with millions of years of chemical weathering that outpaces surface lowering. Broader uplands lie mostly in the Blue Ridge and Plateau, but deep weathering and solution occur in slowly eroding, lower elevation carbonates of the Valley and Ridge at drainage divides. All are remnants of an Appalachian process domain with lower relief and slower erosion rates. They aren’t paleosurfaces and continue to erode, but erosion farther downstream is up to an order of magnitude faster in response to uplift of the Appalachians before the end of the Miocene. In the James River basin, dynamic knickzones of two types characterize the river profiles responding to uplift. The knickzones below the uplands occurs only in the headwaters reaches of the system and contains hundreds of meters of relief. Other systems of knickzones less than 100 m high migrate upstream through the drainage basin, change the slope more than any difference in rock type and create erosion that eclipses the background erosion rate. The high knickzone may be the result of capture of the James River Basin, but the lower, younger knickzones travel upstream through a steeper basin that likely has an unstable base level. The persistence of this erosional signal long after the uplift impulse is possible only because of the longitudinally varied resistance to erosion and long, strike-oriented drainage basins of the Appalachian physiographic provinces. Transition from strike-oriented basins to those oriented down the uplift flanks occurs by captures that take tens of millions of years to leap-frog across provinces and ridges. The signal of uplift is just now reaching the remote ends of the James and other drainage basins with early Cenozoic low-relief terrain.