HR: 0800h
AN: H51E-0804    [Abstracts]
TI: Evidence for knickzone propagation and landscape disequilibrium along the James River, central Virginia Piedmont
AU: * Parker, L B
EM: lbpark@wm.edu
AF: Dept of Geology, College of William and Mary, Williamsburg, VA 23187, United States
AU: Lang, K A
EM: kalang@wm.edu
AF: Dept of Geology, College of William and Mary, Williamsburg, VA 23187, United States
AU: Hancock, G S
EM: gshanc@wm.edu
AF: Dept of Geology, College of William and Mary, Williamsburg, VA 23187, United States
AB: Several lines of evidence indicate landscape disequilibrium in the James River basin in the central Virginia Piedmont. The river longitudinal profile possesses pronounced convexities, and the James flows in a narrow inner valley incised into a discontinuous, low-relief upland. We hypothesize that this disequilibrium is generated by upstream propagation of several pronounced knickzones along the James profile, and we focus here on a major ~five km long knickzone within the western Virginia Piedmont to determine if it is a translational feature. We combine bedrock and surficial mapping, topographic analysis, and cosmogenic radionuclide dating of terraces along the James River to constrain the role of lithology, reconstruct past river profiles, and estimate rates of river erosion and knickzone propagation. Bedrock mapping at 1:12000 scale reveals the knickzone reach to be underlain primarily by moderately resistant schists and phyllites, with narrow bands of more resistant greenstone and diabase, within a complexly deformed melange. Mapping has revealed no clear relationship between rock resistance and river gradient, suggesting minimal lithologic control on knickzone location. We have identified numerous, disconnected terrace deposits, and reconstruction of past longitudinal profiles from these deposits suggests at least three separate terrace levels and hints at terrace generation by knickzone movement. Longitudinal profiles of minor tributaries entering the mainstem within and downstream of the knickzone possess pronounced convexities whose upper elevations generally match that of the mainstem knickzone. We have dated high terraces near but outside the knickzone reach with in-situ ¹⁰Be profiles and a depth integration technique. Dates from three locations along ~100 km of the river are ~1 m.yr., suggesting rapid river incision rates of ~55 m/my. This dating suggests disequilibrium erosion was initiated and has persisted here during the late Quaternary. We hypothesize this recent incision was induced by the shift to more rapid climate fluctuations in the early Pleistocene, leading to base level lowering and knickzone generation
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
SC: Hydrology [H]
MN: 2007 Fall Meeting