HR: 1330h
AN: H52A-1141    [PDF]
TI: Knickpoint retreat and landscape disequilibrium on the James River from the Piedmont through the Valley and Ridge, central Virginia, USA
AU: * Hancock, G
EM: gshanc@wm.edu
AF: Dept of Geology, College of William and Mary, Williamsburg, VA 23187 United States
AU: Harbor, D
EM: harbord@wlu.edu
AF: Dept of Geology, Washington and Lee University, Lexington, VA 24450 United States
AU: Felis, J
EM: jjfeli@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 central Virginia. The river longitudinal profile possesses pronounced convexities and apparently migratory knickzones. Along much of its length, it flows in a narrow inner valley incised into a discontinuous, low-relief upland. In the inner Piedmont, the 150 m wide river channel occupies an incised valley that can be as little as 600 m wide and 75 m deep. Broad fluvial terraces are often found capping the high topography directly adjacent to the valleys, and we use these disconnected terraces to reconstruct paleo-river profiles spanning ~200 km in the Piedmont. These high, extensive terraces disappear in the James River Gap through the Blue Ridge, but reappear with less distinction in the carbonate rocks of the Great Valley. The river remains incised within the Great Valley, with prominent knickzones ~60-100 km above the Blue Ridge gap found on both the mainstem and tributaries. We dated terraces in the Piedmont with in-situ 10Be profiles and a depth integration technique. Dates from the highest terrace level at three locations along ~100 km of the river are ~1 m.yr., suggesting rapid river incision rates of ~55 m/m.yr. We also have documented exceptionally high incision rates along a tributary in the Great Valley. This dating suggests disequilibrium erosion was initiated and has persisted here during the late Quaternary. Assuming the incision is accomplished largely by knickpoint retreat, the dates suggest past retreat rates of up to one m/yr in the Piedmont and 250 m/m.yr. between the outer Piedmont and the Great Valley. We have two hypotheses for how this recent incision was triggered: 1) response to flexural-isostatic uplift generated by drainage basin capture and associated denudation and/or 2) the shift to more rapid climate fluctuations in the early Pleistocene. Additionally, we suggest two interpretations for the generation of the extensive high terrace surfaces. They may be valley floor remnants of a broad alluvial upland incised by a migrating knickpoint, or remnants of a broad valley floor developed at the base of knickpoints that have now migrated upstream along the profile.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2003 Fall Meeting