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Fluvial Terrace Studies and Knickzone Characterization on the James River, Virginia Piedmont
Felis, Jonathan Joseph
Felis, Jonathan Joseph
Abstract
We present correlated longitudinal profiles and 'Be dates for fluvial terraces of the James River in the central and western Virginia Piedmont (~90 to 195 km upstream of Richmond). Terraces are mapped into three levels based on elevation above the modern river. The Tl, T2, and T3 terraces are ~60, 35, and 20 m above the modern river, respectively. Terrace profiles and a significant correlation with the Shores Complex suggests that a modern James River knickzone (~15 m of river elevation change) located 110 km upstream of the Fall Zone is a long-lived, stationary feature. 'Re dates for T1 terraces are 1.10+0.10, 1.16+0.11, and 1.19+0.12 Ma, decreasing in the upstream direction. '°Be dates for T2 terraces are 1.01+0.10 and 1.02+0.10 Ma, decreasing in the upstream direction. Terrace ages are used to calculate incision rates of 35+3 — 59+6 m/Ma averaged over the last ~Ma. These rates are greater than estimates of Piedmont interfluve lowering rates (Pavich, 1989), suggesting a disequilibrium landscape that is growing relief. Terrace ages decrease in the upstream direction in accordance with past knickzone migration as the dominant mechanism for effecting this vertical incision; however age errors are too large to be certain of this. Estimates of past knickzone migration rates are used to suggest the beginning of the Pleistocene and onset of worldwide glaciation and sea level fall as the event initiating past knickzone migration and landscape incision.
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2003-01-01
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Geology