LANDSCAPE EVOLUTION ON THE VIRGINIA PIEDMONT: A NEW SOIL CHRONOSEQUENCE, LUMINESCENCE (OSL AND IRSL) AND COSMOGENIC (TCN) DATING, AND INTRAPLATE SEISMICITY-DRIVEN RIVER INCISION.

Published data from deep residual soils, alluvial terrestrial cosmogenic nuclide (TCN) erosion rates, and long-term river incision rate data all point to a slow pace of landscape evolution for the Appalachian Piedmont west of the Fall Zone measured in the 10⁰ – 10¹ m/My range. New data collected following the 2011 M_w5.8 Mineral earthquake in central Virginia challenge the notion of a uniformly slowly evolving landscape, but also raise new questions about the efficacy of hillslope erosion and fluvial incision processes where the gradients are gentle and the transport of surficial materials has apparently been unsteady. Eight, mostly paired strath terraces (Qt1 – Qt7) are incised into the hanging wall of the Quail Fault along the South Anna River, Louisa County, Virginia. Qt1 and Qt2 lie > 27 m above the South Anna channel (ASAC), are dissected, and characterized by alluvium mixed with residual soil of probable middle-early Pleistocene age. From Qt3 through Qt5, alluvial deposits underlie terrace landforms and preserve distinct, locally cumulic soils. Qt3 (~24 m ASAC) has a deeply weathered, red (2.5YR) gravelly silty clay loam soil with IRSL (infra-red stimulated luminescence) and TCN ages of ~400 Ky. Qt4a (~20 m ASAC) has an orange (7.5 YR) gravelly clay loam soil with a minimum OSL (optically stimulated luminescence) age of 81.5+/-14.4 Ky. Qt4b (~15 m ASAC) has a reddish brown (10YR) silty clay and clay loam soil with IRSL constraining ages of 57.4+/-2.6 – 27.3+/-1.8 Ky. Qt5 (~4 m ASAC) has a brownish yellow (10 YR – 2.5 Y) clay loam soil with an IRSL age of 16.9+/-3 – 18+/-4.9 Ky. Soil morphologies are consistent with the numeric ages, but the resulting river incision rates (~60 – 250 m/My) seem to be too rapid, even allowing for the short late Pleistocene time intervals, given the low stream gradients (~0.0005 m/m) and lack of anomalously rugged topography. Three incomplete explanations emerge: (1) the Pleistocene South Anna River was less sinuous, steeper, and contained more bedload abrasion tools than at present; (2) the hanging wall was mostly underlain by deep saprolite that offered little resistance to incision during a period of rock uplift; or (3) the terrace treads have been resurfaced in response to variations in the late Pleistocene climate, including deposition of loess, resulting in soil and geochronologic age minima.