Extrinsic terrace formation processes and tectonically driven river incision along the South Anna River, Virginia

Abstract Deep residual soils, saprolite, erosion rates inferred from alluvial terrestrial cosmogenic nuclide (TCN), 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 rates of 100 – 101 m/My. New data collected as part of a mapping and geochronologic study following the 2011 M 5.8 Mineral earthquake in Louisa County, central Virginia challenges this notion of a uniformly slowly evolving landscape, and also helps to demonstrate the exogenic impacts of climate change and active tectonics on the South Anna basin. This paper documents the ages and characteristics of the terrace stratigraphy in the hanging wall of the Quail Fault that ruptured in 2011, measured with respect to channel elevation, for the South Anna River. Six mostly paired strath terraces (Qt1 – Qt6) are incised into the hanging wall. Qt1 and Qt2 lie > 27 m in elevation, are highly dissected, and characterized by alluvium mixed with residual soil thought to be middle-early Pleistocene age. From Qt3 through Qt5, alluvial deposits underlie mappable terrace landforms and preserve distinct, locally cumulic soils. The Qt3 terrace strath lies ~25 m and has a deeply weathered, red (2.5YR) gravelly silty clay soil with mid-Pleistocene Infrared Stimulated Luminescence (IRSL) and TCN ages. Qt4a lies ~20 m and has an orange (7.5 YR) gravelly clay loam soil with Optically Stimulated Luminescence (OSL) ages between 120-150 ka. Qt4b lies ~15 m and has a reddish brown (10YR) clayey silt soil with OSL and IRSL constraining ages of 75-100 ka, respectively. Qt5 lies ~3 m and has a brownish-yellow (10 YR – 2.5 Y) loamy clay soil with an OSL age of 45-65 ka. The relatively thick terrace fills and their age distribution are collectively consistent with exogenic climatic changes driving terrace formation. Furthermore, the elevations of the terrace straths in the hanging wall are in sharp contrast to the footwall terrace elevations, where Qt4a-Qt6 all lie close to the modern river level. Long-term river incision varies from ~17 - 88 m/my in both the hanging wall and footwall when averaged over middle Pleistocene time scales (~400 ka). However, distinct exogenic tectonic forcing on incision unsteadiness is apparent in an acceleration of rapid incision at rates of ~340 m/My recorded only in the hanging wall from ~100 - 60 ka.