FLUVIAL RESPONSES TO BASE LEVEL, CLIMATE, AND ACTIVE TECTONICS: SOUTH ANNA RIVER, CENTRAL APPALACHIAN PIEDMONT, LOUISA COUNTY, VIRGINIA

Rivers and their valleys are shaped by unsteady and nonuniform incision and aggradation processes recorded in alluvial deposits preserved in and along valley walls. The 2011 Mineral, VA earthquake opened an opportunity to explore these processes for the South Anna River in the central Virginia Piedmont, a region considerably south of the continental glacial margin, but still influenced by glacial-interglacial cycles, regional base level fall, and intraplate seismicity. Detailed sedimentologic, stratigraphic, soil stratigraphic, geomorphic map-based, and optically-stimulated luminescence (OSL) geochronology data collectively suggest that the South Anna experienced a large sediment flux ~60-80 ka that built a well-preserved terrace (Qt4). This terrace is characterized by a sandy, quartz-pebble gravel (channel) that laterally grades into a gray, micaceous sand (channel and point-bar) both ~1-2 m thick, overlain by ~4 + m, of gray clayey, silty, fine sand (overbank). Several OSL dates constrain the age of Qt4 which can be traced for ~40 km along the valley on the basis of soil stratigraphy. The Qt4 deposit is significantly thicker than Holocene deposits, including legacy sediments, particularly in the proximal footwall of the fault that ruptured during the 2011 earthquake. In fact, the terrace is thicker and more widespread than a younger, inset terrace preliminarily assigned to the latest Pleistocene as well as an older, middle (?) Pleistocene alluvium that it alternately buries and insets, depending on proximity to the fault. Although the climate variability associated with oxygen isotope stage (OIS) 4 is usually not thought of as driving changes in sediment flux and discharge that build and incise big alluvial valley fills like the Qt4 deposit, some combination of latitude, mean elevation, access to sediment liberated from hillslopes, and accommodation space have made this the case for the South Anna River. One testable hypothesis is that the regolith and residuum-saprolite generated on hillslopes during full glacial-interglacial stages (OIS 6-5) are effectively stripped under the prevailing climate conditions of OIS 4, underscoring not only the impact of the climate change, but also the importance of antecedent hillslope processes.