EVIDENCE FOR ENHANCED SINKHOLE DEVELOPMENT AND CHEMICAL WEATHERING IN AREAS COVERED BY UNCONSOLIDATED DEPOSITS IN THE VALLEY AND RIDGE GEOLOGIC PROVINCE OF VIRGINIA

Sinkholes are common karst features in Virginia’s Valley and Ridge Geologic Province and are hydrologically significant because they often capture surface water that recharges local aquifers. The Virginia Department of Energy, Geology and Mineral Resources Program used LIDAR-derived DEMs to accurately map sinkholes in an eighteen 7.5-minute quadrangle area along Interstate 81 and U.S. Route 11 in north-central Virginia as part of a derivative geologic mapping project through the National Cooperative Geologic Mapping Program. In a separate effort, the Virginia Department of Environmental Quality compiled available water well construction data for a twenty-two 7.5-minute quadrangle area that largely overlaps the sinkhole study area. Statistical analysis reveals a higher density of sinkholes in carbonate bedrock areas overlain by elevated fluvial terrace deposits. Additional sinkhole clusters can be observed in elevated areas along rivers where terrace deposits have not previously been mapped. These sinkholes may have developed in the past when terrace deposits were still present. A preliminary statistical analysis of well data in carbonate bedrock portions of the study area indicate that areas covered by terrace (n=82) and debris- or alluvial-fan (n=135) deposits have greater depth to bedrock, casing depth, and yields than wells in carbonate areas where these surficial deposits are not present (n=779). These trends are consistent with a hypothesis that enhanced recharge through overlying permeable fan or terrace deposits promotes greater chemical weathering of underlying fractured-carbonate bedrock units, resulting in increased sinkhole development and/or higher well yields. Clusters of sinkholes in areas where overlying deposits are thin or no longer present may represent the most mature phase of this process and help identify sensitive recharge areas or favorable targets for high yielding wells.