GEOLOGIC FACTORS INFLUENCING SHALLOW AND DEEP KARST FORMATION IN THE SHENANDOAH VALLEY, VIRGINIA

The karst of the Shenandoah Valley has features indicative of both shallow and deep solutional formation. The degree to which shallow and deep solutional processes intersect is dependent upon surficial cover, bedrock structure, and local hydraulic gradients. Where alluvial deposits of siliciclastic sediments cover carbonate bedrock, shallow karstification is enhanced by calcite-undersaturated aquifers, resulting in near-surface fluviokarst development. However, caves preserved in isolated hills show little evidence of fluviokarst processes, and instead are entirely phreatic systems that were drained and exposed during denudation. Recent geologic mapping and LiDAR elevation data has facilitated interpretive associations between bedrock structure, silicification and brecciation of host carbonate bedrock, and the location of several caves and karst springs. Phreatic caves are associated with sub-vertical fracture and fault zones that enable deep circulation (>500 m) of meteoric groundwater. Most caves formed in such settings have been overprinted by later circulation of shallow groundwater and/or fluviokarst processes, thus removing evidence of the history of earliest inception; however, several isolated caves do preserve evidence of deeper primary origin. These isolated caves contain phreatic deposits of calcite and fine grained sediments, and generally lack sedimentary evidence of having been invaded by surface streams. Geochemical and temperature data from springs support two end-member types: those affected primarily by shallow near-surface recharge, and those sourced deeper in the karst aquifer. Springs that gain substantial flow from sinking surface runoff are associated with siliciclastic alluvial deposits, are highly responsive to recharge events, and/or are located near to incised streams where local hydraulic gradients are steepened. Springs that show less influence from rapid surface water infiltration are located along fault and fracture zones, tend to be farther removed from thick alluvial cover deposits, and exhibit muted temperature and geochemical variability in response to recharge events. Many springs fall between these end-members. Thus, the present karst system results from an overprint of surficial processes on an earlier, deeper phreatic karst system.