PATTERNS OF KARST HYDROLOGICAL SYSTEMS DEVELOPED IN ORDOVICIAN-AGED CARBONATES OF THE SOUTHWESTERN VIRGINIA VALLEY AND RIDGE

Recent tracer dye studies in southwestern Virginia revealed patterns of water flow through well-developed karst systems in middle-ordovician aged limestones (Ols). Interpretations are based on 1) flow path delineation using activated charcoal sampler packets, 2) analysis of dye concentration breakthrough curves, and 3) existing geologic maps. The Ols sequence supports a well-developed karst topography characterized by a lack of surface drainage and numerous sinking streams, sinkholes, extensive caves, and large springs. Baseflow to these systems is provided largely by perennial streams that sink near the upper contact of Ols, along the scarp slope of ridges capped by Silurian aged orthoquartzites, separated from Ols by several hundred meters of interlayered sandstone, shale, and limestone. These first order streams are fed by perennial springs associated with both talus slopes and perched aquifers developed in thin limestone beds. These waters resurge at springs in Ols or in the underlying Knox group dolostones, where base flow ranges up to several cubic feet per second. Elevation differences between insurgences and springs range up to 300 meters, with 100-200 meters being typical. Flow vectors of up to 5 kilometers are typical. Transit times range from a few days to several weeks (drought), yielding velocities of up to several kilometers per day. Subsurface flow paths commonly crossed surface topographic divides, following geologic strike in the vadose zone within Ols. These vadose conduits either 1) discharge at large springs along strike, or 2) connect to major fracture zones that cut across the geologic section, with springs at base-level exposures. The portion of these fracture zones crossing the Knox Group dolostones is generally inaccessible to direct exploration. Dye breakthrough curves from several similar-sized springs developed in the limestone (along strike) and dolostone (across strike), are similar, and do not suggest a significantly larger phreatic or diffuse flow component in the dolostone springs. Along strike within the Ols hydrological system, springs discharge both at base level streams and at perched elevations up to 50 meters above base level, reflecting either differing degrees of vertical conduit development or sediment infilling of deeper karst conduits. Perched springs commonly sink again and emerge at base level springs.