GEOLOGIC MAPS OF TWO WATERSHEDS IN THE PIEDMONT AND BLUE RIDGE OF NORTH CAROLINA AS PART OF AN INTEGRATED REGIONAL HYDROGEOLOGIC STUDY

Areas of the Appalachian Piedmont and Blue Ridge provinces underlain by crystalline (igneous and metamorphic) bedrock face a critical demand for ground water, but our knowledge of ground-water flow, storage and chemical behavior in these areas is deficient, largely because ground-water flow is mostly confined to subsurface fractures that are difficult to characterize. Detailed understanding of the bedrock geology, including the subsurface distribution and fracturing characteristics of lithologic units, is thus vital. A cooperative study by the USGS and the North Carolina Department of Environment and Natural Resources is attempting to understand the ground-water characteristics of the major lithotectonic belts in the NC Piedmont and Blue Ridge by intensive investigations in small areas that are representative of these belts. Several of these areas are watersheds, and new USGS 1:24,000-scale geologic maps have been made for two of them: the Upper Wolf Island Creek watershed in the Piedmont, near Reidsville, NC, and the Upper Cullasaja River watershed in the Blue Ridge, around Highlands, NC. Accompanying GIS databases include extensive fracture information in addition to standard geologic data. The Upper Wolf Island Creek watershed is underlain by gently south dipping layered metamorphic rocks of the Neoproterozoic to early Paleozoic Milton terrane, including felsic gneiss, biotite gneiss, schist, amphibolite, ultramafic rocks, and a mylonite zone. Cross-strike, down-slope transects of monitoring wells in the watershed are being utilized to study the cut-slope vs. dip-slope behavior of ground water along its flow path from recharge to discharge point—knowledge that is transferable to other areas underlain by layered metamorphic bedrock. The Upper Cullasaja River watershed is underlain by highly folded gneiss, schist, and amphibolite of the Neoproterozoic to early Paleozoic Ashe Metamorphic Suite that have been intruded by sheet-like bodies of trondhjemite of the 465 Ma Whiteside pluton. Down-slope transects of monitoring wells will also be used here to characterize the contrast in ground-water behavior between areas underlain by schist and gneiss vs. those underlain by plutonic rock.