BEDROCK GEOLOGY OF THE BROWNS COVE 7.5' QUADRANGLE, BLUE RIDGE PROVINCE, VIRGINIA

The Browns Cove 7.5' quadrangle is located in the central Virginia Blue Ridge province and includes ~40 km² of Shenandoah National Park. The quadrangle is underlain by 1) Mesoproterozoic basement rocks, 2) Neoproterozoic siliciclastic and volcanic cover rocks, and 3) the Neoproterozoic to Early Cambrian Chilhowee Group. The basement includes four mappable units: 1) a charnockite, 2) an aerially extensive megacrystic granitoid containing older, map-scale inclusions of 3) biotite granitoid gneiss and 4) leucogranitic aplitic gneiss. The charnockite and the older basement units commonly contain a high-temperature foliation defined by a penetrative fabric. These basement units are cut by several greenstone and diabase dikes in the eastern part of the quadrangle. The Swift Run Formation unconformably overlies the basement complex and includes conglomerate, meta-arkose, and phyllite along with minor layers of greenstone. The thickness of the laterally discontinuous Swift Run typically ranges from 0-50 meters, but in Sugar Hollow in the southwestern part of the quadrangle, it exceeds ~350 meters. The Catoctin Formation, 350-450 meters thick, is dominantly amygdaloidal metabasalt with minor thin beds of metasedimentary rocks. A thin, laterally discontinuous unit of spotted, volcanogenic phyllite occurs at the top of the Catoctin Formation. The Weverton and Harpers Formations of the Chilhowee Group unconformably overlie the Catoctin Formation in the northwest corner of the quadrangle. The Chilhowee Group is ~300 meters thick and is composed of meta-conglomerate, quartz arenite, and phyllite. In Sugar Hollow, at least two Neoproterozoic, syndepositional normal faults cut basement rocks and define an east-stepping basin where the Swift Run Formation thickens dramatically. Cover rocks in Sugar Hollow form a set of asymmetric folds that are buttressed against the basement normal faults. A set of anastomosing, northeast/southwest-striking high-strain zones cut units in the eastern part of the quadrangle. These mylonites record a top-to-the-northwest (reverse) sense of shear. Structures preserved in the Browns Cove quadrangle are the result of Mesoproterozoic deformation, Neoproterozoic extension, and Paleozoic contraction.