BEDROCK GEOLOGY OF THE SWIFT RUN GAP 7.5’ QUADRANGLE, BLUE RIDGE PROVINCE, VIRGINIA

The Swift Run Gap 7.5’ quadrangle is located in the Blue Ridge province and includes ~45 km² of Shenandoah National Park. The Mesoproterozoic basement complex includes three mappable units: 1) granitoid gneiss 2) blue quartz and biotite-bearing granitoid and 3) charnockite. The granitoid gneiss crops out in the southern part of the quadrangle and commonly displays a high-temperature fabric with biotite ± pyroxene. The blue quartz and biotite-bearing granitoid is characterized by a high Ti content and crops out in the east-central part of the quadrangle. The charnockite is typically massive with up to 15% orthopyroxene, crops out in the northern part of the quadrangle, and also occurs as dikes within the granitoid gneiss. The basement is unconformably overlain by metasedimentary and metavolcanic rocks of the Swift Run and Catoctin Formations. The Neoproterozoic Swift Run Formation is a heterogeneous metasedimentary unit comprised of phyllite, meta-arkose, and meta-arkosic conglomerate ranging in thickness from 0- 75 meters. The Neoproterozoic Catoctin Formation is a ~300 meter thick unit of predominantly metabasalt with thin layers of phyllite and meta-arkose. Metabasalt also occurs as dikes in the basement. The Catoctin Formation is overlain by the Chilhowee Group, a package of Neoproterozoic to Early Cambrian siliciclastic metasedimentary rocks ~1000 meters thick. The Chilhowee Group crops out in the northwestern part of the quadrangle. NE striking, SE dipping mylonite zones cut the basement in the southeastern part of the quadrangle and consistently record top-to-the-NW (reverse) sense of shear. A NNE striking fault in the south-central part of the quadrangle is interpreted to be a Neoproterozoic normal fault that was later reactivated as a thrust. A NE trending open anticline with basement in its core dominates the north-central part of the quadrangle. The Chilhowee Group crops out in a SW plunging syncline in the northwest corner of the quadrangle. Most of the rocks in the quadrangle have a SE dipping foliation defined by greenschist facies minerals. An earlier high-temperature foliation, defined by aligned mineral aggregates of quartz and feldspar, is preserved in the older basement units and strikes EW. A Mesozoic NNW striking high-angle fault offsets units in the west-central part of the quadrangle.