GEOLOGY OF THE CROZET 7.5' QUADRANGLE, BLUE RIDGE PROVINCE, VIRGINIA

The Crozet 7.5' quadrangle, centered around the town of Crozet in Albemarle County, VA, is located in the Blue Ridge geologic province. The landscape includes steep mountainous terrain to the northwest and modest rolling topography to the east and south. The quadrangle is underlain by 1) Mesoproterozoic basement rocks, 2) Neoproterozoic metasedimentary and metabasaltic rocks that unconformably overlie the basement, and 3) tabular dikes. The basement rocks include five suites, differentiated by age and composition; these include layered gneiss (in part paragneissic), charnockitic gneiss, leucogranite, megacrystic granitoid, and biotite-bearing granitoid. The oldest cover rocks are the phyllitic and meta-arkosic rocks of the ~700 Ma Mechum River Formation. The Swift Run Formation, composed of phyllite, meta-arkose, and meta-conglomerate, is a laterally discontinuous unit that ranges from 0-60 m thick. The Swift Run Formation is overlain by the contemporaneous ~570 Ma Catoctin Formation, comprised of greenstone interlayered with at least two thin metasedimentary layers that are compositionally similar to the Swift Run Formation. Several thin diabase and porphyritic basalt dikes of presumed Jurassic age strike NNW. The Mechum River Formation crops out in a NE-trending belt forming a structural inlier bounded on the SE by a steeply dipping, NE-striking reverse fault. The Swift Run and Catoctin formations crop out in the northwest and in smaller NE-striking fault-bounded inliers and eroded synclines. The Rockfish Valley high-strain zone forms a 2-3 km wide, NE-trending belt of mylonitic rocks that extends across the quadrangle. Kinematic indicators are consistent with NW-directed contractional deformation. Most rocks display a SE-dipping greenschist facies foliation produced during Paleozoic metamorphism and deformation. Layered gneiss and charnockite also display an older high-temperature foliation produced by amphibolite-granulite facies recrystallization. Surficial deposits include: 1) a broad, alluvial apron of debris flow to fluvial deposits extending up to 2 km from the base of Blue Ridge mountains, 2) sets of isolated fluvial strath terraces above major modern streams, and 3) modern alluvium.