GEOLOGIC MAP OF MESOPROTEROZOIC – CAMBRIAN ROCKS FROM JAMES RIVER SOUTH TO LITTLE RIVER, CENTRAL PORTION OF THE BLUE RIDGE PARKWAY, VIRGINIA

Geologic mapping has been completed on twelve 7.5-minute quadrangles within a 2-mile-wide corridor centered on the Blue Ridge Parkway, from Mile Post (MP) 63 at James River, Bedford County, to MP 145 near Callaway, Franklin County.

From James River south to Bearwallow Gap (MP 91), the Parkway transects Mesoproterozoic rocks: 1) mylonite (MP 63-66) of the Rockfish Valley high strain zone, which exhibit Paleozoic chlorite-grade and NW-directed contractional foliations; 2) layered garnet (gt) + orthopyroxene (opx)-bearing gneisses with syn-kinematic Shawinigan (1.14 Ga) foliation; 3) megacrystic opx-bearing metagranitoid with NW-dipping, amphibolite-facies ductile foliation that may be Neoproterozoic in age; and 4) undeformed fine- to medium-grained hypidiomorphic opx-bearing metagranitoid at Peaks of Otter. This granitoid is the youngest Mesoproterozoic intrusive rock recognized in the Blue Ridge (unpublished U-Pb zircon age); dikes related to this pluton, some exhibiting chilled margins, intrude layered gneisses as far north as MP 68. Very garnetiferous and graphitic layered paragneiss is cut by undeformed pegmatite near MP 90; the possibly early Neoproterozoic paragneiss is structurally overlain by gt+opx-bearing orthogneisses.

From Bearwallow Gap south to Stewart Knob (MP 110), rocks as young as Cambrain Elbrook Formation are exposed as the Parkway transects the Blue Ridge thrust sheet at the southwestern end of the Goose Creek re-entrant. Mesoproterozoic rocks near Roanoke (MP 113) include opx-bearing felsic orthogneiss with porphyroblastic garnet as much as 1-cm in diameter. This felsic gneiss is intruded by medium-grained mesocratic opx-bearing metagranitoid, and megacrystic metagranitoid with K-spar phenocrysts as much as 20-cm in length. South of a 1.5 km-wide high-strain zone at Adney Gap, porphyroblastic gt-bearing layered gneisses transition into muscovite + biotite granitoid gneisses, with Paleozoic mylonitic foliations and locally preserved Mesoproterozoic foliations. Neoproterozoic(?) metasedimentary rocks and amphibolites overly these altered granitic gneisses at MP 144.5; these rocks were metamorphosed at amphibolite-facies in the Paleozoic and have been previously assigned to the Lynchburg Group and Ashe, Alligator Back, and Wills Ridge formations.