GEOLOGIC MAP OF THE NORTHERN PORTION OF THE BLUE RIDGE PARKWAY, CENTRAL VIRGINIA

The US Geological Survey, in cooperation with the National Park Service, has just completed year one of a multi-year task to conduct detailed geologic mapping along the Blue Ridge Parkway in Virginia. Mapping has been completed in nine 7.5-minute quadrangles within a 2-mile-wide corridor centered on the Parkway, from Mile Post (MP) 0 near Afton, southward to MP 63 at the James River. Western Blue Ridge stratigraphy in this area ranges from Mesoproterozoic orthogneisses and metagranitoids to lower Cambrian Chilhowee Group.

In the northernmost portion (MP 0–16), Neoproterozoic Catoctin Formation greenstone unconformably overlies Swift Run Formation phyllite and Mesoproterozoic rocks, and is unconformably overlain by Weverton Formation meta-arkose and phyllite. The contact between Catoctin greenstone and Chilhowee Group rocks is marked by distinctive purplish-green metavolcaniclastic mudstone and conglomerate. Southward to Tye River Gap (MP 24), this Mesoproterozoic to lower Cambrian package of rocks is interleaved in a series of regional-scale, west-dipping blocks; though previously interpreted to be Neoproterozoic-Cambrian growth faults, kinematic indicators are lacking, and the regional structure can also justifiably be interpreted as a window into a complex west-dipping duplex.

In the central portion (MP 24–45), Mesoproterozoic rocks are juxtaposed along a 4.5 km-long north-striking fault: four massive to weakly foliated orthopyroxene-bearing metagranitoids occur on the east block, whereas seven distinct orthogneisses (intruded only locally by very rare orthopyroxene-bearing metagranitoids) occur to the west. Foliations in these orthogneisses likely developed during Shawinigan (~1.14 Ga) deformation, but several of the orthogneisses also exhibit amphibolite-facies ductile foliations that may be Neoproterozoic in age. Penetrative foliations in the metagranitoids developed under greenschist-facies conditions, and are mostly Paleozoic.

In the southern portion (MP 46–63), the Parkway transects the southwestern end of the Oronoco belt, an enigmatic synclinal structure of Mesoproterozoic rocks, Catoctin Formation, and Chilhowee Group rocks, all possibly exposed in another complex tectonic window.