STRATIGRAPHY, STRUCTURAL GEOMETRY, AND DEFORMATION HISTORY OF THE NEOPROTEROZOIC SWIFT RUN FORMATION, CENTRAL VIRGINIA BLUE RIDGE

The Neoproterozoic Swift Run Formation is a metasedimentary unit that unconformably overlies Mesoproterozoic granitoid basement in the western Blue Ridge of central Virginia. Metabasalts of the 570 to 550 Ma Catoctin Formation conformably overlie the Swift Run Formation. The unit is composed of matrix-supported arkosic phyllite, arkosic metasandstone and metaconglomerate, metasiltstone/phyllite, and minor metabasalt. The mineralogy, texture, and sedimentary structures are consistent with fluvial and lacustrine depositional environments in a mechanical weathering-dominated setting. The Swift Run Formation's thickness is highly variable, ranging from 0 to ~300 m over distances of a few km. Original topographic highs are preserved where the Swift Run Formation is locally absent. In at least a few locations, the Swift Run Formation was deposited in rift-related half grabens bounded by NE-striking normal faults.

In Albemarle County, the Swift Run Formation crops out in a series of outliers to the east of the main Swift Run/Catoctin belt, and is surrounded by the Mesoproterozoic basement complex. The NE-trending Pigeon Top, Lickinghole Creek, Little Yellow Mountain, and Stony Run outliers are 3 to 11 km long and <0.7 km wide, with areas 0.5 to ~5 km². Previous workers have interpreted these outliers to be synclinal infolds or tectonic windows through a supposed Rockfish Valley thrust sheet. Our field data are consistent with reverse faults on the SE and unconformities on the NW contacts of the outliers. In this model, NW-translation of the hanging walls occurred as the Blue Ridge thrust sheet was translated northwest up a tectonic ramp. The tectonic window hypothesis is refuted because the supposed thin Rockfish Valley thrust sheet is rather a network of anastamosing high-strain zones with moderate dips. Bedding orientations are inconsistent with the syncline model.

The Swift Run Formation developed a pervasive SE-dipping foliation of moderate to mylonitic character during greenschist facies deformation and metamorphism. ⁴⁰Ar/³⁹Ar ages from syntectonic white mica yield integrated and plateau ages between 340 and 320 Ma, consistent with early Alleghanian deformation. Original Neoproterozoic normal faults were rotated and reactivated during this event.