Date Thesis Awarded
Honors Thesis -- Open Access
Bachelors of Science (BS)
Christopher M. Bailey
Brent Edward Owens
The Neoproterozoic Swift Run Formation is a metasedimentary unit that unconformably overlies Mesoproterozoic granitoid basement in the western Blue Ridge in central Virginia. Metabasalts of the 570-550 Ma Catoctin Formation conformably overlie the Swift Run Formation. The unit is composed of metasiltstone/phyllite, matrix-supported arkosic phyllite, arkosic metasandstone and metaconglomerate, and minor metabasalt. The mineralogy, texture, and sedimentary structures are consistent with fluvial, lacustrine and alluvial depositional environments in a mechanical weathering-dominated setting. The Swift Run Formation's thickness is highly variable, changing from 0 to ~300 m over distances of a few km. Neoproterozoic topographic highs are preserved where the Swift Run Formation is locally absent. Locally, the Swift Run Formation was deposited in half grabens bounded by NE-striking normal faults. To the east of the main basement-cover unconformity the Swift Run Formation crops out in a series of outliers 3-11 km long and <0.7 km wide. Previous workers have interpreted these outliers to by synclinal infolds or tectonic windows through a supposed Rockfish Valley thrust sheet. Field data is consistent with reverse faults bounding these structures on the SE and unconformities on the NW contacts of these outliers. In this model, NW-translation of the hanging walls occurred either ductily during greenschist-facies deformation and/or brittlely during the Blue Ridge thrust event. The Swift Run Formation developed a pervasive SE-dipping foliation of moderate to mylonitic intensity during greenschist facies deformation and metamorphism that also caused pervasive NE-trending folds and reactivation of original normal faults. 40Ar/39Ar ages from syntectonic white mica yield ages from 340-320 Ma, consistent with early Alleghanian deformation. Paleomagnetic analysis for paleolatitude information was inconclusive.
Gattuso, Adam, "Tectonic Significance of the Late Neoproterozoic Swift Run Formation and Basement-Cover Unconformity in the Virginia Blue Ridge" (2009). Undergraduate Honors Theses. William & Mary. Paper 307.
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