THE EVINGTON GROUP RESTORED: A MAJOR STRATIGRAPHIC UNIT IN THE EASTERN BLUE RIDGE, CENTRAL VIRGINIA

In central Virginia, a heterogeneous sequence of layered low-grade metamorphic rocks crops out to the east-southeast of the Ediacaran Catoctin Formation along the boundary between the eastern Blue Ridge and western Piedmont. Brown (1953) named this sequence of pelitic rocks interlayered with quartzite, marble, and greenstones the Evington Group. In contrast, Conley (1985) and Henika (1991) argued that the “Evington Group” is not a legitimate unit, and that much of the sequence is the older Alligator Back Formation (Lynchburg Group) repeated across the Bowens Creek Fault. The 1993 Geologic Map of Virginia, the most recent regional compilation, follows this interpretation. Our study works to resolve these differing interpretations.

Our mapping along the Blue Ridge – Piedmont transition near Gladstone, Virginia indicates that there are several NE-SW trending belts of phyllite, quartzite, marble and greenstone that overlie the Catoctin Formation. The sequence is >3 km thick, and includes (from bottom to top) phyllite, marble, quartzite, phyllite, greenstone and phyllite, with layers repeated by folding and faulting. The greenstone is chemically similar to the underlying Catoctin Formation. Carbonate rocks include: a blue-gray tremolite-bearing micaceous to quartzose ± graphite marble and a milky white to pink marble. Bulk geochemical analysis of the marbles reveals a higher MgO in the white marbles (17%) than the blue-gray marbles (2%). The depositional environment was likely a distal muddy marine shelf below wave base with occasional detrital input.

In central Virginia, the Evington Group is a legitimate and major stratigraphic unit in the eastern Blue Ridge. Evington Group strata were deposited on the Laurentian continental margin, and although distal, these deposits received Grenvillian detritus with occasional periods of carbonate production and sea floor volcanism. The Bowens Creek Fault, as portrayed in southern Virginia, is not present in the Gladstone area, nor do characteristic Alligator Back Formation lithologies occur in this region. Evington Group rocks experienced at least two episodes of deformation, and regional metamorphism reached mid-greenschist conditions. U/Pb dating of detrital apatite and rutile is ongoing in an effort to elucidate the Paleozoic thermochronology of these rocks.