UNDERSTANDING THE BLUE RIDGE-GREAT VALLEY BOUNDARY: GEOLOGY OF THE ELKTON EAST 7.5' QUADRANGLE, VIRGINIA

The Elkton East 7.5’ quadrangle straddles the boundary between the Blue Ridge geologic province to the east and the Great Valley subprovince of the Valley and Ridge province to the west in northwestern Virginia. Approximately 20% of the quadrangle lies within Shenandoah National Park.

The Blue Ridge is underlain by Mesoproterozoic basement rocks and a Neoproterozoic to early Cambrian cover sequence. Cambrian to Ordovician carbonates and shales crop out in the Shenandoah Valley. Basement rocks include granitoid gneiss, charnockitic gneiss, and younger charnockite and granite. Rift-related metasedimentary and metavolcanic rocks of Swift Run and Catoctin formations unconformably overlie the basement complex. Siliciclastic rocks of the Chilhowee Group, include the Weverton, Harpers, and Antietam formations, were deposited in non-marine to marine conditions during the early Cambrian. The Blue Ridge basement and cover sequence experienced greenschist facies during mid to late Paleozoic deformation.

In contrast to previous workers, we recognize a tectonic contact separating Blue Ridge rocks from those in the Great Valley. The Elkton Embayment, an isolated eastward curve in the western edge of the Blue Ridge Mountains, is framed on the east by the low-angle Blue Ridge Fault that places Mesoproterozoic to Cambrian Blue Ridge rocks over the Cambro-Ordovician carbonate sequence in the Shenandoah Valley. In competent units tectonic breccias are localized immediately above the Blue Ridge Fault. Other faults in the quadrangle include a suite of steeply-dipping, north-northwest striking transverse faults, including the Harris Cove fault, that cut older structures. Transverse faults are interpreted to be Mesozoic extensional structures.

Surficial deposits cover much of the western part of the quadrangle and include variably dissected alluvial fans extending westward from the Blue Ridge Mountains and at least three sets fluvial terraces associated with an ancestral South Fork of the Shenandoah River system.