THE VALLEY AND RIDGE TO BLUE RIDGE PROVINCE TRANSITION NEAR LURAY, VIRGINIA: GEOLOGIC MAP & CATACLASTIC BRECCIAS

The Luray 7.5 minute quadrangle showcases a stratigraphic column spanning nearly 1.2 billion years, incorporating the transition from the Blue Ridge Province to the Valley and Ridge Province. The formations within consist of pre-Cambrian basement gneisses, Cambrian clastic rocks, Cambro-Ordovician carbonates, and Siluro-Devonian clastics. The region contains a system of numerous west-directed low to moderate angle thrust faults transected by northwest-striking transverse faults. Most notably, the Vaughn Fault represents the main Blue Ridge thrust in this area. The Stanley Fault trends southwest-northeast, across both Valley and Ridge and Blue Ridge lithologies, and likely represents the last phase of Alleghanian thrusting in this area.

Current mapping covers the eastern portion of the Luray 7.5 minute quadrangle, focusing on the Chilhowee Group (Weverton, Harpers, Antietam Fms.) Their lithology varies somewhat throughout the mapping area, in particular the degree of deformation and weathering of the Antietam quartzite. Additionally, the Harpers Formation is transitional between its confining units and exhibits a range of grain sizes and mineralogy. The presence of ore-bearing brecciated siliciclastic outcrops is another feature of interest. The breccias vary in the degree of cataclastic fracturing and matrix content. They occur frequently in the Antietam formation and likely represent Alleghenian thrust fault activity. The fact that they are not laterally continuous, however, suggests that they may be, in part, vertical features created through dilation and/or collapse processes. Fracture dilation could have occurred as hydraulic injections into vertical jointing and other planes of weakness. Collapse of fractured bedding could also have formed some of these features through implosion of void spaces created during deformation. Better constraints on these lithologic and deformational features have improved delineation of formation contacts and fault locations in the region, with broader implications for the nature of Alleghanian orogenesis in western Virginia.