Previous studies in southwestern Virginia indicate a major decollement at or near the base of the Rome Formation. Recent geologic mapping in the structurally complex southeastern corner of the Amonate quadrangle, southwestern Virginia, identified as much as 800 ft of Shady Dolomite and 600 ft of Rome Formation above the St. Clair thrust fault. The Shady Dolomite, which contains several chert and shale beds up to 1 ft thick, is light gray to white, finely to coarsely crystalline, and thick bedded to massive. The upper part of the unit contains interbeds of grayish-red and reddish-brown shale and siltstone and is in normal stratigraphic sequence with the overlying Rome Formation. “Sunbursts” of barite crystals and pyritized fractures occur approximately 300 ft below the top contact. Above the St. Clair thrust fault, the Shady Dolomite is highly deformed and brecciated, and it contains nappe structures. The dolomite is thrust over Devonian shale, which is overturned and dips 35° to the southeast. The shale is in sequence with strata as young as Late Mississippian. The dolomite dips 55° to the southeast and is truncated by the thrust approximately 1 mi southwest and 0.5 mi northeast of “The Jumps” on State Route 637 at Rourkes Gap, Virginia. This occurrence of Lower and Middle Cambrian strata indicates that the decollement extends locally below the base of the Rome Formation. Fractured zones associated with ramp and splay faults and probable truncated stratigraphic and structural traps may be favorable for the accumulation of hydrocarbons beneath the decollement.

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