Two small oil fields in Lee County, Virginia, and one small gas field in Scott County, Virginia, have been discovered in the Valley and Ridge province of the Southern Appalachians. Very little drilling has been done, however, except in proximity to these fields and, similarly, little seismic work has been done. Hence, the intermediate and deep subsurface structure in this structurally complex belt is little known.

The two oil fields are within the Cumberland overthrust block, and the gas field is close to it. One deep well drilled in 1967, near the crest of the Powell Valley anticline and near the Rose Hill oil field, penetrated the Pine Mountain overthrust at shallow depth, as expected. However, it also penetrated a previously unknown fault, later named the Bales fault, at 6,400 ft, below which 1,450 ft of beds were duplicated. The probable position of basement there is -10,400 ft, which is about 1,500 ft deeper than its probable position at wells 12 mi farther northwest and normal to the regional strike. Thus, the basement surface seems to be southeastward sloping and relatively smooth, and basement movement does not seem to be involved in the arching of the Powell Valley anticline. Data from a recent seismic line across the Valley and Ridge province support this interpretation.

The Bales fault is interpreted to be an imbricate fault, ramping upward from a sole fault in Lower Cambrian rocks, that probably extends northwest at least to the northwest edge of the Cumberland block and southeast beneath most of the Valley and Ridge province. It may extend even farther southeast beneath the Blue Ridge province. The large slice of the crust above the sole fault is believed to have slid northwestward as a unit down a very gentle slope before internal stresses in the moving slice caused rupturing by many imbricate faults.

Structural traps for hydrocarbons should exist in the Valley and Ridge province. If the interpretation advanced here of the mechanism of faulting is correct, compressive stresses during mountain building should not have been strong enough to squeeze out the hydrocarbons.