NEW INSIGHTS INTO THE STRATIGRAPHY AND STRUCTURE OF THE SILURO-DEVONIAN SECTION ON BULLPASTURE MOUNTAIN AS REVEALED BY BEDROCK MAPPING IN THE MONTEREY SE QUADRANGLE, HIGHLAND COUNTY, VIRGINIA

The Monterey SE 7.5’ quadrangle is dominated by Bullpasture Mountain, a northeast-trending 41 km-long by 3 km-wide anticlinorium that exposes a stratigraphic section from the Silurian McKenzie sandstone to the Devonian Millboro shale. New bedrock mapping documents a northeast-trending, upright, symmetrical anticlinorium, which contrasts with more typical northwest verging fold structures. The southeast summit of Bullpasture Mountain between the Hupman valley syncline and the Cowpasture River is an anticline capped by limestone of the middle and lower members of the Silurian Tonoloway Limestone. These members of the Tonoloway are common on topographic highs, which is unexpected because they are karstic, thick to thin-bedded lime mudstones to grainstones. The Wills Creek Shale, exposed along US 250, becomes thinner and sandier to the south, and it is essentially absent at the Bullpasture River Gorge. Other stratigraphic changes occurring southward across the quadrangle include (1) disappearance of the lower Keyser Formation; (2) a facies change in the middle Keyser where the 5 m thick Big Mountain Shale passes southward into the 11 m thick Clifton Forge Sandstone, and (3) general thickening and coarsening of the Williamsport and McKenzie sandstones.

Erosional windows through the Tonoloway and underlying strata along the limbs of the southeasternmost anticline expose older units as far down as the McKenzie sandstone, and reveal that competent Williamsport and McKenzie sandstones are responsible for preserving the primary anticlinal structure of Bullpasture Mountain. These more massive sandstones exhibit abundant parasitic folds along the SE limb of the anticline, whereas the thinner bedded Tonoloway Limestone and Wills Creek Shale are tightly folded to crumpled in the cores of the parasitic folds, absorbing much of the structural deformation. In this region, the Wills Creek shales typically display the most extensive deformation of the Silurian units, but because it is absent or extremely thin in this area, the deformation is, in turn, taken up by the lower Tonoloway limestones. No evidence of major faulting has yet been found, so the thinning of the Wills Creek Shale in the Bullpasture Mountain area, as well as the fluctuations in the thicknesses of other units, is interpreted as a stratigraphic pinch-out.