CONTRAST IN DEFORMATION STYLES WITHIN THE SILURIAN – DEVONIAN SEQUENCE, VALLEY AND RIDGE PROVINCE, HIGHLAND COUNTY, VIRGINIA

Bedrock mapping during the past two years in the Williamsville 7.5’ quadrangle and nearby areas of Highland and Bath Counties, Virginia, has clarified some structural relationships in the Silurian and Devonian sequence of this region.

The major valleys in this area are synclines with Devonian mudrocks along their axes. These shales and siltstones of the Needmore, Marcellus, and Millboro Formations are incompetent and structurally weak. In contrast, the underlying competent Oriskany Sandstone has functioned as the strong and more brittle unit. In the more complexly folded areas that were mapped in detail, the Oriskany exhibits chevron folds, with Devonian mudrocks semi-chaotically folded and faulted within the confining folds of the Oriskany. The deformed mudrocks have in fact been deformed on a much smaller scale than the Oriskany, with localized shear zones, small-scale duplexes, and meter-scale anticlines and synclines of varying trends. The shales appear much more deformed due to space restrictions in the nose regions of the Oriskany folds. In the Williamsville area the small-scale deformation of the mudrocks does not appear to be replicated at the regional scale, where the dominant features are broad anticlines and synclines. In areas studied in detail, joints in the Needmore and Marcellus, which are of interest to gas exploration farther west in the Appalachian basin, mostly precede the folding and faulting of those units.

The major ridge in the mapped area is Bullpasture Mountain, a northeast-trending anticlinorium. Exposures on both limbs of the anticline in the Bullpasture River Gorge show that the Silurian sandstones (Keefer, McKenzie, Williamsport) have behaved as the brittle units. The Tonoloway Formation, particularly its lower and upper members with their thin-bedded lime mudstones and few thin sandstones, is the most deformed of the major stratigraphic units so that in the Bullpasture Gorge, the Tonoloway is observed to have been kink folded and crumpled to varying extents. The Wills Creek Formation, which in exposures to the north along strike is the most deformed unit in this part of the section, is absent or extremely thin (1-2 m) in this area and thus the thin-bedded Tonoloway exhibits some of the deformation style that elsewhere is concentrated in, and is typical of, the shalier units of the Wills Creek.