SURFACE AND SUBTERRANEAN MAPPING DOCUMENTS A REGIONALLY SIGNIFICANT ALLEGHANIAN THRUST SYSTEM IN THE MILLBORO QUADRANGLE, BATH COUNTY, VIRGINIA

Previously unmapped, regionally significant, westward verging thrust faults cross-cut a “breadloaf”-shaped, doubly-plunging anticline along State Highway 42 in Bath County, VA about 6 miles south of Millboro Springs. A localized thrust that dips shallowly to the southeast, where it reaches the surface, placed chert-free quartzose limestones of the upper Licking Creek over the stratigraphically younger Needmore Shale. Investigation of the structural and stratigraphic relations in nearby Porter’s Cave shows that the cherty lower Licking Creek is present in normal sequence, indicating that the subhorizontal fault exposed along State Highway 42 cuts down section southeast of the road. We interpret the fault as a flat-to-ramp thrust that dips progressively more steeply from west to east. Constraints on the lateral extent of the fault are limited, but we hypothesize that the fault projects southeast under the Rough Mountain syncline, which is cored by clastics of the Devonian Brallier Formation.

One mile northeast of Porter’s Cave, an extensive roadcut bisects the anticline and exposes shallowly-dipping thrust splays that place the Healing Springs Sandstone over the stratigraphically younger Licking Creek Limestone. The Tonoloway Limestone, which underlies the Keyser in this area, is truncated along a minor fault, as it is locally present and then disappears abruptly. The Clifton Forge and underlying limestones of the Byers Island Member of the Keyser, also in the hanging wall of the lower thrust, appear to wedge out to the west. Deformation including large-scale folds and micro-folds in the Clifton Forge Sandstone Member of the Keyser Formation is associated with faulting. Oriented thin sections of the upper Licking Creek and other units reveal multiply fractured quartz grains, with fractures parallel to the direction of principal stress during faulting.

Though a fairly localized, kilometers-scale structure, this fault bend anticline provides an example of thin-skinned deformation features that predominate in the southeastern Appalachian region. As with similar features in the region, deformation is interpreted to derive from the northwest-directed collision of Gondwana with eastern Laurentia during the Alleghanian orogeny.