Geology of the Big Walker Mountain-Crockett Cove Area, Bland, Pulaski, and Wythe counties, Virginia

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Virginia Polytechnic Institute


The Big Walker Mountain-Crockett Cove area lies in the Valley and Ridge province of southwestern Virginia and is made up of Middle Cambrian-Lower Mississippian strata which are exposed in two separate northeast-trending strike belts. The area is bounded on the northwest by the northwestern limb of the Greendale syncline which has been overridden from the southeast by rocks of the Saltville block. The northeast-trending Saltville thrust probably dips less than 25° in the area near Bland, Virginia, where Middle Ordovician rocks are exposed in a fenster. The rocks exposed in the fenster are probably part of the northwest limb or near-trough portion of the Greendale syncline.

The major structure of the Saltville block is the walker Mountain homocline which has subsidiary folds that plunge eastward into a much larger structure - the Blacksburg synclinorium - which is located just east of the area studied.

The Saltville block is broken at its southern limit in most of the area by the Tract Mountain reverse fault which essentially parallels the trace of the Saltville fault. Stratigraphic displacement along the Tract Mountain fault decreases from a maximum of about 8,000 feet near its southwestern terminus where the Pulaski block overrides it from the south,to less than 1,000 feet nearly 18 miles away at the eastern border of the area studied. The Tract Mountain block, which is bounded on the northwest by the Tract Mountain fault, is made up of a series of northeast-trending folds which plunge eastward toward the Blacksburg synclinorium.

Mapping of the Tract Mountain block and stratigraphic studies of two of its larger folds, the Crockett Cove anticline and the adjacent Queens Knob syncline, show that there is no appreciable thickness or lithologic change in Middle Cambrian-Lower Ordovician rocks between syncline and anticline. However, the basal Champlainian Series in the more southern fold, the Queens Knob syncline, has an aggregate thickness of about 4,400 feet, whereas the same interval near the crest of the Crockett Cove anticline is less than 2,000 feet thick. Most of the beds present near the trough of the syncline are markedly more clastic and less pure than the corresponding beds on the crest of the adjacent anticline which is less than 2.5 miles up structure to the north. Based on this evidence, it is concluded that the present structural axes are identical to axes of maximum and minimum differential subsidence of the sea floor during Middle Ordovician-Late Silurian time. The synclinal trough was the site of maximum subsidence and the anticlinal axis was the site of minimum subsidence. The date of inception of these two folds must correspond to the beginning of pronounced vertical movement of the sea floor which started in early Champlainian time.

The anticline which must have at one time lay adjacent to and southeast of the Queens Knob syncline was probably eliminated as a large fault slice during movement along the Pulaski fault that strikes obliquely across the axis of the Queens Knob syncline, which is the southernmost structural element of the Tract Mountain block. The leading edge of the Pulaski block forms the southern border of the Big walker Mountain-Crockett Cove area.which contains approximately 160 square miles that was mapped on a scale of 2 inches to the mile.