Abstract: Stratigraphy as Key to Arch-Related Origin of Little North Mountain Structural Front, Virginia and West Virginia

Eugene K. Rader, William J. Perry, Jr.

The Little North Mountain structural front along the western margin of Shenandoah Valley separates the eastern and western Valley and Ridge structural belts in Virginia and West Virginia. Here Cambrian and Ordovician carbonate rocks of the North Mountain thrust sheet lie in fault contact on younger rocks on the west. Stratigraphic units ranging in age from Late Ordovician to Middle Devonian are exposed in a line of ridges and hills collectively termed Little North Mountain, just west of or in the fault zone.

Upper Ordovician to Middle Silurian rocks provide most of the data for the following interpretation. A normal stratigraphic sequence west of Little North Mountain (Judy Gap area, West Virginia) is Ordovician Reedsville Shale, Oswego Sandstone, Juniata Formation (red beds), and Silurian Tuscarora Sandstone, Rose Hill Formation, and Keefer Sandstone. In Massanutten synclinorium on the east, the sequence is Ordovician Martinsburg Formation (chiefly turbidites) overlain by Silurian Massanutten Sandstone. Five segments of Little North Mountain show four distinctly different stratigraphic sequences. The Gerrardstown, West Virginia, segment has a sequence very similar to that of the western Valley and Ridge. The Saumsville and Freemason Mountain, Virginia, segments contain an Oswego section ith a tongue of Juniata red beds. In Rockingham County, Virginia, the Brocks Gap segment has a 400 to 600 ft (122 to 183 m) thick sandstone between the Martinsburg and Rose Hill Formations, and Juniata red beds are absent. Conformable Martinsburg and Tuscarora are present in the Buffalo Gap segment, Augusta County, Virginia. Only the Buffalo Gap and Gerrardstown segments are rooted; the intervening segments are tectonic slices transported differentially westward so that all are roughly in line, forming the discontinuous segments of Little North Mountain. As stratigraphic sequences of the segments are compared with each other and with those east and west of the front, the relative westward transport of each and the configuration of a Late Ordovician through Early Devonian axis of thinning may be established. The Brocks Gap segment has been transported farthest west from an initial position on or east of the axis, and the Gerrardstown and Buffalo Gap segments the least. The Brocks Gap segment requires approximately 5 mi (8 km) of transport over the Devonian black shales in the North Mountain fault system. This indicates the presence of Silurian and Lower Devonian sandstones which have associated probable fracture porosity and possible natural gas under part of the North Mountain thrust sheet in the western Shenandoah Valley.

AAPG Search and Discovery Article #90975©1976 GCAGS- GC Section SEPM Annual Meeting Shreveport, Louisiana