What Came First: Folding, Faulting, and Fracturing in the Devonian Needmore and Marcellus Shales of Highland County, Virginia

John T. Haynes¹, Seldon M. Walker¹, Steven J. Whitmeyer¹, and Keith E. Goggin^2
1James Madison University, Dept. of Geology & Environmental Science, Harrisonburg, VA 22807, [email protected], [email protected], [email protected]
²Weatherford Laboratories, Houston, TX 77064, [email protected]

Recent mapping of Devonian shales along the SE and NW limbs of the Bullpasture Mountain anticline, Highland County, Virginia, reveals a complex history of folding, faulting, and fracturing. Unraveling this history is potentially of great significance in the understanding of Marcellus Shale natural gas production in the Appalachian basin subsurface.

On both limbs of the anticline, exposures of the Needmore and Marcellus shales exhibit pervasive fracturing, with subvertical joint sets striking NW (~306, ~335) and NE (036-055). Pencil cleavage is ubiquitous in both shales. In the Marcellus, joints transect foliation in the shales and (less pervasively) the few calcareous beds. Joints are commonly attenuated by nodules. Fractured nodules are less common, and septarian nodules with internally-confined, radial fracturing are less prevalent than nodules with through-going foliation planes.

NW-directed shear zones cross-cut foliation AND fractures in several Marcellus outcrops. Shear zone thickness varies from 1-2 m on the NW limb of the anticline, to an extensive 10-15 m thick zone of complex folding and shearing on the SE limb. This complexly deformed horizon, the base of which is 5 m stratigraphically above the Needmore-Marcellus contact, includes many vertical nodules with abrupt terminations, suggesting disaggregation along pre-existing fracture planes. It is probable that this deformed zone is coincident with the weak interval in the basal Marcellus where the Tioga K-bentonite occurs throughout the region. We conclude that the regional joint sets in the Marcellus interval developed prior to Alleghenian folding and shearing, at least in this region of western Virginia.

AAPG Search and Discovery Article #90095©2009 AAPG Eastern Section Meeting, Evansville, Indiana, September 20-22, 2009