Determining the Controls on Extensional Fracturing in the Wyoming Foreland
University of Wyoming Department of Geology and Geophysics Laramie, Wyoming
The timing and origin of extensional fractures (joints) in the Laramide Foreland are highly debated. Previous studies have interpreted NW-striking foreland fractures as being pre- or syn-Laramide (Hennings et al., 2000; Bergbauer and Pollard, 2004; Bellahsen et al., 2006). But, similar NW-striking fractures have been observed in post-Laramide rocks in several foreland basins (Erslev, verbal communication; Ryan Thompson, verbal communication).
Fractures create permeability anisotropy that profoundly effects hydrocarbon production. Insight into fracture orientations, densities, and sealing requires knowledge of the origin of fractures. Ultimately, this study will give insight into fracture orientations and sealing at depth, contributing to our understanding of the controls on the productivity of hydrocarbon reservoirs.
Minor fault and joint orientation data was collected at Oil Mountain Anticline, Alcova Anticline, Bessemer Mountain, and in the eastern-most boundary of the Wind River Basin. Minor strike-slip and thrust fault data collected on the backlimb of the Laramide Arches corresponds with regional Laramide compression of 067°. Dip-slip faults with strikes of 095°-110° on the hinge of Oil Mountain and Alcova Anticline may be from outer-arc stretching, localized arch collapse, thrust back-sliding, or erosional unloading. Post-Laramide and syn-Laramide rocks are fractured by west-northwest striking joints. Hypothesized jointing mechanism include regional tectonic deformation, localized arch collapse, thrust back sliding, erosional unloading, and flexural unfolding of the lithosphere.
In conclusion, the Laramide compression direction of 067° holds true throughout the arches in this study. The controversial west-northwest joint set found the post-Laramide rocks must have occurred after Laramide tectonics ceased.
AAPG Search and Discovery Article #90094 © 2009 AAPG Foundation Grants in Aid