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AAPG Bulletin, V.
Morphology of the Casper Mountain uplift and related, subsidiary structures, central Wyoming: Implications for Laramide kinematics, dynamics, and crustal inheritance
Donald S. Stone1
16178 South Lakeview Street, Littleton, Colorado, 80120; email: [email protected]
Donald S. Stone is an independent geologist and consultant living in Littleton, Colorado. He received his M.A. (1951) from Cornell University, with majors in structural geology and stratigraphy, and began his career as a petroleum geologist with Chevron in California (onshore and offshore). He has spent the last 35 years working the subsurface geology of the Rocky Mountain area with particular emphasis on interpretation of foreland structure.
Many organizations and individuals supported this investigation with contributions of critical data. I thank Amalgamated Explorations, Inc.; Texaco U.S.A.; Union Pacific Resources; Amerada-Hess; Mobil; and Wold Oil for their contributions. Earth Satellite Corporation provided the image in Figure 2A. Special thanks are due to Murray Dahill, owner of the Iron Creek oil field, who supplied critical data not available anywhere else and served as a guide on two field trips through the Casper Mountain area. Amgad Youres and Bill Muehlberger provided copies of several important papers consulted during the formulation of the concepts developed here. An early draft of this article was reviewed by Murray Dahill, Eric Erslev, Rick Groshong, and Randy Ray, and their critical comments and suggestions were carefully considered in early revisions. Final revision of the manuscript greatly benefited from AAPG reviews by Jon Olson, Jim Lowell, and Peter Hennings.
The general east-west trend of the regional-scale, fault-related Cas per Mountain uplift in central Wyoming reflects a preexisting Pre cambrian fabric along which there was Laramide compressional re activation. Initial fault displacement on the south-dipping Casper Mountain fault zone probably predates displacement on the inter secting, northwest-trending, northeast-dipping Casper arch thrust that forms the northeastern border of the Wind River basin. Later phase, incremental, Laramide displacements occurred along both fault zones: left-oblique slip on the Casper Mountain fault zone, dip slip on the crosscutting Casper arch thrust. Basement-involved thrust generation of the subsidiary Laramide, northwest-trending Iron Creek and Emigrant Gap anticlines along the north (footwall) side of the Casper Mountain fault zone also occurred during this later phase Laramide deformation.
Perturbation of far-field, northeast-southwest, Laramide max imum horizontal paleostress trajectories (s1) to local, nearly fault-normal orientation at Casper Mountain, together with strain par titioning, is proposed to explain the apparent coeval development of divergent hanging-wall and footwall, basement-involved, fault-related structures. This kinematic and dynamic interpretation may be applied conditionally to other east-west-trending Laramide up lifts in the central Rocky Mountain foreland province.
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