ABSTRACT

Understanding the diversity of structural trends in the Laramide foreland of the conterminous United States is important to understanding the location, geometry, and fracturing of hydrocarbon reservoirs. East-west basement-cored arches in central Wyoming are oblique to the average northwesterly trend of foreland faults and folds. Tectonic models predict that these arches formed by one of the following mechanisms: north-south–directed thrust faulting; sinistral strike-slip faulting; or northeast-southwest–directed, oblique-slip thrust faulting. In the eastern Owl Creek Mountains, average slip directions given by slickenline directions trend from N37°E to N57°E. Geometric analysis of conjugate faults and stress inversion of minor fault data indicate nearly horizontal compression trending between N48°E and N65°E. In the east-west Casper Mountain structure, more limited minor fault data are consistent with the northeast-southwest compression seen in the eastern Owl Creek arch and indicate an additional stage of extension by normal faulting.

The northeast-southwest compression documented by minor faults suggests oblique thrusting with a component of sinistral strike-slip on the underlying, east-west–striking Owl Creek and Casper Mountain thrusts. In this area of the Laramide foreland, east-west arches probably formed during a single stage of oblique slip on thrust ramps connecting northwest-trending arch culminations. This conclusion indicates that trap geometries and reservoir characteristics of foreland hydrocarbon accumulations are dependent on their obliquity to the regional stress field.

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