Structural analysis of basement-involved anticlines along the western margin of the Bighorn Basin, Wyoming

The western margin of the Bighorn basin, Wyoming, is marked by a series of basement-cored anticlines formed along steep, easterly-dipping reverse faults. The major faults in the western Bighorn basin are generally steep reverse faults. Some of these reverse faults are interpreted to detach to the brittle-ductile transition area at deep continental crust. These master reverse faults and associated structures are termed first-order faults/structures. There are steep reverse faults branch from these first-order faults and are termed as second-order faults. The surface anticlines associated with the second-order faults, such as Rattlesnake Mountain, Little Buffalo Basin, Grass Creek and Hamilton Dome anticlines, are characterized by short steeply-dipping forelimbs and long, gently-dipping back limbs.;In most cases basement is faulted but relatively undeformed by penetrative shear (Mode 1). However, in some cases, basement is deformed within a limited zone of penetrative deformation (Mode 2). In both modes of deformation the cover rocks typically deform within a upward widening quasi-triangular zone of deformation (Trishear zone). The cover rocks involved in the deformation consist of alternating clastic and carbonate sediments of Cambrian to late Cretaceous age above Precambrian granitic basement. Based on regional geologic mapping, structural cross section construction, restoration and 3-D modeling, the second-order structures along the western margin of the Bighorn basin are interpreted as back-limb structures on large first order anticlines, such as the Oregon Basin anticline. In some cases first-order anticlines are broader structures and show lower structural relief compared to the corresponding second-order structures. Backlimb and forelimb dips of these anticlines can be interpreted to be linked with the synclinal and anticlinal bends of blind thrusts within the basement.;The Rattlesnake Mountain anticline, controversial in the literature, is interpreted here as a forelimb deformation related to the Line Creek/Oregon Basin master fault system. This second-order structure is interpreted to have formed by Mode 2 type basement deformation on a backthrust branching from the Line Cree/kOregon Basin master fault system. This interpretation does not require sub-horizontal dip of major backthrust within shallow crustal depth as some previous interpretations have hypothesized. Also 3D geologic model used in this study accounts to better consistency of interpretation of this and other structures described in this study.;The Hamilton Dome and the Grass Creek anticlines show strong attenuation of Cretaceous shales along the forelimb. Structural thinning and stretching of the forelimb accommodates the displacement of the basement fault which is unexposed at the surface. Relatively smaller scale secondary detachment folds, which detach in the lower Cretaceous shales and the Triassic Chugwater Formation, are present in this area. These include several producing structures such as the Pitchfork, Spring Creek, North and South Sunshine fields, which produce from the Cretaceous Frontier, Permian Phosphoria, and Pennsylvanian Tensleep formations. Understanding the 3-D geometry of these basement-involved structures is critical in unraveling the structural evolution of the Bighorn Basin and in interpreting trap geometries in the Rocky Mountain Foreland basins.