Structural analysis of basin margin laramide folding and fault reactivation in the south-central Wind River Basin, Schoettlin Mountain quadrangle, Fremont County, Wyoming
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] The southwestern margin of the Wind River basin contains a series of southwest verging, left-stepping, en-echelon Laramide-aged folds that fold Paleozoic and Mesozoic strata dipping off the uplifted Precambrian core of the Wind River Mountains. The origin of such basin margin folds and their mechanics of formation are significant to the understanding of Laramide fold mechanics and the structural localization of hydrocarbons. Geologic mapping of the southernmost, up-plunge end of these basin-margin fold structures shows the region to be divided into two structurally distinct zones: a northern zone containing Sheep Mountain anticline (ShMA) and a southern zone containing Schoettlin Mountain anticline (ScMA). The two structural zones contain fundamentally different structural orientations and basement-cover fault displacements. The ShMA is a SW vergent fold that plunges shallowly toward ~N15̊°W and is similar in geometry and structural orientation to other Laramide folds that extend to the northwest along the basin-margin fold trend. Folding of ShMA is interpreted to be controlled by a blind basement-involved thrust with a vertical throw of ~190 feet. Conversely, the Schoettlin Mountain anticline (ScMA) in the southern structural zone, is a variably plunging, ENE-WSW trending, basement cored anticline that is bounded to the south by the Beaver Creek thrust (BCT). The complex geometry of the ScMA is interpreted to be the result of fault-related folding along the BCT; however, the fault displacement along the BCT has an estimated vertical throw of ~3,000 feet, which is substantially larger than the ~190 feet inferred for ShMA. The clear boundary between the two distinct structural zones, the Clear Creek fault (CCF), is a steep southerly dipping, ENE-WSW trending fault with a south-side-up sense of displacement in the study area. Using results from geologic mapping, balanced cross-sections, fold-related fracture analyses, and a 2-D seismic reflection and refraction experiment, this study argues that both the ShMA and ScMA fold structures are fault-related forced folds. Furthermore, the CCF is interpreted to represent a pre-Laramide feature reactivated late in the overall development of the local folds. The sharp contrasts in fold orientations and fault offsets between the two structural zones are argued here to be a result of complex stress perturbations created by the reactivation of both the CCF and BCT.
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