GEOLOGIC MAPPING TO EVALUATE LARAMIDE BASEMENT INVOLVED FOLDING IN THE SOUTHEASTERN WIND RIVER MOUNTAINS

The southwestern margin of the Wind River basin contains a series of southwest verging, left stepping en-echelon folds that fold Paleozoic and Mesozoic strata dipping off the uplifted Precambrian core of the Wind River Range. Sheep Mountain anticline is the southernmost, up-plunge end of the fold sequence.  As such, it exposes deeper stratigraphic units and structural levels than other folds in the sequence, and may provide clues to the importance of basement faulting in the folding process. Detailed geologic mapping of the southern part of Sheep Mountain and associated faults suggest that basement faulting along the southern margin of Sheep Mountain may have contributed to complex variations in the local fold geometries. The Clear Creek fault (CCF), a steep southerly dipping, NE-SW trending regional reverse fault, marks the boundary between two structurally distinct zones. The northern zone contains Sheep Mountain anticline, which is SW vergent and plunges shallowly toward  ~N15°W. This fold is similar in geometry and structural orientation to the folds that extend northward along the basin-margin fold trend. The southern structural zone contains the Schoettlin Mountain Anticline (ScMA), which is bounded to the north by the CCF and to the south by the basement involved Beaver Creek thrust (BCT).  The BCT trends NW-SE and is south vergent along the southern margin of the ScMA, but abruptly curves northward becoming west vergent along the western margin of the fold. The ScMA is a variably plunging, NE-SW trending, basement cored anticline, with a complex fold geometry interpreted to be the result of fault related folding during offset on the BCT. Termination of the BCT and southern Sheep Mountain anticline at the CCF suggests that the CCF was present prior to folding both to the north and to the south of the fault. However, local drag folding along the south side of the CCF (northern limb of the ScMA) suggests that the youngest offset along the fault post-dates the earliest stages of folding in both the northern and southern fold zones.  These interpreted timing relationships and incompatible basement offsets between the ScMA and the Sheep Mountain anticline suggest that the BCT is not the surface expression of the controlling thrust along this en-echelon fold line.