Evidence of Late Quaternary Faulting along the Northeastern Segment of the Stagner Creek Fault in the Northwestern Wind River Basin, Wyoming

The Stagner Creek fault, located along the southern margin of the Owl Creek Mountains in central Wyoming, is one of several east-west striking Quaternary faults that may correspond with scattered, present-day, intraplate seismicity in the region. These Quaternary faults are peculiar in that they strike nearly orthogonal to the Basin and Range faults observed in western Wyoming, and their orientation may reflect the influence of inherited, Laramide structures. These east-west striking faults are inferred to be predominantly normal faults. Previous studies have documented the Quaternary activity of the Stagner Creek fault affecting 6 distinct alluvial fan southeast of the Boysen Reservoir. This study aims to expand on these prior efforts using geomorphic and geophysical analyses. In this area, the Stagner Creek fault is expressed as a prominent scarp, visible in air photos and satellite imagery. Microtopographic mapping was accomplished using a real-time kinematic GPS surveying. The resulting DEM provides detailed scarp morphology, as well as documenting possible knick points within the alluvial surfaces upstream from the scarp. The alluvial surface has a regional slope of 2-3 degrees, and the scarp face has a relatively shallow slope of 6-12 degrees. Scarp heights range from 0.4 to 2 meters. The minimum scarp may reflect a single faulting event, corresponding with a magnitude 6.5 - 6.7. These surfaces are believed to be Late Pleistocene and younger, and geochronological constraints are pending. A shallow seismic reflection profile acquired across the scarp imaged structure to depths of about 500 meters. The profile suggests a steep fault coincident with the surface scarp that offsets Tertiary reflectors several 10s of meters in the subsurface. The steep dip is similar to Laradmide structures. These results suggest that the Stagner Creek fault is capable of generating moderate magnitude earthquakes in this low-strain tectonic environment.