ABSTRACT

The Big Trails fault is traditionally defined as a northeast-trending, basement-controlled, high angle fault extending nearly 53 miles along the crest of the southern Bighorn Mountains. However, the authors have observed several features along this fault typical for strike-slip or oblique-slip fault models. The fault begins at the intersection of the Owl Creek and the Casper arch thrusts on the south and terminates on the east-west trending Tensleep fault to the north.

In an area mapped by the authors, the main fault trace is an eighth to a half mile wide, and characterized in map view by multiple braided fault strands. Variable displacement and reversals of displacement occur on the main fault trace and on subsidiary faults. Horizontal to near horizontal slickensides were measured along the fault; plunge of the striations ranged from 10-55° in a predominantly northerly direction. "Horsetail" fault splays mapped in the west fault block indicate drag from right oblique-slip. Horst and graben features identified in the fault trace correlate with confining and releasing bends and the trace of the main fault is near vertical in profile. Vertical offset, i.e., stratigraphic separation, which is down to the west along the majority the fault, shifts to down to the east near the north end. Separation ranges from almost zero on the north end, where vertical offset shifts to down to the east, to a maximum of more than 4500 feet near the middle of the fault. Horizontal offset in the form of right-slip is estimated at 2-3 miles.

The Big Trails fault is basement controlled and coincident with zones of weakness. These zones are defined by northeast trending quartz diorite, mafic, and amphibolite dikes of early Proterozoic and Archean age. We suggest that the fault may continue north of the Tensleep fault and connect with the Crazy Woman "tear" fault. The Crazy Woman fault forms the southern boundary of the Sisters Hill segment of the large east verging thrust feature along the eastern flank of the Bighorn Mountains, west of the town of Buffalo. The Big Trails fault is a result of a northeast-southwest oriented horizontal compressional event in the Rocky Mountain foreland during the Laramide orogeny.