Abstract

The south margin of the Powder River Basin is arcuate in shape and concave to the north. The southeastern segment from Lance Creek field to Orin Junction is in general related to the western flank of the Hartville Uplift. The margin from Orin Junction northwestward lies along the north end of the Laramie Mountains and subsidiary structures. The trend then changes to southwest and parallels the northwest flank of the mountains in the head waters of Deer Creek.

At two localities, Lance Creek field and South Glenrock field, thrust faults crop out and have been drilled in the subsurface. At one other locality, between Orin Junction and Flat Top field, thrust faulting is probably present but has not been confirmed by drilling. Maps and cross sections of these three areas indicate that the vertical separation on these faults ranges from 7000 ft to 12,000 ft at the level of the Precambrian basement.

The evidence suggests that either a single continuous thrust fault, or a system of reverse faults dipping toward the bounding highlands defines the southern margin of the Powder River Basin.