ABSTRACT

The rocks along the southeast flank of the Bighorn Mountains range from Precambrian to Quaternary, but include no Ordovician, Silurian, or Devonian. Two Jurassic formations characteristic of central Wyoming, the Nugget sandstone and Gypsum Spring formation, are lacking. Tertiary deposits are present along the southern margin and at places higher in the mountains.

The top of the Middle Cambrian Flathead sandstone makes an upward migration in time toward the northeast of 22 feet per mile. The Upper Cambrian Du Noir member of the Gallatin formation tongues out eastward within the area. The presence of remnants of the basal Phosphoria Nowood member implies early Permian deposition and erosion prior to deposition of younger Phosphoria strata. Eastward-extending dolomite and limestone Phosphoria tongues alternate with westward-extending red shale tongues. A restricted use of the name Crow Mountain sandstone member is applied to the Triassic Chugwater interval between the Alcova and the Popo Agie members.

A high-angle reverse fault in the southern part of the area is probably related to the southward thrusting of the Bighorn Mountains during the earliest Eocene. Intense faulting in Mesozoic rocks at the southern margin of the mountains suggests proximity to a main thrust zone covered by younger Tertiary deposits. Northwest-trending folds and faults probably developed later than the thrust faulting.

Five oil tests, including two within the area, are dry holes. Four of these tests had oil staining in the Phosphoria; one had staining in the Tensleep. A gentle dome in the southeastern part of the area has not yet been tested.

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