Structural Geology of the Piney Creek Thrust Area, Bighorn Mountains, Wyoming

Abstract

The Piney Creek thrust fault is a major secondary flank structure on the eastern side of the Bighorn Mountains, northwest of Buffalo, Wyoming. Precambrian crystalline rocks and overlying sedimentary rocks of the Piney Creek thrust fault and adjacent areas were investigated in structural detail to determine the nature and the extent of Precambrian control during Laramide deformation. The thrust fault has carried a wedge-shaped block of Precambrian and younger sedimentary formations to their present position, some 2.5 miles east of the normal position of the mountain front. Stratigraphic displacement is estimated at 10,000 feet. A prominent conjugate joint system is developed, normal to the bedding, in the brittle Paleozoic sedimentary formations. In the South Fork Rock Creek area, south of the thrust fault, joint sets have mean strikes of N. 40° E. and N. 66° E. On the Piney Creek thrust, mean fracture directions are N. 32° E. and N. 73° W. Comparison of the attitudes of the conjugate joint sets suggests that they were formed by a common direction of maximum stress at a time when the beds were essentially horizontal. This direction corresponds with a maximum stress direction of N. 54° E., which can be postulated for deformation, suggesting further that early Laramide deformation was compressive, and that later, vertical uplift predominated. In the Precambrian rocks, foliation is concluded to have no control on the trend of the range in this area, and minor, if any, control on the trend of faulting. Lineation is poorly developed throughout the area. Fracture cleavage, or closely spaced jointing, is locally very well developed and at some places can be definitely associated with shear movement. Very well-developed zones of jointing and fracture cleavage striking N. 17° E., probably guided Laramide movement along portions of the southern right-lateral tear fault of the Piney Creek thrust. No structure was found in Precambrian rocks to indicate control of the northern left-lateral tear fault. Eight joint sets can be distinguished within the Precambrian. Most are nearly vertical, and six are prominent. Three sets, which occur in almost every area studied are: (1) N. 70°–88° W.; (2) N. 40°–56° W.; and (3) N. 21°–44° E. Sets 1 and 3 correlate with similar trending sets in the sedimentary formations, indicating a probable Laramide age. Set 2 is also considered to be Laramide, resulting from the eastward thrust movement. Three other joint sets are locally prominent: (4) strikes N. 10°–17° E. and is a prominent shear direction in Precambrian rocks which guided Laramide faulting; (5) strikes N. 45°–58° E. and is questionably considered Precambrian; and (6) strikes N. 37°–47° W. and is a prominent Precambrian sheeting surface near the sedimentary contact.