Seismic expression of Buffalo deep fault, Buffalo, Wyoming

A concealed, west-dipping, high-angle reverse fault, one of a pair first reported by N.H. Foster, P.E. Goodwin, and R.E. Fisher in 1969, has been interpreted from seismic profiles in the area south and west of Buffalo, Wyoming. The fault, named the Buffalo deep fault (BDF) by D.L. Blackstone, Jr., in 1981, trends generally north-northwest and dips westward along the deepest part of the Powder River basin near the western margin. The projected surface trace lies as much as 14 mi east of the nearest outcrop of Precambrian rocks on the eastern flank of the Bighorn Mountains. Offset on the BDF decreases upward through the Paleozoic section into younger rocks that have been folded into the synclinal bend of an east-facing monocline. The monocline, the BDF, and another reverse fault appear clearly on a high-quality seismic profile that trends east-west about 8 mi southwest of Buffalo, Wyoming. This profile is one of many that have been surveyed across the mountain front. Maximum throw across the BDF is about 4500 ft from the base of the Phanerozoic section. This throw, plus the steepened dips on the upthrown western block, account for only part of the large uplift on the Bighorn Mountains. Themore » remainder of the uplift probably is the consequence of movement on younger thrust faults, strike-slip faults, and normal faults, many of which are exposed. The BDF, monocline, thrust faults, and strike-slip faults were formed by horizontal compression, a dominantly eastward component of movement for the Bighorn Mountain block with respect to the Powder River basin.« less