Laramide Thrusting of Bighorn Mountains Onto Powder River Basin Near Buffalo, Wyoming: ABSTRACT

Abstract

Recent seismic surveys and exploratory drilling by industry for subthrust oil and gas prospects beneath the Bighorn Mountain front along the western edge of the Powder River basin near Buffalo, Wyoming, reveal a basement-involved thrust of considerable magnitude. A deep test for oil and gas, the ARCO 1-4 Kinney Ranch borehole, was drilled 13 km (8 mi) west of Buffalo and penetrated 750 m (2460 ft) of Precambrian granite gneiss before penetrating the thrust and entering 1475 m (4838 ft) of the Paleocene Fort Union Formation and another 2199 m (7214 ft) of Mesozoic and Paleozoic sedimentary rocks. The Gulf Granite Ridge 1-9-2D borehole, which was drilled 31 km (19 mi) north-northwest of the ARCO borehole and 5 km (3 mi) northwest of Story, Wyoming, penetrated 1768 m (5800 ft) of granite before entering Upper Cretaceous strata. This borehole penetrated a total of 3021 m (9911 ft) of Mesozoic and Paleozoic sedimentary rocks. Chevron U.S.A., Inc., supplied to the USGS two very high-quality seismic reflection profiles near the Kinney Ranch and Granite Ridge boreholes. These profiles have been reprocessed by the USGS and integrated with surface geologic mapping, gravity surveys, and other geologic studies by the USGS in progress inmore » the Powder River basin. The seismic profiles near the Kinney Ranch and Granite Ridge boreholes clearly show that sedimentary rocks of the Paleozoic through the Paleocene, which occur beneath the thrust fault, extend more than 11 km (7 mi) west ward from the eastern edge of the basement thrust. The fault plane at the base of the Precambrian granites and gneisses dips 30/degrees/ to the west.« less