--> ABSTRACT: Laramide Thrusting of Bighorn Mountains Onto Powder River Basin Near Buffalo, Wyoming, by J. A. Grow, E. N. Hinrichs, J. J. Miller, M. W. Lee, and S. L. Robbins; #91033 (2010)

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

J. A. Grow, E. N. Hinrichs, J. J. Miller, M. W. Lee, S. L. Robbins

Recent seismic surveys and exploratory drilling by industry for sub-thrust 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 (2,460 ft) of Precambrian granite gneiss before penetrating the thrust and entering 1,475 m (4,838 ft) of the Paleocene Fort Union Formation and another 2,199 m (7,214 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 1,768 m (5,800 ft) of granite before entering Upper Cretaceous strata. This borehole penetrated a total of 3,021 m (9,911 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 in 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) westward from the eastern edge of the basement thrust. The fault plane at the base of the Precambrian granites and gneisses dips 30° to the west. Although exploration for oil and gas plays beneath basement overthrusts along the Bighorn Mount in front has not been successful to date (fall, 1987), information derived from this effort provides valuable insight into the tectonic processes along Laramide overthrust zones and the techniques required for future exploration.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988