ABSTRACT

A seismic profile across the central western rim of the Big Horn basin, Wyoming, provides evidence of major flank thrusting beneath the Oregon Basin anticline. Vertical separation at the Precambrian basement level on this westerly dipping thrust zone is at least 6 km (20,500 ft), as verified by a deep footwall test of a spurious subthrust structural closure. The displacement on the Oregon Basin thrust is the same order of magnitude as that measured on the Casper Arch thrust and about half of that on the Wind River thrust. Actual fault plane dip cannot be accurately determined on the seismic profile because of poor seismic returns from the subthrust block, but based on data from other seismic line crossings of the thrust it appears that the fault plane dips west at about 4 °. Under this assumption the amount of overhang at the Precambrian-sediment contact is at least 3 mi (4.8 km), and could be as much as 5 mi (8.0 km) if the fault plane is listric. Forward seismic modeling has helped in the selection of the most likely fault-plane solution and in understanding the attenuation of footwall seismic data.

The seismic profile traverses three major thrust-fold trends in the hanging-wall block (all with westerly asymmetry), each of which contains an important oil accumulation. From east to west, these are: Oregon Basin, Horse Center, and Half Moon.

In the footwall block of the thrust, Mesozoic and Paleozoic rocks beneath the Tertiary unconformity dip uniformly west at low angles, and this regional homoclinal dip continues for about 30 mi (48 km) across the Big Horn basin to the first line of thrust-folding (e.g., Garland trend) on the eastern basin flank.

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