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AAPG Rocky Mountain Section Meeting

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Structural Divisions of the Bighorn Mountains and Western Powder River Basin

Abstract

The Bighorn Mountains and the western portion of the Powder River Basin are divided into 4 structural provinces based on a structure contour map drawn on top of the “Massive” Member of the Bighorn Dolomite. The Northern Province stretches from Muddy Creek on the Crow Reservation in Montana south to Little Goose Creek in Wyoming and is dominated by westdirected thrusting. The west flank of the Powder River Basin consists of homoclinal beds dipping at rates of 10 to 35 degrees on average. Depth to the Bighorn Dolomite is roughly -7,000 to -9,000 feet. The Central Province stretches from Little Goose Creek south to North Fork Crazy Woman Creek and is dominated by east-directed thrust-faulting. Dips along the west flank of the Powder River Basin are very steep to slightly overturned. Depth to the Bighorn Dolomite is between -10,000 and -11,500 feet. The Transition Province extends from North Fork Crazy Woman Creek south to Middle Fork Crazy Woman Creek and contains both east and west-directed thrusting. Westdirected thrusting is confined to the west flank of the Powder River Basin. East-directed thrusting is best exemplified by the Buffalo Deep Thrust. Depth to the Bighorn Dolomite is roughly -11,500 feet. The Southern Province runs from Middle Fork Crazy Woman Creek southeast to the terminus of the Powder River Basin. It is dominated by west-directed thrusting. The west flank of the Powder River Basin is again dominated by homoclinal dips of 10 to roughly 35 degrees. Depth to the top of the Bighorn Dolomite is roughly - 12,000 feet. At present horizontal production from the Niobrara Shale is confined to the Transition and the Southern structural provinces.