--> The Cat Creek Anticline, Central Montana: Recurrent Faulting and Oil Migration, by W. J. Nelson; #90986 (1994).

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Abstract: The Cat Creek Anticline, Central Montana: Recurrent Faulting and Oil Migration

W. John Nelson

The Cat Creek anticline is a Laramide fault-propagation fold that overlies a south-dipping, slightly listric, high-angle reverse fault in crystalline basement. The Cat Creek fault has undergone at least five episodes of displacement. It originated as a normal fault (southern block downthrown) along the northern margin of a Proterozoic failed rift (Montana aulacogen) during deposition of the Belt Supergroup. Then it was reactivated as a reverse fault (southern block upthrown) in Late Cambrian time; and again near the end of the Devonian Period. Normal faulting, with as much as 1,500 feet of throw, took place between Middle Pennsylvanian and Middle Jurassic time. Finally, during the Laramide orogeny (early Tertiary) the Cat Creek fault underwent oblique left-lateral/reverse displacement The dip-slip component was as great as 4,000 feet, while the strike-slip component was roughly 5,000 feet. A belt of en echelon domes and normal faults developed along the crest of the anticline in response to wrenching.

Montana's first significant oil discovery (1920) was at Cat Creek. Only 4 out of 10 domes have yielded oil, although all have closure and suitable reservoir rock. Timing of oil migration evidently was the critical factor. Maximum burial and hydrocarbon generation probably occurred in Cretaceous time; therefore oil is found only where traps existed both before and after Laramide deformation. Still a mystery is the source rock of Cat Creek's unique high-gravity crude.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994