The east-west structural complex, known as the Big Snowy anticlinorium, is believed to be mid-Miocene in age. It was a stable area during the Laramide orogeny, which terminated during the Eocene. Injection of igneous material accompanied and followed the deformation in central Montana. The then increase of crustal thermal conditions probably aided the structural growth, which was principally vertical in expression rather than arcuate and compressional.

Jurassic formations overlie the Amsden, of Pennsylvanian age, throughout the structural province. Paleozoic structure is at variance with the observable surface geology (involving Paleocene through Jurassic formations). This ancient tectonic control is more significant in areas not affected by the mid-Miocene deformation that formed the big and obvious features like Porcupine dome, the Big Snowy Mountain dome, and Woman's Pocket anticline.

Generation and accumulation of oil and gas occurred under structural conditions that existed before the mid-Miocene tectonic pulsations. The present distribution of petroleum pools is due to traps that were not materially affected by the post-Laramide deformation, or traps that were due to secondary migration into newly formed structures. The latter condition has been subject to severe attrition by the increasing effect of artesian waters. The search for new oil should be guided by the more subtle geologic factors of Jurassic and pre-Jurassic stratigraphy and tectonics.

End_of_Article - Last_Page 416------------