Rocks of Devonian age underlie all but the south-central part of a 200,000 square mile area that includes

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the southern Williston basin in North Dakota, South Dakota, and eastern Montana and adjacent parts of central Montana and northern Wyoming. They crop out in the Bighorn, Pryor, Absaroka, Beartooth, Big Snowy, and Little Rocky Mountains.

These Devonian rocks consist predominantly of marine carbonates, evaporites and shales, and attain a maximum thickness of about 2,000 feet in northwestern North Dakota. Lower Devonian rocks assigned to the Beartooth Butte formation, an estuarine channel-fill deposit as thick as 150 feet, crop out at many isolated localities in north-central Wyoming and south-central Montana. Middle Devonian rocks underlie the central Williston basin but are not present at the surface. They reach a maximum thickness of about 870 feet in north-central North Dakota. The Middle Devonian series is divided into the Winnipegosis and Prairie formations of the Elk Point group and the overlying Dawson Bay formation. Upper Devonian rocks underlie most of the area studied and make up most of the outcrops. They at ain a maximum thickness of about 1,250 feet in northern Montana. The Upper Devonian series is divided, in ascending order, into the Souris River formation; the Jefferson group, consisting of the Duperow and Birdbear formations; and the Three Forks formation.

Several major anticlines and a large reverse fault have been interpreted from the isopach maps of the Devonian formations. These structural features were formed during Middle and Late Devonian and earliest Mississippian time and then buried beneath thick deposits of the Madison group (Mississippian). They controlled Devonian sedimentation and probably trapped large quantities of oil and gas in Devonian and older strata. Minor oil production has already been established from reservoir rocks in every Middle and Upper Devonian formation except the Prairie. Most of the Devonian oil fields are located on the trends of the pre-Madison features which diverge slightly from the trends of related Laramide features.

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