Distribution and Thickness of Devonian Rocks in Williston Basin and in Central Montana and North-Central Wyoming
Sandberg, Charles A.
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The report area includes approximately 200,000 square miles in the southern part of the Williston basin of North Dakota, South Dakota, and eastern Mon- tana and in central Montana and north-central Wyoming. Devonian rocks underlie all but the south-central part of this large area, and they crop out on the flanks of the Bighorn, Pryor, Beartooth, Big Snowy, and Little Rocky Mountains and the Absaroka Range. The Devonian rocks consist predominantly of marine carbonate, evaporite, and shale beds, which have a maximum thickness of 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, generally less than half a mile in length, in the Bighorn, Pryor, and Beartooth Mountains and in the Absaroka Range. Middle Devonian rocks underlie the central Williston basin but do not crop out in Montana or Wyoming. They reach a maximum thickness of about 870 feet in north-central North Dakota. The Middle Devonian series is divided into the Elk Point group and the overlying Dawson Bay formation. The Elk Point group consists of the Winnipegosis and Prairie formations. The Winnipegosis and Dawson Bay formations are composed mainly of limestone and dolomite, but the Prairie is largely halite and anhydrite with a little sylvite. Upper Devonian rocks underlie most of the area studied and attain 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, overlain by the Bakken formation of Late Devonian ( ?) and Early Mississippian age. The Souris River,Duperow, and Birdbear formations were deposited in similar sedimentary cycles and their lithologies consist of various quantities of limestone, dolomite, dolomitic limestone, silty and argillaceous dolomite and limestone, and anhydrite with thin interbeds of shale, siltstone, and sandy argillaceous dolomite. The Three Forks formation contains calcareous and dolomitic shale with interbeds of limestone, dolomite, anhydrite, and siltstone. The lower half of the formation is commonly anhydritic, and sandstone is present locally at the top. The Bakken formation consists of two black carbonaceous shale beds separated by gray sandstone, siltstone, and dolomite. Tectonic activity occurred along several major anticlines and one large re- verse fault during Middle and Late Devonian and earliest Mississippian time. These structural features were eroded in Early Mississippian time and buried beneath thick Mississippian deposits; they are ancestral to but have slightly divergent trends from similar Laramide features. The ancestral structural features greatly influenced Devonian sedimentation and may have trapped large amounts of oil and gas in Devonian and older strata. Minor oil production has already been established from the Winnipegosis, Dawson Bay, Souris River,Duperow, and Three Forks formations, and many near-commercial oil shows have been found in the Birdbear formation.