ABSTRACT

The Deadwood Formation in the North Dakota portion of the Williston Basin ranges in thickness from zero to more than 270 m (900 ft), and represents the earliest recorded Paleozoic deposition in the North Dakota part of the Williston Basin. Six correctable lithostratigraphic units, here informally described as members of the Deadwood Formation, can be recognized. The Deadwood Formation consists largely of siliciclastic rocks, principally quartz arenites, quartz wackes, and siltstones, and lesser amounts of carbonate rocks, with textures from mudstones to grainstones, and was deposited under marginal marine to offshore conditions.

The Winnipeg Group unconformably overlies the Deadwood Formation over most of North Dakota, and attains a maximum thickness of about 135 m (440 ft) in the center of the basin. The Winnipeg Group consists of three formations, in ascending order, the Black Island, Icebox, and Roughlock Formations. The Black Island Formation is composed of sandstones with subordinate amounts of shale, and represents fluvial/deltaic deposition in the lower part, which occurs only in western North Dakota, and shallow marine deposition in the upper part. The Icebox Formation is largely shale, with some local sandstone lithosomes, and was deposited in a deeper marine environment than was the Black Island. The Roughlock consists of argillaceous, fossiliferous limestone, and was also deposited in a deeper marine environment. The three formations are, in part, lateral facies equivalents.

The initial subsidence of the Williston Basin in North Dakota occured at a time no later than during deposition of member D of the Deadwood Formation (late Tremadocian), and perhaps even earlier, and does not appear to have been associated with major orogenic events elsewhere in North America. The area of greatest subsidence seems to have been initially elongate, and to have become more circular by Winnipeg time.

The Deadwood Formation was deposited during a gradual major eustatic sea level rise, which included several smaller-scale transgressive-regressive episodes. At the end of the Early Ordovician (latest Llanvirnian), there was a major eustatic sea level drop. Marine deposition in North Dakota ceased, and Deadwood strata were eroded from the newly exposed areas of the basin. By Late Ordovician time (early Caradocian), sea level was rising again. In North Dakota, the first evidence is the fluvial-deltaic deposits of the lower Black Island Formation, which were deposited in the area where the greatest amount of subsidence had taken place. With continued sea level rise, the deltaic deposits were covered, and the shallow marine sandstones of the upper Black Island were deposited, followed by the deeper marine shales of the Icebox Formation and, finally, by the argillaceous limestones of the Roughlock Formation.