Abstract

The Deadwood Formation (Cambro-Ordovician) ranges in thickness from zero to more than 900 feet (270 m), and represents the earliest recorded Paleozoic deposition in the Williston Basin. Six correlatable lithostratigraphic units, informally described as members A through F, can be recognized in the Deadwood Formation. 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 lower members, A and B, are of Cambrian age (Dresbachian-Trempealeauan), and represent the initial deposits of the Late Cambrian transgression in the basin. They are present over a larger area than the other members, and underlie most of North Dakota, northern South Dakota, eastern Montana, extreme southwest Manitoba, and southern Saskatchewan. The upper members of the Deadwood Formation, C to F, are of Early Ordovician age (Tremadocian-Arenigian). The areas over which they occur are less extensive than for the lower members, and their areal extent becomes more limited with higher stratigraphic position; the uppermost (F) only occurs in the deepest part of the basin in western North Dakota.

The Winnipeg Group (Middle Ordovician) attains a maximum thickness of about 448 feet (137 m) in the center of the basin. It unconformably overlies the Deadwood Formation over most of the Williston Basin, except in extreme eastern North Dakota, most of Manitoba, and eastern Saskatchewan, where it lies directly on Precambrian basement rocks. The Winnipeg Group consists of three formations, in ascending order the Black Island, Icebox, and Roughlock formations. The Black Island Formation is predominantly sandstone with subordinate amounts of shale; in North Dakota, two members of the Black Island can be recognized: the lower Hawkeye Valley Member, deposited in fluvial and deltaic environments, and the upper Garland Member, a shallow marine unit. The Icebox Formation is a fossiliferous, bioturbated shale, with some sandstone lithosomes in Manitoba and eastern North Dakota, and was deposited in an offshore marine environment. The Roughlock is a fossiliferous, calcareous shale, and was deposited farther from shore than the Icebox. The Roughlock is transitional between the shales of the Icebox and the carbonates of the overlying Red River Formation. The three formations are, in part, lateral facies equivalents.

The Deadwood Formation was deposited as the sea rose over the eroded Precambrian basement. During this sea level rise, there were several smaller-scale transgressive-regressive episodes. In mid-Ordovician time (latest Llanvirnian), there was a major sea level fall. 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 had risen enough to begin deposition of the Winnipeg Group. The first evidence of this is the fluvial-deltaic deposits of the Hawkeye Valley Member of the 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 remainder of the Black Island was deposited as a transgressive systems tract over the eroded Deadwood Formation, As sea level continued to rise, the offshore shales of the Icebox Formation and the calcareous shales and argillaceous limestones of the Roughlock Formation were deposited.

The Deadwood-Winnipeg interval spans the time during which the Williston Basin began to subside, and became a recognizable basin. The initial subsidence of the Williston Basin in North Dakota occurred at a time no later man during deposition of member D of the Deadwood Formation (late Tremadocian), and perhaps even earlier. The initial subsidence seems to have occurred in a north-south elongate zone in western North Dakota; subsidence rates were comparatively high by the end of Deadwood time, and remained high throughout Winnipeg deposition.