Depositional environments and history of the Winnipeg Group (Ordovician), Williston Basin, North Dakota

The Winnipeg Group (Ordovician) in the Williston Basin, North Dakota, contains three formations. They are, in ascending order, the Black Island Formation, herein informally divided into lower and upper members, the Icebox Formation, and the Roughlock Formation. Strata of the Winnipeg Group (maximum thickness 400 ft., 122 m) represent the initial deposits of a Middle Ordovician craton-wide transgression. Throughout most of North Dakota, the Winnipeg is unconformably underlain by the Deadwood Formation (Upper Cambrian-Lower Ordovician) and conformably overlain by the Red River Formation (Upper Ordovician). The strata of the lower member of the Black Island Formation consist of a red-bed lithofacies, containing reddish brown and dark greenish gray clayshale and reddish brown hematite-cemented quartz arenite, and a green quartz wacke lithofacies, composed of greenish gray, friable, quartz wacke. The upper member of the Black Island has been divided into a quartz arenite lithofacies, composed of thickly bedded, medium to light gray, quartz arenite and bioturbated quartz wacke, and a green quartz wacke which is similar in lithology to the green quartz wacke of the lower member. The Icebox Formation is composed mostly of greenish gray, noncalcareous, in places bioturbated and fossiliferous clayshale. The Roughlock Formation consists mostly of argillaceous, fossiliferous, nodular limestone. Rocks of the red-bed lithofacies of the lower member of the Black Island Formation are interpreted to have been deposited on a deltaic plain. The quartz arenite lithofacies of the upper member of the Black Island is interpreted to be a nearshore deposit. The green quartz wacke lithofacies of both the lower and upper members is considered to be transitional between the deltaic plain and nearshore deposits and may be the initial deposits of the Middle Ordovician transgression. Rocks of the Icebox Formation are interpreted to have been deposited in an offshore environment. Bioturbation and fossils indicate that mild oxidizing conditions existed in the uppermost Icebox sediments. Rocks of the Roughlock Formation were also deposited in an offshore environment. Reduction in detrital sedimentation permitted the transition from the clayshale of the Icebox to the argillaceous limestone of the Roughlock. A further reduction in detrital input resulted in the transition from the Roughlock to the limestone of the Red River Formation. The formations of the Winnipeg Group are at least partial facies equivalents of each other. Intertonguing between the lower and upper members of the Black Island and between the upper member of the Black Island and the Icebox substantiates the existence of a facies relationship between these units. Isopach maps, sandstone-shale ratio maps, and kaolinite-illite ratio maps suggest that the Middle Ordovician marine connection to North Dakota was to what is now the southeast through Minnesota rather than to the present southwest, as was previously assumed. This implies a sag or break in the Transcontinental Arch at that time. Cross sections of the Deadwood Formation and of formations of the Winnipeg Group, together with isopach maps of those units, suggest that the initiation of subsidence of the Williston Basin coincided with the beginning of Winnipeg sedimentation. The area of greater thickness of the underlying Deadwood Formation near the center of the Williston Basin is due, at least partially, to post-depositional erosion of the Deadwood rather than to subsidence during Deadwood deposition