ABSTRACT

The Winnipeg Group (Middle Ordovician) unconformably overlies the Deadwood Formation (Cambrian and Early Ordovician) in most of North Dakota, except in the extreme eastern part, where it lies directly on the Precambrian basement rocks. The Winnipeg is conformably overlain by the Red River Formation (Late Ordovician), and reaches a maximum thickness of 448 ft (137 m) in the center of the basin. The Winnipeg Group consists of three units, the Black Island, Icebox, and Roughlock Formations. Herein are proposed names for previously unnamed members of the Black Island Formation, the Hawkeye Valley and Garland.

The Black Island Formation consists of sandstone and lesser amounts of shale and is interpreted as deltaic and shallow marine deposits. It unconformably overlies the Deadwood Formation, except in extreme eastern North Dakota where it lies directly on Precambrian basement. The Icebox Formation is a fossiliferous, bioturbated shale with minor sandstone, and is an offshore deposit. It has gradational contacts with both the underlying Black Island and the overlying Roughlock. The Roughlock Formation is a fossiliferous, calcareous shale deposited in a deeper marine environment than the Icebox.

At the end of the Early Ordovician there was a major sea level drop, causing erosion of much of the Deadwood Formation. Sea level rose again during the Early Ordovician, and the basal Black Island was deposited unconformably on the Deadwood. As sea level continued to rise, more Black Island was deposited, followed by offshore marine Icebox shales, more distal Roughlock calcareous shales, and finally Red River limestones.