Date of Award
Master of Science (MS)
The Winnipeg Group (Upper Ordovician) unconformably overlies the Deadwood Formation (Cambrian - Lower Ordovician) over most of North Dakota, except in the extreme eastern part, where it lies directly on Precambrian basement rocks. The Winnipeg is conformably overlain by the Red River Formation (Upper Ordovician). The Winnipeg reaches a maximum thickness of 448 feet (136.6 m) in the center of the basin, Williams County. The Winnipeg Group consists of three formations, in ascending order, the Black Island, Icebox, and Roughlock Formations. The Black Island Formation is, herein, formally divided into the Hawkeye Valley and the Garland Members.
The Hawkeye Valley Member consists of two lithofacies: a red-bed lithofacies and a green quartz wacke lithofacies. The red-bed lithofacies consists of two lithotypes: a red quartz arenite and a red clayshale. The Hawkeye Valley Member reaches a maximum thickness of 128 feet (39 m) in Williams County and is restricted to the western half of North Dakota and the immediately surrounding areas. Abundant desiccation cracks and the distinctive red color strongly suggest that the red-bed lithofacies was deposited in a subareal environment. The red-bed lithofacies represents a fluvial/deltaic environment and the green quartz wacke lithofacies represents a.nearshore marine or lagoonal environment. The Garland Member consists of two lithofacies: a quartz arenite lithofacies and a green quartz wacke lithofacies. The green quartz wacke lithofacies is similar in character to the green quartz wacke lithofacies within the Hawkeye Valley, which represents the initial deposits of the Late Ordovician transgression. The quartz arenite lithofacies consists of three lithotypes: a bioturbated quartz arenite, a structured quartz arenite, and a structureless quartz arenite. The Garland Member reaches a maximum thickness of 169 feet (52 m) in McKenzie County. Prominent sedimentary structures and good sorting in the structured quartz arenite indicate that deposition occurred in an active foreshore or nearshore environment. The high degree of bioturbation and lack of sedimentary structures in the bioturbated quartz arenite lithotype indicate deposition in a relatively low-energy environment below normal wave base. The lithofacies found within the Garland Member thus represents a shallow marine environment.
The Icebox Formation is a fossiliferous, bioturbated shale with occasional sandstones, and represents an offshore deposit. The Icebox Formation reaches a maximum thickness of 167 feet (51 m) in Grand Forks County; the maximum thickness in the center of the basin is 156 feet (48 m) in McKenzie County. The Roughlock Formation is a fossiliferous, calcareous shale deposited in a deeper marine environment. The Roughlock reaches a maximum thickness of 95 feet (29 m) in the extreme eastern part of North Dakota and grad1.µUly thins to the west. The contacts between all of the units within the Winnipeg are intertonguing and gradational.
At the end of the Llanvirnian (Middle Ordovician) there was a major sea level drop, causing erosion of much of the Deadwood Formation. Sea level rose again during the Chazyan Stage (latest Middle Ordovician), and the Hawkeye Valley was deposited unconformably on the Deadwood. As sea level continued to rise, the Garland was deposited, followed by offshore marine Icebox shales. Local topographic highs probably had sufficient relief to cause the formation of sand bodies during Icebox deposition. Still farther offshore, where the influx of clays was minor, the calcareous shales of the Roughlock were deposited, and finally deposition of the Red River limestones occurred.
Ellingson, Jonathan B., "Depositional environments and paleogeography of the Winnipeg Group (Ordovician), Williston Basin, North Dakota" (1995). Theses and Dissertations. 78.