Date of Award
Master of Science (MS)
The Frobisher-Alida interval is a subsurface, marker-defined unit, one of several units designated for rocks of the Madison Group (Mississippian) in the Williston Basin of North Dakota. The Frobisher Alida conformably overlies the Tilston interval and is conformably overlain by the Ratcliffe interval. 1t attains a maximum thickness of a little over 400 feet (123 m) and thins to an erosional edge along the eastern margin of the Basin.
The Frobisher-Alida is composed of limestones, dolomitic limestones, and dolostones of the Mission Canyon Formation and anhydrites of the overlying Charles Formation which exist in a complex facies relationship within the interval. Thin-section and core analyses indicates the presence of eight lithofacies: skeletal wackestone and packstone, skeletal grainstone, calcisphere wackestone and packstone, intraclast grainstone, fenestral wackestone and packstone, pisolite ooid wackestone and grainstone, dolomudstone, and anhydrite. These lithofacies were deposited in open marine, protected marine, and restricted marine envirorunents, tidal flats, coastal sabkhas, and inner sabkha salt pans. The marginal marine and sabkha environments existed in a digitate morphology instead of parallel, linear, environmental belts common in other paleogeographic models. The Frobisher-Alida interval represents a lime mud to sabkha cycle deposited during a regression of the Madison sea in the Williston Basin. This regression was part of several transgressive-regressive cycles evident in the Madison Group.
Eogenetic, mesogenetic, and telogenetic stages of diagenesis affected rocks of the Frobisher-Alida interval. Internal sediments, some calcite cement types, and dolomite replacement by evaporative pumping and seepage refluxion occurred during eogenetic stage diagenesis. Other calcite cement types, sulphate and saddle dolomite cement, pressure solution, some dissolution, and mineral replacement by anhydrite and some dolomite occurred during mesogenetic stage diagenesis. Some dissolution, calcite cementation, and dolomite replacement may have been the result of telogenetic stage diagenesis, possibly involving the mixing of connate and telogenetic meteoric waters.
Obelenus, Thomas J., "Depositional environments and diagenesis of carbonates and associated evaporites, Frobisher-Alida interval, Madison Group (Mississippian), Williston Basin, northwestern North Dakota" (1985). Theses and Dissertations. 215.