The Ashern Formation (Middle Devonian) in the Williston Basin, North Dakota

• Main Author: Lobdell, Frederick K.
• Format: Text
• Language: unknown
• Published: UND Scholarly Commons 1984
• Subjects: Petrology > North Dakota; Geology; Stratigraphic > Devonian; Geology > Williston Basin; Northwest Territories
• Online Access: https://commons.und.edu/theses/176; https://commons.und.edu/context/theses/article/1175/viewcontent/Lobdell__Frederick_K.__MS_Thesis_1984.pdf

The Ashern Formation (Middle Devonian) is the basal unit of the Kaskaskia Sequence in North Dakota. It unconformably overlies the Interlake Formation (Silurian) and underlies, probably paraconformably, the Winnipegosis Formation (Middle Devonian). The Ashern is present in the northwestern one-half of the state, and attains a maximum thickness of 180 feet (55 meters) The Ashern Formation may be subdivided into a lower red member and an upper gray member. The latter directly overlies the Interlake Formation where the red member is absent. Both members are argillaceous, microcrystalline dolostones containing minor quartz silt. Anhydrite, present in both members, is much more abundant in the red member, frequently forming beds or layers of nodules. The red member is present in a north-south trending, linear band in western North Dakota. This area probably was weakly positive and remained emergent during the initial Ashern transgression. Coastal sabkhas developed on this peninsula, or large island, in the early Ashern sea, which was located between 10 and 20 degrees south latitude. These sabkhas are represented by the red microcrystalline dolostones and nodular anhydrite of the red member. As the transgression continued, the sabkhas were inundated by rising sea level and the entire area became a restricted embayment These subtidal sediments became the microcrystalline dolostones of the gray member. Continued transgression produced the normal-marine, biogenic sediments of the Winnipegosis Formation Except for calcispheresin the gray member, no fossils have been found. However, stratigraphic relationships and datable faunas in central and northern Saskatchewan and Alberta and southern Northwest Territories strongly suggest an Eifelian (early Middle Devonian) age for the Ashern.