The Ashern Formation (Middle Devonian) is the basal unit of the Kaskaskia Sequence in North Dakota. It is unconformably underlain by the Silurian Interlake Formation, and overlain, generally conformably, by the Winnipegosis Formation of Middle Devonian age. The Ashern is present in the northwestern one-third to one-half of North Dakota. Beyond the limits of the overlying Winnipegosis Formation, the Ashern is indistinguishable on electric logs from the basal argillaceous members of the Dawson Bay and Souris River formations and, together with these, must be considered undifferentiated basal Devonian.

The Ashern Formation is composed of two members. The lower red member is predominantly an argillaceous microcrystalline dolostone, containing nodular anhydrite and thin shale partings. The upper dark gray member is predominantly a featureless microcrystalline limestone. Together, these two members range in thickness from about 10 m near the limit of the Ashern Formation to about 50 m near the center of the Williston basin.

The red member owes its color to a reworking of a lateritic soil on top of the Late Silurian/Early Devonian erosional surface. The fine-grained dolostone and its nodular anhydrite imply supratidal deposition in a sabkha environment. The featureless gray member was deposited in a low intertidal to subtidal environment. There is an occasional suggestion of bioturbation, though no fossils have been found. No porosity is evident in the formation, except for some partly anhydrite-filled fractures near the top in one core.

End_of_Article - Last_Page 595------------