The Swift-Rierdon nomenclature of the Ellis group in Montana is applied in eastern Wyoming to formational units of the Sundance group. Indications are that the names are applicable to the marine Upper Jurassic throughout most of Wyoming and western South Dakota, but the term “Sundance” should be retained because of historical significance.
Thickness maps indicate that the distribution of the Rierdon sediments in eastern Wyoming was influenced by a minor depositional basin, the “Powder River embayment,” corresponding in general with the present Powder River Basin, and a positive-trending feature, the “Sheridan arch,” present across north-central Wyoming during the early part of Upper Jurassic time. Micropaleontologic studies indicate the presence of two distinct Rierdon ostracode biofacies whose distribution is apparently governed by environmental factors associated with presence of the Sheridan arch.
The Swift sediments are lithologically and paleontologically distinct from those of the Rierdon, but show a less definite pattern of distribution, probably because of ineffectiveness of the Sheridan arch during their deposition. For a similar reason, only a single ostracode assemblage is present in the Swift formation. Two ostracode zones are recognized in this assemblage.