Abstract

Correlation of 16 Frontier Formation surface sections and two wells extending from the western Wind River Basin, Wyoming to the eastern Powder River Basin, and from south-central Wyoming near Rawlins to southern Montana is shown on four cross-sections. Correlation is based on the first occurrence of Deflandrea (Rigaudia) cf. D. acuminata, along with four dinoflagellate cyst species present in the Lower Cretaceous, but which became extinct in the lower part of the Frontier and Belle Fourche Formations. These four species are Palaeoperidinium caulleryi, Ascodinium, cf. A. verrucosoum, Dinoflagellate A, and Coniferatium frontierensis. The first occurrence of Dinogymnium heterocostatum near the base of the overlying Cody and at the base of the Greenhorn Formation at Newcastle, Wyoming, parallels the Deflandrea datum and enforces postulated time-stratigraphic relationships.

The mean Frontier shoreline during any 100-foot time rock interval is located along the western margin of the study area by constructing six 100-foot slice maps, and identifying the aggregate continental thickness based on palynomorph or microplankton recovery. Interpretation of each 100-foot interval reveals areas of maximum and minimum sedimentation suggesting delta building, and subsequent destruction.