DISENTANGLING CONFLICTING STRATIGRAPHIC SUBDIVISIONS THROUGH AMMONOID BIOSTRATIGRAPHY, THE SHARON SPRINGS MEMBER OF THE PIERRE SHALE FORMATION (UPPER CRETACEOUS), WESTERN KANSAS

The Upper Cretaceous Pierre Shale Formation deposited in the Western Interior Seaway is composed of gray to black marine shale in Western Kansas. Stratigraphic correlation of the Pierre Shale is complicated due to the east-west facies changes and the inconsistent subdivision of the Sharon Springs Member, which has been subdivided into two or three units in western Kansas. A biostratigraphic study based on the ammonoids in fossil-bearing concretions in the Sharon Springs Member has clarified the stratigraphic subdivision to aid in regional correlation. The member spans 6 ammonite zones. Abundant calcitic concretions are present in the middle part of the member. They are commonly about 2 m in diameter and 0.5 m in thickness. The concretions contain abundant ammonoid Baculites mclearni (540 specimens), and some fish scales and fragments of large mollusks Inocernamus sp. Baculites mclearni has a moderately complex suture pattern, widely spaced flank ribs, and a smooth to weakly ribbed venter, and varies in size from 2 to 35 mm. This is the first finding of Baculites mclearni in the Sharon Springs Member in Western Kansas. In addition, the abundance of phosphatic nodules, sepatarian concretions, cone-cone structures, and color varies on the outcrop section. The faunal and stratigraphic features can be used to clearly subdivide the Sharon Springs Member into lower, middle, and upper units. The three-unit subdivision suggests variations in the water chemistry, water depth, and ocean circulation pattern during the deposition of the Sharon Springs Member in western Kansas.