WHY AREN’T THE NEBRASKA MEMBERS OF THE TECUMSEH SHALE (SHAWNEE GROUP, VIRGILIAN) RECOGNIZED IN THE TYPE TECUMSEH SHALE OF KANSAS?

The Tecumseh Shale overlies the Lecompton Limestone and underlies the Deer Creek Limestone, all formations that were named in Kansas and recognized in Nebraska. In both states, the Tecumseh overlies the Avoca Limestone Member (named from Nebraska) at the top of the Lecompton, and underlies the unspecified basal unit of the Deer Creek. In Nebraska, the Tecumseh was subdivided into the Kenosha Shale, Ost Limestone, and Rakes Creek Shale members (in ascending order), but the Ost has not been identified in Kansas, hence the others have not been differentiated there. Three possibilities might explain why the Ost Limestone has not been identified in Kansas: 1) The Ost may grade southward into unfossiliferous shale within the Tecumseh, a facies change that is unlikely because the marine basin from which the Ost would have transgressed is to the south. 2) The Ost may descend southward to join the top of the Avoca Limestone as the possibly deltaic Kenosha Shale pinches out basinward. 3) It is a semantic problem because the subdivision of the overlying Deer Creek Limestone into members is different between the two states. In Kansas, the basal two members of the Deer Creek are the Ozawkie Limestone overlain by the Oskaloosa Shale. The Oskaloosa is overlain by the distinctive Rock Bluff Limestone—black phosphatic Larsh Shale couplet, both named from Nebraska. In Nebraska, the original base of the Deer Creek was defined at the base of the Rock Bluff Limestone. Therefore, the Ost Limestone of Nebraska is in the stratigraphic position of the Ozawkie Limestone of Kansas and could be equivalent. Early workers were aware of this difference in the definition of the base of the Deer Creek Formation between the two states, but identified a thin shaly limestone just below the Rock Bluff in Nebraska as the Ozawkie Limestone, and thus did not recognize the possible equivalency of the Ost and Ozawkie limestones. Further study is underway to distinguish between possibilities 2 and 3, because either is plausible, based on regional patterns of deposition. In any case, the Rakes Creek sandstone facies beneath the Rock Bluff Limestone in Nebraska should be retained in the nomenclature specifically as a sandstone unit, because of its unique lithology in the succession.