NEW INTERPRETATION OF THE TECUMSEH SHALE AND LOWER UNITS OF THE DEER CREEK LIMESTONE IN THE MIDDLE SHAWNEE GROUP (VIRGILIAN), NORTHERN MIDCONTINENT, USA

We present a new stratigraphic and sedimentologic analysis of the Tecumseh Shale and lower units of the Deer Creek Limestone based on outcrops and cores in Nebraska, Iowa, and Kansas. The Tecumseh Shale in Nebraska is subdivided into the Kenosha Shale, Ost Limestone, and Rakes Creek Shale members. The Kenosha consists chiefly of calcareous mudstone, which is locally rooted and blocky. The overlying Ost is a skeletal wackestone-packstone to oolitic grainstone. A regionally extensive paleosol separates the Ost from the overlying Rakes Creek, which is a coarsening-upward succession of heterolithic shale and rooted, very fine sandstone interpreted as a transgressive bayhead delta beneath the major transgressive Rock Bluff Limestone. In Kansas, the undivided Tecumseh is up to 21 m of microlaminated gray shale with ubiquitous pyritized root traces and carbonaceous plant matter, a stressed trace fossil suite, and a few marine body fossils. Plant-bearing sandstones, locally conglomeratic at the base, are reported from this unit in Kansas. The Tecumseh is overlain by the marine, dominantly oolitic Ozawkie Limestone, followed by the mostly unfossiliferous Oskaloosa Shale, which contains blocky mudstone that represents lowstand exposure. This is overlain by the major transgressive Rock Bluff Limestone. Based on tracing of marine units and intervening paleosols and exposure surfaces, we correlate the Tecumseh Shale of Kansas with the Kenosha Shale of Nebraska, the Ozawkie Limestone of Kansas with the Ost Limestone of Nebraska, and the Oskaloosa Shale of Kansas with the Rakes Creek Shale of Nebraska. Our revised correlations reveal different thickness patterns that were previously unknown: the Tecumseh/Kenosha detrital unit thins northwestward toward the Nemaha Arch, whereas the Rakes Creek sandstone thickens and coarsens in the same direction. Unlike the underlying Tecumseh/Kenosha detrital systems, the transgressive Rakes Creek bayhead delta was apparently deflected westward toward the paleotopographic high, perhaps by marine currents. This resulted in strong regional partitioning of the greatest detrital sediment volumes between the Tecumseh/Kenosha lowstand and the later, early transgressive Rakes Creek sandstone facies prior to the major Rock Bluff transgression.