PRELIMINARY CONODONT-BASED CORRELATION OF LOWER CHEROKEE MARINE UNITS (ATOKAN-DESMOINESIAN, MIDDLE PENNSYLVANIAN) IN IOWA WITH THOSE OF THE TRADEWATER FORMATION IN THE ILLINOIS BASIN

Restudy of conodonts from cores and outcrops in south-central Iowa allows correlation of several units with those in the Illinois Basin. Although Iowa is at the north end of the Midcontinent Basin, it contains older Atokan rocks at the base, which occur also in the base of the Tradewater in the southern Illinois Basin, but which are missing across the Bourbon Arch in east-central Kansas and adjacent Missouri, helping to define the Forest City Basin as active during that time in southwestern Iowa and adjacent region. About 80 cores cut the Pennsylvanian, 3/8 of them cut Atokan coals based on spores, and 1/5 cut marine zones containing Idiognathoides, an Atokan genus that allows correlation with the Lead Creek Limestone at the base of the Tradewater. Where complete in western Kentucky, the Lead Creek comprises four separate beds, of which the upper two contain Idiognathoides, the upper three contain Idiognathodus gibbus, and the lowest bed, including the type Fulda Limestone of Indiana, contains the genus Declinognathodus along with Idiognathoides. Most of the Iowa cores contain Idiognathoides alone, so correlate with either of the upper two Lead Creek beds, but two cores contain Declinognathodus as well, in one core below a higher Lead Creek equivalent with Idiognathoides alone. The next higher marine unit in Iowa, above the Blackoak Coal, contains Idiognathodus amplificus, Neognathodus bothrops, N. bassleri, and in one core, Gondolella cf. G. gymna, suggesting correlation with the Seville, Perth and Curlew limestones in the Illinois Basin, close to the poorly defined Atokan-Desmoinesian boundary. The next higher marine unit in Iowa above the Cliffland Coal contains the types of both Idiognathodus amplificus and Neognathodus caudatus, suggesting correlation with the marine unit above the Buffaloville Coal of southwestern Indiana. The next higher marine zone in Iowa, associated with the succession of Laddsdale coals, contains fine-ribbed species of Idiognathodus compatible with those of the Stonefort cyclothem in the southern Illinois Basin. Above this level in Iowa, associated with coals in the upper Floris Formation, are marine zones dominated by coarse-ribbed Idiognathodus rectus and I. attenuatus, and containing Gondolella pulchra, allowing correlation with the Carrier Mills Shale of the Illinois Basin.