CONODONT CORRELATION OF UPPER ATOKAN AND LOWER DESMOINESIAN (MIDDLE PENNSYLVANIAN) MARINE UNITS IN MIDCONTINENT AND ILLINOIS BASINS AND PLACEMENT OF THE ATOKAN-DESMOINESIAN BOUNDARY

Conodonts from lower Desmoinesian and upper Atokan beds in Iowa and the southern Illinois Basin allow correlation of marine units between those regions. Below the widespread Midcontinent Verdigris cyclothem and Illinois Basin equivalent centered on the Mecca Quarry Shale, the next older major unit, the Midcontinent Upper Tiawah cyclothem correlates with the Carrier Mills Shale, based on dominance of coarse-ribbed Idiognathodus rectus and I. attenuatus, and the Gondolella pulchra acme zone. Below this, the Midcontinent Inola cyclothem correlates with a marine zone in the Laddsdale coals in Iowa and the Stonefort cyclothem in Illinois, based on dominance of fine-ribbed species of Idiognathodus. Below this, a lenticular unit above the Cliffland Coal in Iowa and above the Buffaloville Coal in Indiana contains first appearance of Neognathodus caudatus. Below this, the widespread equivalent Seville-Perth-Curlew limestones of the Illinois Basin contain the fusulinid Fusulinella iowensis and first appearance of N. bothrops. In Iowa, the marine unit above the Blackoak Coal contains type F. iowensis, N. bothrops, and also G. gymna, found elsewhere in the Seville Limestone. Lower marine zones in the Illinois Basin are four beds of Lead Creek Limestone, which contain Idiognathoides, a typical Atokan genus, and the lowest bed also contains Declinognathodus. Iowa cores cut marine zones with Idiognathoides, and two contain Declinognathodus at the base. In the Appalachian Basin, the Upper Mercer Limestone contains first appearance and type N. bothrops, within the range of F. iowensis there. In eastern Oklahoma, Idiognathoides is not reported below the basal Desmoinesian terrestrial Hartshorne Formation, where the type upper Atokan marine zones are mainly sandstone. Above the Hartshorne, the McCurtain Shale contains N. caudatus, which correlates it with the marine unit above that containing N. bothrops. Nonetheless, first appearance of N. bothrops, which is in the most widespread marine unit above the units containing Idiognathoides in the central to eastern basins, would pragmatically define the Atokan-Desmoinesian boundary. However, because of disconformity at this level in that region, more continuous successions in New Mexico are under study to more accurately determine ranges and establish a stratotype section.