UPDATED CONODONT-BASED CORRELATIONS OF TRADEWATER (ATOKAN-DESMOINESIAN, MIDDLE PENNSYLVANIAN) MARINE CYCLOTHEMS IN THE ILLINOIS BASIN

More conodont work on type sections, type areas and cores of marine units in the Tradewater Formation in Illinois, Indiana, and western Kentucky has established a better framework for correlating marine cyclothems of all scales, many of which have received local names in each state. In the upper Tradewater, the Carrier Mills cyclothem is largely black shale overlain by local limestone; it contains the acme zone of Gondolella pulchra and dominance of coarse-ribbed Idiognathodus rectus and related forms, with Neognathodus aff. asymmetricus. Below the Carrier Mills, the Stonefort cyclothem is also largely black shale more widely overlain by limestone, and marked by dominance of fine-ribbed I. cf. amplificus and related forms, and also by N. asymmetricus. Both are major cyclothems traced easily around most of the basin as spikes on gamma-ray logs. Above the Carrier Mills, the type Yeargins Chapel Limestone of Kentucky contains a similar fauna consistent with its position at the top of the Tradewater there. Below the Stonefort, the Creal Springs and older Mitchellsville limestones were named in Illinois, and the Holland Limestone was named in Indiana. Although all three contain sparser faunas dominated by smaller forms of I. cf. amplificus, the Holland and Creal Springs also contain N. asymmetricus, thus may be correlative. Below the Holland, the unnamed limestone above the Buffaloville Coal is dominated by I. amplificus and N. caudatus, whereas below the Creal Springs, the Mitchellsville contains other species of Neognathodus. Farther down, in the Fusulinella iowensis Zone close to the poorly defined Atokan-Desmoinesian boundary, the type Seville cyclothem of west-central Illinois, the Perth cyclothem of Indiana, and the type Curlew cyclothem of western Kentucky contain diverse fine-ribbed broader forms of I. cf. amplificus, including I. cf. obliquus, and also robust specimens of N. colombiensis, N. bassleri, and N. bothrops. Below this level are several limestones termed Lead Creek in Kentucky, with Ferdinand above Fulda named as beds in Indiana, and at least three separate unnamed beds recognized in Kentucky. The upper two contain Idiognathoides, an Atokan genus, and Declinognathodus appears in an older Lead Creek bed. Questions remain as to how many separate marine horizons exist in the Tradewater.