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

Recent conodont work on type sections and type areas of marine units in the Tradewater Formation and from cores and a mine in Illinois, western Indiana, and western Kentucky is establishing a framework for correlating the marine cyclothems of various scales, many of which have received local names in each state.  In the upper Tradewater, the acme zone of Gondolella pulchra along with dominance of coarse-ribbed Idiognathodus rectus and related forms in the type black Carrier Mills Shale of southern Illinois, the black Logan Quarry Shale of western Indiana, and the unnamed limestone in northwestern Illinois from which G. pulchra was named confirms correlation of these three units with each other. Below the Carrier Mills, dominance of fine-ribbed Idiognathodus cf. amplificus and related forms in the Stonefort Limestone and its underlying black shale (Stonefort cyclothem) of southern Illinois distinguishes it from the Carrier Mills. Both cyclothems are traced easily around the central and southern parts of the basin as spikes on gamma-ray logs. Below the Stonefort, the Creal Springs and Mitchellsville limestones in Illinois contain sparser faunas dominated by smaller forms including fine-ribbed I. cf. amplificus, as does the Holland limestone of Indiana, but with no definite basis yet for exact correlation. Farther down, the type Seville Limestone and underlying black shale of west-central Illinois (Seville cyclothem) contain a diversity of broader fine-ribbed Idiognathodus cf. amplificus, including I. cf. obliquus, and also type Gondolella gymna, but the latter species only in a north-trending band in northwestern Illinois. Based on content of Fusulinella iowensis, the Seville is considered equivalent to the Perth Limestone of western Indiana and Curlew Limestone of western Kentucky. Both contain a similar diversity of Idiognathodus, but no Gondolella, which suggests some isolation of the Seville. Below this level (which is close to the indefinite Atokan-Desmoinesian boundary) are units that contain Idiognathodus gibbus in Indiana, and Idiognathoides sp. in Kentucky, which indicate Atokan age. Several units in the cores and mine can be correlated with the named units, but questions remain as to the exact number of minor marine units that exist in this succession.