NEW CONODONT-BASED CORRELATIONS OF DESMOINESIAN (MIDDLE PENNSYLVANIAN) MARINE CYCLOTHEMS BETWEEN ILLINOIS AND MIDCONTINENT BASINS

Conodont biostratigraphy in the late 1900s established correlation of Missourian and lower Virgilian marine cyclothems between the Illinois and Midcontinent basins. Recent work on Desmoinesian cyclothems confirms some upper Desmoinesian correlations previously suggested by stratigraphic position, and establishes new correlations of middle and lower Desmoinesian cyclothems that were heretofore unknown. Discovery of Gondolella magna and Swadelina nodocarinata in the type Attila Shale of southern Illinois confirms its correlation with shale in the base of the Lonsdale Limestone of northwestern Illinois, with shale in the middle of the West Franklin Limestone of southeastern Illinois and Indiana, and with the Nuyaka Creek Shale in the Lost Branch cyclothem of the Midcontinent. Discovery of subequal numbers of Swadelina neoshoensis and Idiognathodus expansus in the dark phosphatic shale underlying the type Piasa Limestone of western Illinois confirms its correlation with the Altamont cyclothem of the Midcontinent. The lowest occurrence of Sw. neoshoensis in the basal Farmington dark shale above the Danville Coal in the Peoria area confirms its correlation with the underlying Farlington cyclothem of the Midcontinent. In the older Tradewater Formation, discovery of the acme zone of Gondolella pulchra in the type black Carrier Mills Shale of southern Illinois, in the black Logan Quarry Shale of western Indiana, and in an unnamed limestone in northwestern Illinois (from which G. gymna was named) confirms correlation of these three units with each other and with the upper Tiawah cyclothem of the Midcontinent. Below the type Carrier Mills Shale, dominance of Idiognathodus amplificus in the Stonefort Limestone and its underlying black shale [Stonefort cyclothem] of southern Illinois strongly suggests its correlation with the Inola cyclothem of the Midcontinent. Farther down, the type Seville Limestone and its underlying black shale [Seville cyclothem] contain diverse Idiognathodus and type Gondolella gymna, but the latter species only in a north-trending band in northwestern Illinois. The Perth Limestone of western Indiana contains similar diverse Idiognathodus, but no Gondolella. The Seville is not yet correlated with a marine unit in the Midcontinent.