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Pennsylvanian strata in the Illinois basin comprise a stratigraphic bridge between the well-studied dominantly terrestrial Appalachian succession and the marine Western Interior basin sequences. Within the Virgilian Series, understanding of this transition has been hampered by uncertainty as to stratigraphic order and a generally poor data base in Illinois. Previous studies in Illinois attempted local correlation by means of limestones and coals, both of which are largely thin and discontinuous. This study shows that black "sheety" shales are the most widespread lithologic units within the Virgilian. As many as six different black shales--associated with the Shumway, Bogota, Effingham, Reisner, Woodbury(?), and "Steel Bridge" limestones, respectively--have been recognized in the upper half of the Mattoon Formation in east-central Illinois. Detailed outcrop examination indicates that the black shales associated with the Shumway, Effingham, and "Steel Bridge" limestones are stratigraphically equivalent. Subsurface data confirm this conclusion. In addition to reducing the number of black shale beds to only four, this correlation shows that the lithology directly below the black shales is more diverse than previously thought. Rocks below the black shale associated with the Shumway-Effingham- "Steel Bridge" limestones include coal, gray shale, fossiliferous limestone, and unfossiliferous limestone.
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