ILLINOIS BASIN COAL RESOURCES AND MINING GEOLOGY
Lower Pennsylvanian strata (Morrowan) of the Caseyville and lower Mansfield Formations are mostly restricted to the southern half of the basin and are dominated by thick quartzarenites and few coals, which are thin and local in extent. Early Middle Pennsylvanian strata (Atokan) of the Tradewater, upper Mansfield, Brazil, and Staunton Formations contain more coals, with rock unit extent and lateral consistency increasing up section. Some of these coal have locally low sulfur content.
The Carbondale and Shelburn Formations (Desmoinesian) consist of classic cyclothemic strata arranged in 30–50 ft intervals between coal beds. The units include eight to 11 coal-bound intervals, including the most heavily mined coals in the basin: the Springfield, Herrin, Danville. and Baker. Many Desmoinesian coal beds are overlain by transgressive black shales and regressive-highstand gray shales and siltstones. The Herrin is overlain by black shale, thin limestones, and a local coal in the southern part of the basin. Several lowstand channels are mapped in the intervals between coals, which locally influence coal thickness, quality (including lower sulfur content), and mining conditions. Changes in roof facies, depth, regional faulting, stress fields, and fractures below areas of low cover are important mining factors in different parts of the basin.
Late Pennsylvanian (Virgillian-Stephanian) strata of the Patoka, Bond, and Mattoon Formations are also arranged in cyclothemic patterns, but coals are thin and not of economic importance.