PALYNOLOGICAL RECONNAISSANCE OF LOWER AND MIDDLE PENNSYLVANIAN STRATA IN THE EASTERN INTERIOR (ILLINOIS) BASIN, USA

The Eastern Interior (Illinois) Basin is an intracratonic sag basin that extends across most of Illinois and parts of Kentucky and Indiana. Pennsylvanian strata within the basin ranges from Early to Late Pennsylvanian (Early Permian?) in age, with most of the economic coal deposits occurring in the late Middle Pennsylvanian Carbondale and Shelburn Formations.

Lower Pennsylvanian strata in the Illinois Basin are assigned to the Caseyville and Mansfield Formations. Thin, discontinuous coal beds within these units contain heterogenous palynofloras usually dominated by Lycospora,the dispersed spore genus of several large lycopod trees (e.g., Lepidodendron, Lepidophloios) that were conspicuous elements of mire vegetation during this time. Stratigraphically diagnostic forms include Schulzospora rara, Radiizonates striatus, R. aligerans, Laevigatosporites desmoinensis and Granasporites medius, all of which are indicative of an Early Pennsylvanian (Morrowan, Langsettian) age assignment.

Early and mid-Middle Pennsylvanian strata in the Illinois basin are assigned to the Tradewater Formation in Kentucky and Illinois and the Mansfield, Brazil and Staunton Formations in Indiana. Some Tradewater coal beds attain sufficient thickness and lateral continuity for commercial mining. Early Middle Pennsylvanian coal beds are usually dominated by Lycospora with mid-Middle Pennsylvanian coals containing increased amounts of tree fern spore taxa. Stratigraphically diagnostic forms include Endosporites globiformis, Microreticulatisporites nobilis, Torispora securis, Vestispora fenestrata,Radiizonates difformis, Cadiospora magna, Murospora kosankei and Thymospora pseudothiessenii. Early and mid-Middle Pennsylvanian palynofloras are indicative of Morrowan (Duckmantian and Bolsovian) and Desmoinesian (Asturian) age assignments.

Late Middle Pennsylvanian strata in the Illinois basin are assigned to the Carbondale and Shelburn Formations in Kentucky and Illinois and the Carbondale Group and Shelburn Formations in Indiana. Coal beds in this part of the section are thick, laterally continuous, and co-dominated by Lycospora and tree fern spores. The occurrence of Schopfites, Densosporites and Lycospora, among other spore and pollen taxa, indicate a Desmoinesian (Asturian) age assignment.