Structures active during the Pennsylvanian Period and paleoenvironments in which Pennsylvanian strata were formed strongly influenced the character and location of coal resources in the Illinois Basin. Sediment filling the basin probably originated in the Canadian Shield and perhaps the highlands of the northern Appalachians. Major river systems transported this sediment to the Illinois Basin, where it was deposited in fluvial and deltaic paleoenvironments on a broad coastal plain and adjacent shallow shelf. Loci of deposition were controlled by the La Salle Anticlinal Belt on the east and the Du Quoin Monocline and the Louden and Salem Anticlines on the west. These structures separated the deeper parts of the basin from the shallow shelf areas to the west and east.

Periodic marine invasions interrupted fluvial and deltaic sedimentation. River systems were able to re-establish themselves following these inundations, which resulted in the creation of a series of elongate, sinuous sandstone/siltstone bodies. Some of these bodies were deposited contemporaneously with coal-forming swamps that occupied vast areas of this coastal plain. These rivers were the Pennsylvanian continuation of the Michigan River System, which carried terrestrial sediment from the Canadian Shield and perhaps the Appalachians to the Illinois Basin during the Mississippian Period. Further exploration will undoubtedly reveal additional peat-contemporaneous channel-fill deposits.