The Pennsylvanian System in the Mid-Continent area consists of five series, from the lowest upward--the Morrowan, Atokan, Desmoinesian, Missourian, and Virgilian. In the Arkoma basin the Pennsylvanian rocks were deposited upon Chesterian rocks without apparent unconformity. On the platform area they were deposited unconformably upon an eroded surface of earlier Mississippian rocks; along the Nemaha uplift, upon all earlier systems including the Precambrian, and in the center of the Ozark dome upon Ordovician rocks.

The Arkoma basin is a narrow wedge of thick Morrowan, Atokan and Desmoinesian rocks bordering the Mid-Continent platform. Morrowan strata are thickest in the western part of the basin. They occupy a belt about 100 miles wide in southeastern and eastern Oklahoma, and northern Arkansas. Rocks of Atokan age overlap the Morrowan strata on the southern part of the platform. The Atokan is more than 9,000 feet thick at places in the basin and thins rapidly northward as it grades into a platform type of sediment marked by clastic limestones.

The Desmoinesian Series is ten miles as thick in the basin as it is on the platform. The basinal sediments are predominantly shale with massive sandstone tongues, and some coal beds. The platform facies is markedly cyclical, consisting of repeated sandstone, coal, limestone, shale sequences.

The Arkoma geosyncline ceased to exist at the close of Desmoinesian time and upper Pennsylvanian rocks are no thicker in the Arkoma basin area than they are on the platform. They are clastic and are in a pattern of intricate overlaps, pinchouts, and conglomerate tongues in the southwestern part of the area where sedimentation was influenced by movements in the Arbuckle uplift. On the platform upper Pennsylvanian rocks are predominantly shale with a succession of limestone units, many of which are rich in fusulinids.

The Pennsylvanian-Permian contact is customarily drawn below the first occurrence of Schwagerina, herein, the basal unit of the Council Grove group. The Admire group (below) has been placed in the Permian because the first significant physical break below the Council Grove is at the base of the Admire. The fusulinid boundary is here considered an illogical one, and it is suggested that the Herington limestone-Wellington shale contact is the most desirable boundary.

At the close of Krebs time (early Desmoinesian) a system of northeast-southwest trending normal faults displaced the rocks in and adjacent to the southwestern flank of the Ozark uplift. Uplift and erosion at the end of the Desmoinesian resulted in unconformity and overlap. Uplift in the Arbuckle Mountains became strong in Virgilian time and the Virgilian sediments of the adjacent area reflect the movements in coarse clastics, unconformities and overlaps. In late Permian or post-Permian time the rocks were folded into the present structural features--Prairie Plains homocline, Forest City basin, and local anticlines and synclines.

Pennsylvanian fossils of the area are in general of long-ranging species and are of little stratigraphic use. Fusulinids, some ammonoids, some brachiopods, and spores are useful guide fossils. Rocks of the system yield almost all of the coal of the area, and important amounts of ceramic products, cement, petroleum and natural gas.