A large proportion of the Pennsylvanian sediments in the Mid-Continent area are dark shales, which tend to grade eastward into sandstones, and westward into limestones. Most of this dark shale is believed to have come from a land mass (Llanoria), located to the southeast. The large part of the rocks in this land mass consisted of closely-folded, faulted and uplifted shales, sandstones, and limestones previously deposited in geosynclinal troughs, during Mississippian and earlier Paleozoic periods.

The Pennsylvanian is divided into six series--Springer, Morrow, Atoka, Des Moines-Strawn, Missouri-Canyon, and Virgil-Cisco. Several maps show regional conditions of deposition, the location of land masses, and the source of sediments during each of these series. Since it is impossible at this time to satisfactorily show the Springer, Morrow, and Atoka on separate maps, they are presented together on one map.

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At the beginning of the Pennsylvanian, during Springer time, the sea was quite restricted and sediments of that age were deposited only in southern Oklahoma, in adjacent parts of Arkansas and North Texas, and in the Marathon region of West Texas. During Morrow time, the Pennsylvanian sea expanded somewhat and dark shales and limestones were deposited over a larger area. During Atoka time, conditions changed markedly in the Mid-Continent area, and the Wichita period of orogeny occurred. In addition to the continued uplifting of the Llanoria or Ouachita Mountain land mass to the southeast, three other different kinds of uplift occurred. One set of the new uplifts formed large anticlinal islands, such as the Wichita Mountains and Uncompahgre Uplift, trending northwest-southeast in the ex anding Pennsylvanian sea. These large islands were eroded rapidly and clastics accumulated on their flanks. Some of the other uplifts were smaller and were rapidly buried by later sediments. The sea expanded to form new marine basins between uplifted land-masses.

During Des Moines-Strawn time, the Pennsylvanian sea probably extended to its maximum limit in this region. Thick sands which have produced much oil were deposited on the eastern side of the area and many limestones accumulated in the western area.

During Missouri-Canyon time, there was relatively little uplift or structural movement. Conditions in the seas were most favorable for reef building by calcareous organisms, particularly in West Texas.

During Virgil-Cisco time, another period of orogeny, the Arbuckle, occurred in southern Oklahoma and extended into the Panhandle of Texas. The Arbuckle and Wichita mountains were again uplifted and the thick series of Pennsylvanian shales and sandstones in the Ardmore and Anadarko basins were folded and faulted to form traps that produced many prolific oil fields.

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