By about the Middle Pennsylvanian, folding and thrusting had started in the Ouachita area. This was the Ouachita orogeny, which pushed former basin sandstones, siltstones, shales, and chert formed during the earlier part of the Paleozoic, and moved them some 50 miles north into Oklahoma to create the Ouachita Mountains. This pulse of mountain building will continue on through to the end of the Pennsylvanian.
Once mountains had risen above the sea, vast amounts of clastic sediment were eroded off these highlands, and were deposited in deep basins situated to the north (Arkoma and Anadarko) and to the south (Ardmore). As one goes north out of these basins, shallow shelf areas situated around Tulsa, and surrounding the Ozark uplift were sites of extensive river and swamp environments. Over time, trees within these swamps die, decay, and start to accumulate in the soggy bottoms of the wetlands. Eventually these accumulated dead trees and plants are turned into coal.
As one moves west and slightly north of the Ozark area (toward
Enid and Wichita, Kansas), one would be moving from terrestrial (land) areas
with abundant coal swamp and river sand deposits, into shallow marine environments
characterized by more silt, clay and limestone deposition. Marine invertebrate
life was still abundant in these warm, tropical, shallow seas, and is exemplified
by the numerous fossils that can be collected in rocks of this period.