Allocyclic Controls on Middle Pennsylvanian Sedimentation in Cratonic Depositional Systems (U.S.A.)
Research on cyclic sedimentation in the Pennsylvanian System of the United States began with Udden (1912), who delineated cyclic patterns in Pennsylvanian strata in the Eastern Interior Basin (Illinois, U.S.A.). Udden described a stratigraphic succession associated with four upper Middle Pennsylvanian (Desmoinesian Stage) coal beds in western Illinois where similar lithologic sequences were repeated above each coal bed. The stratigraphic succession, as described by Udden, consists of the following lithologies:
Middle Pennsylvanian tectonics of the United States were dominated by the Laurasia–Gondwana collision and assembly of the Pangea supercontinent. The resulting Alleghanian and Ouachita orogenies along the eastern and southern continental margins as well as the Ancestral Rocky Mountain orogeny in the western interior have typically been linked to this collisional event. Large syntectonic clastic wedges spread westward and northward from the Appalachian and Ouachita highlands into the foreland basins and contributed large volumes of clastics to the midcontinent. Broad intracratonic basins such as the Illinois and Michigan were subsiding at the same time but during the Middle Pennsylvanian probably were not receiving any significant sediment contribution from the east.
Although there is general agreement that the tectonic and depositional activities along the margins can be related to the construction of Pangea, the mechanism and underlying cause of the coeval mountain building and basin filling in the western interior are still enigmatic. Analysis of reflection seismic data along the Uncompahgre Uplift–Paradox Basin margin documents Desmoinesian and Wolfcampian thrust faulting with only minor lateral offset. The results of this and related studies indicate a principal stress direction of northeast–southwest, rather than northwest–southeast as would be expected if it had been the result of compression along the southeastern margin. Recent results from a number of workers suggest that the western and southwestern continental margins were significantly more active in the Pennsylvanian than previously thought and may hold the key to resolving some of the apparent inconsistencies.