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.
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Climate Controls on Stratigraphy
The role of climate as a primary control on stratigraphy is the cornerstone of this volume. The emphasis on climate is in distinct contrast to most previous studies, in which stratigraphic variability has been related to changes in sea level and in tectonic activity. Furthermore, the findings, derived from several years of detailed study of modern and ancient key geologic sections around the world, indicate that traditional depositional models generally do not fully explain the origin of fossil fuels. Although the results of the studies presented in this volume are intended to contribute to the disciplines of sedimentary geology and stratigraphy, the contributors recognize that their results may also contribute to a better understanding of global climate change. The theoretical background of climate control on sediment supply and stratigraphy is presented in the volume. With this background in place, detailed documentation and analysis of climate control on the lithologic variation of a single Middle Pennsylvanian.