The Paradox basin, about 360 miles long in northwesterly trend, and 180 miles wide, subsided obliquely across the high southeast shelf of the Cordilleran geosyncline during mid-Pennsylvanian time after an earlier Pennsylvanian history of shelf deposition of red clastics to form the Molas formation, and gray marine shale and limestone to form the Pinkerton Trail formation. The early autogeosynclinal subsidence of this intracratonic parageosyncline allowed the deposition of the Paradox formation comprising major cyclic deposits of normal marine, penesaline, and saline strata, and simultaneous emplacement of medium-to coarse-grained clastics from the Uncompahgre-San Luis-Penasco highland on the northeast and east, and fine- to medium-grained clastics from the Zuni-Defiance-Kaibab-Emery positives on the south, southwest, west, and northwest. This restricted basin of evaporite deposition underwent rigorous aridity cyclically interspersed with what appear to have been humid conditions, while normal marine waters entered the basin episodically to cyclically through sedimentational passes or marine accessways from the Sonoran geosyncline on the south and the Cordilleran geosyncline on the west.
Increased tectonic intensity in late Pennsylvanian led to sharp subsidence of the northeast flank of the Paradox autogeosyncline, to form the Uncompahgre taphrogeosyncline, into which were dumped many cubic miles of coarse arkosic clastics from the sharply uplifted Uncompahgre highland. These clastics interfinger southwestward with normal marine shale and carbonates of the Honaker Trail formation, deposited as the basin became thoroughly ventilated via additional marine accessways across the shelves of the basin. High on the Paradox-Uncompahgre southwestern shelf, Coconino-type quartzose clastics and Supai-type fine clastics moved into the basin from westerly and southerly directions, which, combined with influx of Uncompahgre-San Luis-Penasco clastics from the northeast and east, drove the seas from the area during latest Pennsylvanian to early Permian time.
The interfingering of invading clastics, combined with the generation of organic deposits along a broad zone of the southwest sedimentational shelf parallel with the strike of the Paradox basinal axis where thick evaporites were formed, created the facies gradations that make the Paradox basin a classic area for the study of Pennsylvanian stratigraphic and sedimentologic complexities.
Figures & Tables
Rocks of the Pennsylvanian System are the bed rock of approximately 10 per cent of the land area of continental United States. These rocks yield 17 per cent of the petroleum, most of the coal, and most of the ceramic raw material of the United States.
Areas of deposition of Pennsylvanian rocks are naturally discriminated as the New England trough, the Appalachian trough, the Eastern Interior basin, the Michigan basin, the Western Interior basin, the Ardmore basin, the Ft. Worth syncline, the Permian basin, the Rocky Mountain geosyncline, and the Cordilleran trough. These genetic areas and the Ouachita fold belt are the regions described.
The Pennsylvanian System in Michigan has been described in detail by W. A. Kelly (Mich. Geol. Survey, Pub. 40, part 2, p. 155-226, 1936). The section is truncated and consists of the Parma sandstone (below), the Saginaw group of cyclical formations, and the Grand River group, which contains red sandstones and gypsum. The Parma and Saginaw are Pottsvillian in age; possibly, Atokan and Desmoinesian. The Grand River, placed by some geologists in the Permian, is probably early Missourian.
The project for a special volume was initiated in 1953, at which time it was decided that the Springer Series was tobeconsidered as Pennsylvanian, the Wolfcamp and Admire-Council Grove-Chase, as Permian. Evidence has since accumulated that the Springer is Mississippian, that the Ouachita Jackfork and Stanley are Mississippian, that the Dunkard is, atleast in part, Pennsylvanian, and that the “Lyon series” and Wolfcamp formation might better be classed