Pennsylvanian and Permian rocks in an area of approximately 240,000 square miles in the Cordilleran area have numerous features in common which justify their consideration together. For this study they are treated in the light of nomenclature, thickness, stratigraphic relationships, correlation, conditions of deposition, and relative significance. Approximately 50 formal names can be applied to units of formation rank in these systems. The sediments were deposited in well-defined basins and troughs, and upon banks adjacent to positive areas within the western part or eugeosyncline, and eastern part or miogeosyncline. During Permian time at least, a volcanic archipelago probably existed west of the eugeosyncline; the latter geosyncline has volcanic and associated materials. The miogeosyncline received mostly marine clastic sediments and various carbonates. Manhattan line separated the two geosynclines, at times only as an arbitrary boundary, and at other times as an epeirogenic to orogenic belt. East of the miogeosyncline was a well-defined platform or shelf which varied in time and space in proportion between the continental craton east of it and the geosyncline to the west.
Individual formations within the Cordilleran area range in thickness from a few hundred feet to some of thousands of feet (26,000 for the Oquirrh formation). Many of these formations straddle systemic boundaries; many are time-rock units. Sedimentation was not everywhere constant nor uninterrupted for the duration of these periods of time. In fact, epeirogeny and orogeny were characteristic (but localized at times). Antler orogeny in Devonian and late Mississippian to early Pennsylvanian time, however, affected much of Nevada and western Idaho on a profound scale. Certain positive areas, such as one in the general region of western Utah and eastern Nevada, were tectonically active during parts of Pennsylvanian time, and late in the period existed as important land masses. Basins and troughs adjacent to this particular positive area, such as the one between it and the Manhattan area to the west, subsided with acceleration at times. Such prominent depocenters as the Oquirrh basin, Wood River basin, Wells basin, Diamond Peak and Ely basin, Bird Spring basin, and others, were dominantly negative parts of this area, and were integral parts of the larger miogeosyncline. Most of these, along with counterparts in the eugeosyncline farther west, persisted as subsiding repositories throughout most of Permian time also. Orogeny in mid-Permian affected much of the eugeosynclinal belt on a grand scale.
The Las Vegas line of southern Nevada acted as an important Paleozoic hinge line, separating shelf facies to the south and southeast from basin facies on the west and northwest, and was particularly effective in the Pennsylvanian and Permian. It is the southern counterpart of the Wasatch line.
Reef complexes composed of bioherms, biostromes, and bioclastic beds are abundant and widespread in both Pennsylvanian and Permian rocks. Many of these sediments have strong hydrocarbon odors and may, therefore, be potential sources and reservoirs of petroleum. Drilling for oil and gas in which these late Paleozoic rocks have been tested is still in no more than preliminary state for most of the Cordilleran area.
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