The type area of the Pennsylvanian System is the central part of the Appalachian Mountains, and specifically, the state of Pennsylvania. The Pennsylvanian rocks of the area are divided into the Pottsville supergroup, the Allegheny group, the Conemaugh group, the Monongahela group; and are overlain by the Dunkard group. The thick, primarily sandy, sequence of the Appalachian basin grades westward into cyclical units. Marine units are but a minor part of the system.
Several biofacies are well developed—those with plant compressions, a large number of fresh-water zones, a few vertebrate communities, and thin, marine invertebrate-bearing limestones and shales.
Structural deformation of the region developed through much of the Paleozoic Era, and the culminating westward thrusting and folding in the miogeosynclinal sediments is attributable to the Allegheny orogeny of post-Pennsylvanian (and perhaps pre-Permian) time.
Pennsylvanian rocks of the area contain many minable coal beds, of which the anthracites, the Pittsburgh seam, and the Pocahontas coals are the more valuable; and the Kittanning and Freeport coals are of major importance. Petroleum and natural gas of local importance are produced in a restricted area within the central Appalachians.
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Pennsylvanian System in the United States
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