An attempt is made to review and present in summary the stratigraphy of the Pennsylvanian System in the central and northern Rocky Mountains. The area considered extends from southwestern Montana southeastward into Colorado, eastward to the Black Hills and westward into northeastern Utah. After a general evaluation of studies to the present, and a brief review of pre-Pennsylvanian paleogeology and the structure of the system, principal formations are presented with a definition, a description of lithology, and a statement of age and paleogeography. Formations of lesser extent, and marginal to the area, are given smaller treatment. The presentation is summarized in 11 figures including a paleogeologic map, 5 maps showing paleogeography and lithofacies, 3 isopach maps, and 2 correlation charts.
Figures & Tables
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