Currently accepted terminology for Pennsylvanian units in the area includes the Tensleep, Amsden, Quadrant, Minnelusa, Hartville, Casper, Ingleside, and Fountain formations. A review of nomenclatural history, lithologies, and lateral continuity of these units results in elimination of the terms Quadrant, Hartville, Casper, and Ingleside. The Minnelusa is raised to group status and is extended to include the strata formerly designated as Hartville. The Amsden is restricted vertically to the carbonate upper portion of the original formation, and Branson’s term Sacajawea is applied to the clastic formation below. The Ten-sleep, Amsden, and Sacajawea formations are extended laterally to include lithogenetically equivalent strata.
Age variations within the units suggest a southern Wyoming land mass connecting with the Black Hills until Morrowan time. Amsden-Sacajawea seas, initiated in Chesterian time to the north and southeast of this land bridge, were later joined. Tensleep sands advanced southward from Montana forcing the Amsden seas to retreat southeastward until overwhelmed by the youngest Tensleep deposits in the Hartville uplift area during Wolfcampian or Leonardian time.
Isopach variations are relatively constant from one unit to another, and show tectonically negative areas in the Lusk embayment and southwestern corner of the map area. The northern zero isopach is interpreted as an erosion edge, but areas of thinning on the Montana-Wyoming border, in the Black Hills, and in southern Wyoming are thought to be related to tectonic instability or position of former land masses. A superjacent strata map illustrates post-depositional erosion intervals by showing the age of the overlying formations.
Lithofacies analyses reveal a southern and southwestern source for the Tensleep, Amsden, and Sacajawea sediments. Reef-structures are postulated in the Amsden on the basis of lithofacies patterns, and coincide with the margins of evaporite distribution. A lithofacies map of the entire sequence emphasizes the masking effect introduced when this technique is used with large units involving long depositional intervals.
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