Pennsylvanian and associated rocks in Wyoming

Abstract

The Pennsylvania!! System in the Middle Rocky Mountains of Wyoming is represented principally by the Amsden and Tensleep Formations. These grade eastward into the Minnelusa, Hartville, Casper, and Fountain Formations, and southward into the Weber and Morgan Formations. The Amsden Formation has three members: the Darwin Sandstone at the base, the Horseshoe Shale (new name) in the middle, and the Ranchester Limestone (new name) at the top. The Darwin is of Chester (Mississippian) age; the Horseshoe is of Chester and Morrow ages; and the Ranchester is of Morrow age at the base but is mostly of Atoka age. The Tensleep Sandstone (and its equivalent, the Quadrant Quartzite, in the Yellowstone Park area) is entirely of Des Moiues age in the Bighorn and Wind River Basins. In central Wyoming, however, the Tensleep contains beds of Missouri, Virgil, and Early Permian ages. The Minnelusa and Hartville Formations in the eastern part of the State contain interbedded sandstone, shale, and carbonate rock ranging in age from Morrow to Early Permian. The Casper is composed of sandstone and carbonate rock and grades southward into red arkosic conglomerate of the Fountain Formation in the Laramie basin and adjacent areas. The Casper and Fountain Formations also range in age from Morrow to Early Permian, but the older strata are missing in a wide area in central Wyoming where the Pathfinder uplift, the northernmost element of the Ancestral Rocky Mountains, dominated the paleogeographic scene.