Abstract

Despite the fact that the stratigraphy of the Pennsylvanian System in Wyoming is essentially simple, the literature contains abundant differences of opinion which are puzzling to the geologist seeking an integrated, coherent concept. By returning to Darton's original definitions of the Tensleep and Amsden Formations and applying his criteria to surface and subsurface sections in central and western Wyoming, it is possible to defend, by sections and by thickness and facies maps of each significant unit, the following statements. The Amsden Formation is a tripartite lithologic unit. Its basal member, the Darwin Sandstone, has irregular limits and thickness but is widespread in the Wind River and Bighorn basins where it lies on a karst topographic surface developed on the Madison Limestone. A middle red shale has nearly constant thickness and lithology. An upper carbonate member is widespread in northwestern Wyoming but is lacking on the Pathfinder uplift (the most northerly of the Ancestral Rocky Mountain uplifts) and grades southward into the Fountain and Casper Formations. The Tensleep Sandstone everywhere overlies the Amsden.

Sources for the sand in the Darwin and Tensleep, either from the northwest as suggested by some, or from the Ancestral Rocky Mountains as alternately suggested, are discarded in favor of a widespread older Paleozoic sandstone in the Hudson Bay region which had been exposed to erosion in Pennsylvanian time.

The Darwin Sandstone an ideal reservoir rock, is composed of exceptionally pure well-sorted quartz sandstone. Production is currently obtained from the Tensleep in numerous anticlinal reservoirs; but may also yield from other types of traps. Facieschange belts, as in the gradation of the Tensleep into the Minnelusa, and that between the Casper. Fountain and Morgan Formations, may have unsuspected possibilities.