Pennsylvanian (Upper Carboniferous) system of Wyoming

Abstract

Pennsylvanian rocks in Wyoming comprise two stratigraphic sequences bounded by regional unconformities. Red mudstone and limestone characterize the strata in the lower sequence, and sandstone and dolomite characterize the strata in the upper sequence. Initial deposits of the lower sequence are-red mudstone probably of no older age than about middle Morrowan, except in the subsurface of southwestern Wyoming where early Morrowan strata may be present. The basal red mudstone lies unconformably upon Mississippian strata except in the vicinity of the ancestral Rocky Mountain Front Range where they lie upon Precambrian rocks. The red beds grade upward to interbedded limestone and mudstone of late Morrowan and early Atokan age; but the limestone strata are absent at many places because of beveling on a Middle Pennsylvanian erosional surface. Initial deposits of the upper sequence are heterogeneous and locally may be thinly bedded red, green, or varicolored mudstone, sandstone, or dolomite of late Atokan to early Des Moinesian age. Basal strata of the upper sequence lie unconformably upon lower Pennsylvanian strata at most places in Wyoming; but locally, where the lower Pennsylvanian strata are absent by erosion or possibly by nondeposition, the upper sequence lies upon Mississippian rocks. The upper sequence in central and northwestern Wyoming comprises mostly dolomite in the lower part but grades upward into mostly sandstone no younger than Des Moinesian. The upper sequence in eastern Wyoming comprises sandstone and dolomite about equally and also includes Upper Pennsylvanian strata as young as Virgilian. Upper Pennsylvanian strata also occur as part of the upper sequence in the thrust belt in western Wyoming. Pennsylvanian strata terminate upward at an erosional unconformity beneath Permian rocks everywhere in Wyoming.