Abstract

Late Paleozoic tectonic movements greatly complicated patterns of deposition and preservation of Pennsylvanian and Permian strata on the eastern part of the Wyoming shelf. These localized movements coincided with development of the ancestral Rocky Mountains in Colorado and adjacent areas and of the Big Snowy trough in central Montana. Isopachs for the rock interval between the top of the Mississippian carbonate rocks (Madison Limestone and Pahasapa Limestone) and the base of the Permian Minnekahta Limestone in eastern Wyoming are deceptively uniform, suggesting late Paleozoic stability of the shelf. The regional structural pattern seems to have been dominated by an orthogonal system of faults that bounded rectangular blocks of terrain that were complexly and alternately elevated, depressed, or tilted relative to each other. Topographic relief on the shelf during the late Paleozoic was minor compared to the relief that has developed since the Late Cretaceous to Early Tertiary Laramide orogeny, but tectonic activity seems to have been greater during the Pennsylvanian than at any other time during the Phanerozoic prior to the Laramide.

The depositional environments and the total thickness of the Pennsylvanian and Lower Permian Minnelusa Formation were essentially uniform throughout eastern Wyoming. However, the formation comprises three unconformity-bounded members that are lithologically similar but are not uniformly thick owing to the differential movements, to penecontemporaneous erosion, and to compensating depositional infilling. Consequently, correlation of the members of the Minnelusa and individual strata within the formation is complicated by their lithologic similarities and their thickness differences.

Unconformities and significant changes in depositional patterns show the shelf was tectonically disturbed several times during the late Paleozoic: (1) in Meramecian (Late Mississippian) time, (2) at the onset of Pennsylvanian deposition, (3) about late Atokan to early Des Moinesian (Middle Pennsylvanian) time, (4) at the onset of Permian deposition, (5) during late Early Permian, prior to deposition of the Goose Egg Formation, and (6) during late Late Permian time before Triassic deposition began.