Abstract

The northeastern Bighorn Basin was the site of differential tectonic uplift in the Late Paleozoic, beginning perhaps as early as Mississippian time and continuing sporadically through the Permian. These paleo-uplifts are coincident with the Laramide anticlines seen today parallel to the basin margins.

Field observations at the Sheep Mountain Anticline suggest that uplift and subsequent erosion of the Pennsylvanian Tensleep Formation resulted in considerable thinning of the unit. Uplift also occurred at Little Sheep Mountain, but the erosion was not as extensive as at Sheep Mountain. Up to 130 feet (40 m) of relief was observed on the Tensleep strata. This topographic relief was filled-in by the Glendo Member of the Permian Goose Egg Formation (Phosphoria equivalent).

Facies transitions in the upper members of the Goose Egg Formation consistently delineate paleo-highs coincident with the present anticlines, suggesting that the Permian relief was due to syndepositional uplift and not inherited from Pennsylvanian tectonism and erosion. Faulting during deposition of the Goose Egg Formation shows the same vergence (eastward) as the Laramide structures, therefore, it is suggested that the basement weaknesses activated during the Laramide Orogeny were also active during the Late Paleozoic.

The discrete paleo-highs formed in the Permian localized the most porous carbonate facies in the Goose Egg Formation. The coincidence of Laramide structural and Permian stratigraphic traps in the northeastern Bighorn Basin has been very important to hydrocarbon production in the region. Understanding these relationships could be of great use in future exploitation of Permian strata in Wyoming.