The mid-Permian Opeche Shale of North Dakota consists of bedded evaporites and red-bed siliciclastics. Detailed core and petrographic study has documented sedimentary and early diagenetic features in order to develop a depositional model, and to refine paleoclimatic data and paleogeographic setting for the late Paleozoic of the U.S. midcontinent.
Lithologies and sedimentary features indicate lacustrine, distal alluvial, and minor eolian deposition, subaerial exposure, and soil formation. Bedded halites consisting of chevron and cumulate crystals, dissolution surfaces and pipes, and mudcracked microcrystalline salt crusts were deposited in a saline pan dominated by flooding, evaporative concentration, and desiccation. Bedded halites containing chevron and cumulate crystals but lacking any dissolution or desiccation features formed in perennial saline lakes. Chaotic halite, composed of red mudstone and siltstone with displacive halite crystals, represents saline mudflat deposits. Red mudstone and siltstone with little or no displacive halite but with abundant cracks and root features suggest deposition in a dry mudflat. Red-bed sandstones and conglomerates, composed of poorly sorted, subrounded quartz grains cemented with halite indicate distal alluvial deposition with possible transport by ephemeral streams, sheet floods, and debris flows. Most deposition took place in halite-dominated shallow perennial and ephemeral saline lakes surrounded by saline and dry mudflats. Evaporation, desiccation, flooding, and wind played significant roles in this environment. Therefore, the Opeche evaporites and red beds are representative of an ancient saline pan system.
An inland playa setting is favored as a depositional model for the Opeche Shale. The abundance of soil features and halite dominance, as well as lack of nearshore carbonates and lack of restricted marine fossils, suggest a closed-basin nonmarine setting for the mid Permian of the U.S. midcontinent.