Wyoming Geological Association: 1982 Luncheon Meetings Casper, Wyoming: Abstracts of Papers

The Williston basin of North Dakota, Montana, South Dakota, and south-central Canada (Manitoba and Saskatchewan) is a major producer of oil and gas, lignite, and potash. Located on the western periphery of the Phanerozoic North American craton, the Williston basin has undergone only relatively mild tectonic distortion during Phanerozoic time. This distortion is largely related to movement of Precambrian basement blocks.

Sedimentary rocks or cratonic sequences Sauk through Tejas are present in the basin. Sauk (Cambrian-Lower Ordovician), Tippecanoe (Ordovician-Silurian), and Kaskaskia (Devonian-Mississippian) sequence rocks are largely carbonate, as are the major oil and gas producing formations. Absaroka (Pennsylvanian-Triassic) and Zuni (Jurassic-Tertiary) rocks have more clastic content, but carbonates are locally important. Clastics of the Zuni sequence (Fort Union Group) contain abundant lignite. Tejas (Tertiary-Quaternary) sequence rocks are not significant in the production of minerals and energy, although glacial sediments cover much of the region.

Oil exploration and development in the United States portion of the Williston basin in the time period 1972 to present has given impetus to restudy of basin evolution and geologic controls for energy resource locations. In consequence, oil production in North Dakota, for instance, has jumped from a nadir 19 million bbls in 1974 (compared to a previous zenith of 27 million bbls in 1966) to 32 million bbls in 1979 and 40 million bbls in 1980. Geologic knowledge of carbonate reservoirs has expanded accordingly.

Depositional environments throughout Sauk, Tippecanoe and Kaskaskia deposition were largely shallow marine. Subtidal and even basinal environments were developed in the basin center, but sabkha deposits were abundant near the basin periphery. Evidence of subaerial weathering was commonly preserved in structurally high areas and on the basin periphery, especially in late Kaskaskia rocks. Some pinnacle reefs were developed in Kaskaskia deposition, morphologically similar to the Silurian pinnacle reefs of the Michigan basin.

Clastic sediments were transported into the southern (U.S.) part of the basin during Absaroka sequence deposition, a product of erosion of ancestral Rocky Mountain orogenic structures. Continental and shallow-marine clastic sediments were deposited during Zuni sedimentation until deeper Cretaceous marine environments were established. Laramide orogenesis to the west provided detritus that was deposited in fluvial, deltaic, and marginal-marine environments, regressing to the east. Major lignite deposits are part of this post-orogenic, regressive rock body.

Major structures in the basin, and the basin itself, may result from left-lateral shear along the Colorado-Wyoming and Fromberg zones during pre-Phanerozoic time. Deeper drilling in the basin has revealed several major structures with indications of others. Most structures probably resulted from renewed movement or “tensing” of pre-Phanerozoic faults. Meteorite impact events have been suggested as the origin for one or more structures.