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The Jurassic system in the Williston Basin is divided into the thin non-marine Morrison formation at the top and the thick marine Ellis group below. The Ellis group includes the Upper Jurassic Sundance formation and the Middle Jurassic Piper formation. No Lower Jurassic rocks are known. Over most of the basin the Piper overlies Triassic Spearfish rocks, except in the north, where it overlies Mississippian or Devonian.
The center of Jurassic deposition was in western North Dakota, and a less extensive Jurassic basin was in southern Saskatchewan.
Grain size and relative amount of clastic material decrease eastward, and limestones and calcareous shales are more abundant in the areas of thickest accumulation.
The lower part of the Piper formation contains red shales and gypsum, and in places it is difficult to separate from underlying Triassic strata.
Members of the Sundance formation defined in the Black Hills can not now be recognized in the basin, and the Rierdon formation of the Ellis group in Montana is not a readily mappable unit in North Dakota.
Oil has been produced in Wyoming, Montana, and Saskatchewan from rocks equivalent to the Sundance and Piper. Sundance sandstones and Piper limestones have suitable porosity and may be found to be productive on structure elsewhere in the basin.
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