Stratigraphy and depositional environments of the Three Forks Formation (Upper Devonian), Williston Basin, North Dakota

The Three Forks Formation (Upper Devonian) is present in the subsurface in the western two-thirds of North Dakota and is generally conformable with the underlying Birdbear Formation and the overlying Bakken Formation. The Three Forks attains a maximum thickness of 265 feet (81 meters) in the central basin, east and south of the Nesson Anticline, and thins to an erosional edge in eastern North Dakota. The Three Forks is composed of micrite and dolomicrite, which may be fcssiliferous and argillaceous. From the study of core samples and detailed petrographic analysis of thin sections, five lithofacies were recognized and their extent determined. These lithofacies are: micrite, argillaceous micrite, dolomicrite, argillaceous dolomicrite, and argillaceous biomicrite. Major criteria used in categorization of each lithofacies were fossil biota, sedimentary structures, and other lithologic features. Sedimentation of the Three Forks Formation occurred in an epeiric sea setting. The five lithofacies represent deposition in supralittoral, littoral, and low-energy sublittoral environmen~s. These environments were formed approximately parallel to the periphery of the basin. Of the three energy zones depicted for an epeiric sea depositional setting, the Three Forks is interpreted to have been deposited solely within the most landward, lowest energy zone. Extreme sea level fluctuations and resultant progradations are responsible for the widespread distribution of argillaceous material. Allochthonous material was distributed by wind-generated waves and currents. Repeated transgressions and regressions caused the lateral migration of environments and produced the complex mosaic of lithe, facies. Major diagenetic features within the Three Forks include dolorritization, anhydritization, and calcite recrystallization (neorrorphism). Dolomitization is best accounted for by the evaporative pumping model, which may also have been responsible for anhydrite formation in the supralittoral environment. Eight structural types of anhydrite are present in those rocks interpreted as being deposited in an arid, supralittoral environment. These structural types are: nodular, distorted nodular, bedded nodular, distorted bedded nodular, distorted nodular mosaic, bedded nodular mosaic, distorted mosaic, and bedded massive. Calcite recrystallization involves microspar and pseudospar formation