Sedimentary Depositional Cyclicity, Mission Canyon and Charles Formations (Mississippian), Williston Basin, North Dakota

Workers within the Williston Basin have repeatedly reported the presence of sedimentary cyclicity within the carbonate and evaporite succession of the Mississippian Madison Group. Descriptions of cyclicity are diverse and commonly conflict. Previous work has been exclusively qualitative. This study used a group of statistical techniques in a method designed specifically for the objective identification and evaluation of cyclicity in the lithofacies sequence of the Mission Canyon and lower Charles Formations in North Dakota. Twelve cores were sampled at every visually distinguishable lithologic unit. Forty-six lithologic components were counted in each sample on a presence-absence basis. The phi association coefficient was used to calculate similarities between all pairs of lithologic units in a data set. Similarities were clustered; significant clusters were used as lithofacies. The succession of lithofacies was examined for cyclicity using embedded Markov analysis. Mutual substitutability analysis identified pairs of lithofacies that could be combined to permit further iteration of cyclicity analysis. The final succession was examined using entropy analysis. A variety of carbonate and evaporite lithofacies, deposited under subtidal to supratidal, open- and restricted-circulation conditions, were identified. Specific interpretation of depositional environments was rarely permitted by the data. Both lithofacies and cyclicity reflect increased evaporite deposition upward in the succession and from the basin center to the margin. After analysis of data sets from individual wells, data representing selected stratigraphic intervals over larger geographic areas were combined and examined for cyclicity. Homogeneity tests of the combined data sets indicate probable areal stationarity within the succession, although the occurrence of abrupt cyclicity changes in the succession indicates a lack of vertical stationarity. Cyclicity exists in the succession but is not everywhere present, nor uniform in character throughout the basin. Cycles are simple, dominantly asymmetric, and of minor importance on the flanks of the basin; they are more complex, dominantly symmetric, and dominate the succession near the basin center. The cyclicity is more compatible with deposition of lithofacies as a mosaic, than as narrow facies belts, and indicates the need for new facies models