Within the Williston basin, thickness variations of the Prairie Formation are common and are interpreted to originate by two processes: differential accumulation of salt during deposition and differential removal of salt by dissolution. Unambiguous evidence for each process is rare because the Prairie/Winnipegosis interval is seldom cored within the US portion of the basin. Therefore, indirect methods, using well logs, provide the principal method for identifying characteristics of the two processes. The results of this study indicate that the two processes can be distinguished using correlations within the Prairie Formation. Several regionally correlative brining-upward and probably shoaling-upward sequences occur within the Prairie Formation. Near the basin center, the lowermost sequence is transitional with the underlying Winnipegosis Formation. This transition is characterized by thinly laminated basal carbonates that become increasingly interbedded with anhydrites of the basin-centered Ratner member. The remainder of the sequence progresses up through halite and culminates in the halite-dominated Esterhazy potash beds. Two overlying sequences also brine upward; however, these sequences lack the basal anhydrite and instead begin with halite and culminate in the Belle Plaine and Mountrail potash members, respectively. A fourth sequence is indicated by several feet of halite capping the Mountrail member in some partsmore » of the basin. Subsequent erosion or dissolution prior to burial may have removed the upper portion of this sequence. Cross sections show that the lower Prairie gradually decreases in thickness from the basin to its margins. This thickens variation is most simply explained by decreasing accommodation potential due to decreased basin topography away from the basin depocenter and by depositional onlap of the Prairie toward the basin margins.« less