Depositional and Dissolutional Processes and Their Resulting Thinning Patterns Within the Middle Devonian Prairie Formation, Williston Basin, North Dakota and Montana

Chris A. Oglesby

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 United States' 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 n some parts 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 thickness 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. A depositional onlap pattern is characteristic of the basin's southern margin. In this case, a progressive increase in areal extent of the formation occurs from the base to the top. By correlating the four sequences within the Prairie, some local thickness variations are shown to originate by depositional onlap onto Winnipegosis reefs. Other thickness variations occur in response to minor vertical movements across basement faults. In particular, the Brockton-Froid fault zone and the Nesson anticline have affected deposition of the Prairie and Winnipegosis Formations.

In other areas, such as along the western and eastern basin margins, the basal parts of the formation are more extensive and completely preserved, but upper parts are progressively diminished in areal extent from the top down. This pattern is explained by progressive dissolution of Prairie evaporites from the top downward, suggesting that the aquifers contributing to the dissolution overlie the Prairie Formation. Although no distinct linear pattern emerged, the coincidence between tectonic features such as the Brockton-Froid fault system, the Nesson anticline, and the Superior/Churchill contact with major dissolution areas suggests that dissolution is controlled primarily by tectonic fracturing.

Dissolution was also recognized over the Winnipegosis pinnacle reefs. This dissolution preferentially removed the overlying potash horizons and resulted in only partial dissolution of the Prairie. These observations suggest that dissolution resulted from an upward-directed flow of waters localized by the reef facies.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988