ABSTRACT

Abrupt lateral thickness variations within the Prairie Formation are common and caused by two disparate but distinguishable processes; differential accumulation of salt during deposition and differential removal of salt by dissolution. Direct differentiating evidence, such as the presence of collapse breccias is preferred; however it is not often observed because the Prairie/Winnipegosis interval is seldom cored. Therefore indirect methods, utilizing well logs, provide the primary method for interpreting occurrences of the two processes. The results of this study indicate that the two processes can be distinguished using Prairie intraformational correlations.

There are several regionally correlative shoaling upward sequences within the Prairie Formation. The lowermost sequence begins with the transition from the Winnipegosis Formation laminated carbonates to the basin-centered Ratner anhydrite, followed by halite and culminating in the Esterhazy potash beds. Two overlying sequences also display shallowing upward trends; however, these sequences begin with halite and culminate in the Belle Plaine and Patience Lake (Mountrail) potash Members respectively. A fourth sequence is indicated by several additional feet of halite capping the Patience Lake in some parts of the basin. Complete desiccation and subsequent erosion or dissolution prior to burial may have removed the upper portion of this sequence.

Cross-sections show a regional, gradual decrease in thickness in the lower Prairie away from the basin depocenter. This is most simply explained by decreasing the subsidence rates away from the basin depocenter and by depositional onlap of the Prairie toward the basin margins. This depositional onlap pattern occurs along the basin's southern margin. By correlating the four sequences within the Prairie, some local thickness variations are shown to originate by depositional onlap onto Winnipegosis reefs. In this case, there is a progressive increase in areal extent of the formation from the base to the top. In other instances, such as along the western and eastern basin margins, the basal parts of the formation are preserved but the upper parts are progressively restricted. This pattern is explained by progressive dissolution of Prairie evaporites from the top downward.