The spatial distribution of the Shell Lake Member and its relationship with the Upper Winnipegosis reef mounds in the Saskatchewan Sub-basin record an important aspect of the transitional history of the Elk Point Basin from fully marine to desiccation. The Shell Lake Member in the study area is composed of anhydrite and laminated dolomite, and produces a good reflection on seismic profiles. The top of many Winnipegosis reef mounds gives a seismic reflection similar to the Shell Lake Member, leading to the interpretation in the literature that the Shell Lake Member is a continuous unit overlying the Winnipegosis reef mounds in the Sub-basin. Data presented in this paper demonstrate that the Shell Lake Member is not continuous because it terminates against the fully developed reef mounds. The anhydrite on top of the reef mounds previously interpreted as the Shell Lake Member is diagenetic anhydrite that occurs as replacement of the Winnipegosis carbonate at the top of reef mounds and as cement in the vugs. This interpretation is based on differences recorded in the depositional, lithological and diagenetic features of these two units. The formation of the diagenetic anhydrite of the Winnipegosis reef mounds is distinct from the primary depositional anhydrite in the Shell Lake Member based on the lithologic and diagenetic features, spatial distribution and stratigraphic relationship of the Winnipegosis carbonate and the Shell Lake Member in the study area. The spatial relationships of various lithologic units may serve as a predictor for the location of new Winnipegosis reef mounds and potash deposits.