The Middle Devonian lower Prairie Evaporite Formation of central Saskatchewan
Reinson, Gerald E.
The lower Prairie Evaporite of central Saskatchewan consists of three distinct members, which are named, in ascending order, the Ratner, Whitkow and Shell Lake Members. The Ratner Member consists of interlaminated and interbedded carbonate and anhydrite, the Whitkow Member of halite with anhydrite in the lower part, and the Shell Lake Member of anhydrite with a medial carbonate unit called the Quill Lake marker beds. The carbonate platform of the lower Winnipegosis member is overlain locally by carbonate banks with up to 300 feet of relief, which are included in the upper Winnipegosis member. Basinal areas between these banks became the sites of accumulation of the lower Prairie Evaporite which reached a maximum thickness of 350 feet. The upper 20 feet of the carbonate banks consist of algal-derived pisolitic and stromatolitic material, and of some pisolitic and stromatolitic material which probably formed in the vadose zone during subaerial exposure. These shallow water and supratidal-subaerial carbonates extend into the adjacent anhydrite of the Shell Lake Member in off-bank areas. Thus, the upper portions of the Winnipegosis banks appear to be equivalent to the uppermost part of the lower Prairie Evaporite. The Ratner Member averages 45 feet in thickness and contains eight distinct units with laminated carbonate and, or, anhydrite, which can be traced throughout the northern part of the study-area. Sub-millimeter carbonate laminae, in one of the units, can be correlated exactly over a distance of two miles. The remarkable uniformity and lateral continuity of layers in the Ratner Member suggest deposition in a basinal environment with relatively quiet water conditions. This is contrary to previous investigations which have inferred a supratidal origin for these sediments. Layers rich in organic matter have been preserved in these sediments by precompaction calcitization and dolomitization. Thick sequences of nodular mosaic and massive anhydrite, of the Shell Lake and Whitkow Members, formed on the flanks of carbonate banks. These anhydrites accumulated preferentially on the southeast sides of carbonate banks. If the regional flow was from the northwest, maximum evaporite deposition occurred on the leaward sides of banks. These thick anhydrites are lateral equivalents of, and interbedded with halite of the Whitkow Member. The relationship of nodular mosaic and massive anhydrite (Shell Lake-Whitkow Member) on bank-flanks, and interlaminated carbonate and anhydrite (Ratner Member), and halite (Whitkow Member), in basinal areas, is analogous to other ancient evaporite-carbonate occurrences, such as the Zechstein of Germany. Note:This thesis contains maps that have been sized to fit the viewing area. Use the zoom in tool to view the maps in detail or to enlarge the text.