Abstract: Carbonate Buildup-Evaporite Relations in Middle Devonian of Saskatchewan--Reinterpretation
The Winnipegosis and lower Prairie Evaporite Formations of Saskatchewan provide evidence for: (1) alternations between carbonate and evaporitic depositional episodes, (2) equivalency between other carbonate and evaporite units, (3) replacement of some carbonate materials by evaporites, and (4) regional dolomitization of Winnipegosis and lower Paleozoic carbonate deposits during deposition of Prairie evaporites.
Winnipegosis carbonate buildups are composed of partly coalescent, stromatactis-bearing mud mounds with intervening, steeply dipping lithoclastic and lithoskel-bearing flank beds. In basins between mud-mound complexes, buildups grade into thin bituminous mudstones. These starved-basin deposits are overlain conformably by carbonate and anhydrite laminites that are unrepresented stratigraphically in the mud-mound complexes. The mud-mound depositional phase is thus pre-evaporite.
Mound complexes are flanked by younger carbonate and anhydrite laminites and by "vadose" pisolitic sediments that commonly are replaced by mosaic anhydrite. Replacement was penecontemporaneous with carbonate deposition and involved growth of large, sediment-enclosing, gypsum crystals. The replaced carbonate rocks overlie the basin-central laminites and interfinger basinward with chevron-halite units.
Mound complexes are terminated by complex sequences of loferites, "vadose" pisolitic and lithoclastic breccias, and probable travertine deposits. These beds extend basinward over halite and pass gradationally into pelleted muds or anhydrite replacements and interfinger with halite.
The lower Prairie evaporites are believed to have been precipitated from groundwaters that entered the basin by means of the Winnipegosis buildups. Groundwater seepage first deposited travertine on mound tops, then caused massive growth of interstitial sediment-replacing and sediment-displacing gypsum in mound-flanking beds. Concentration and processing of brines during their downward migration allowed final precipitation of chevron halite in salt flats on the basin floor. Groundwaters also caused regional dolomitization of pre-Prairie Evaporite carbonate materials which lay along the flow paths. Dolomitization of lower Winnipegosis blanket carbonate deposits occurred prior to lithification (during compaction) and is consistent with the suggested dolomitization model.
AAPG Search and Discovery Article #90964©1978 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah