Abstract: Carbonate-Evaporite Relations in Middle Devonian of Saskatchewan
Like the Silurian of the Michigan basin, the late Eifelian to Gedinian Winnipegosis and Prairie Evaporite Formations of the Elk Point basin have provoked controversy about the stratigraphic relations between carbonate buildups and enclosing evaporites. In the Saskatchewan portion of the basin there is evidence for (1) alternations between carbonate and evaporitic depositional episodes, (2) equivalency between certain carbonate and evaporite units, and (3) penecontemporaneous replacement of some carbonate units by evaporites.
Winnipegosis carbonate buildups (up to 370 ft or 110 m high and more than 50 mi or 80 km across) are composed of partly coalescent, stromatactis-bearing mud mounds (locally extensively replaced by formerly aragonitic spherulitic masses) with intervening, steeply dipping lithoclastic and lithoskel-bearing flank beds associated with algal-coral-stromatoporoid boundstones. In basins between the mud-mound complexes the buildups grade into thin (20 ft or 6 m or less) bituminous mudstones with local carbonate turbidites. These starved-basin deposits are overlain conformably by laminated (varvitic) carbonate rocks and anhydrites which are, in part, replaced oil shales. They appear to be unrepresented stratigraphically in the mud-mound complexes.
The mud-mound complexes are flanked by younger laminated carbonate rocks that commonly have been replaced almost entirely by nodular anhydrite. Replacement was penecontemporaneous with carbonate deposition and involved growth of large, sediment-enclosing gypsum crystals. The replaced carbonates overlie the center-basin laminites and interfinger basinward with chevron-halite units of the Prairie Evaporite. Mound complexes are terminated by complex sequences of loferites (containing teepee structures), vadose-pisolitic and lithoclastic breccias, and probable travertine deposits. These beds pass basinward and come to overlie halite. They become partly replaced by nodular anhydrite, pass gradationally into pelleted muds (and anhydrite replacements of the same), and interfinger with halite. Prairie Evaporite deposition continued with basin-wide halite precipitation (which finally buried the Winnipegosis carbonate buildups) and localized precipitation of potash-bearing salts.
AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma