Diagenesis of the Middle Devonian Winnipegosis and Ratner deposits in Southern Saskatchewan, Canada

The Middle Devonian Winnipegosis and Ratner deposits in the Elk Point Basin of Saskatchewan have been greatly altered by diagenesis. The objective of this study is to characterize the diagenetic features of the Winnipegosis and Ratner rocks with emphasis on subaerial diagenesis, dolomitization, and sulfate replacement and cementation, by integration of petrography and geochemistry.; Dolomitized calcrete (dolocrete) profiles and paleokarst structures are the most striking subaerial diagenetic features of Winnipegosis mounds. Up to three discrete calcrete profiles are present in the top parts of the mounds. The calcrete profiles are interpreted to be pedogenic in origin and to have formed by diagenetic alteration of the host carbonate deposits in meteoric vadose zones of the mounds. Extensive solution features, fractures, cavity sediments and speleothems characterize the Winnipegosis paleokarst. Periodic subaerial exposure and repeated changes in water-level have led to the development of karst features at various depths corresponding to the occurrence of meteoric vadose, phreatic and mixing zones at different levels in the mounds.; The Winnipegosis and Ratner sequences have undergone extensive dolomitization. Two major types of replacive dolomite are distinguished based on petrography observation: microcrystalline to finely crystalline dolomite and medium to coarsely crystalline dolomite. Petrographic, stratigraphic and Sr isotopic constraints suggest that microcrystalline to finely crystalline dolomite occurred syn-depositionally by the downward flux of evaporative brines. The dense dolomitizing brines may have not effectively seeped through the gypsum/anhydrite beds in the Ratner laminite but rather could have breached the seal through fractures. Recrystallization of early-formed dolomite occurred at elevated temperatures of approximately 70-83°C related to burial and a late Paleozoic thermal anomaly. Medium to coarsely crystalline dolomite has higher 87Sr/86Sr ratios and lower delta18O values than the corresponding Middle Devonian marine carbonate, and is interpreted to have precipitated from upward-migrating basinal fluids. This late stage of dolomitization postdated or occurred coevally with early stylolitization during burial.; Five types of diagenetic anhydrite and gypsum have been identified: crystallotopic anhydrite, acicular anhydrite, blocky anhydrite, macrocrystalline gypsum and selenite gypsum. The various types of anhydrite and gypsum formed at different stages and from different fluids, as suggested by their petrographic evidence and geochemical data.