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Keywords:

  • Canada;
  • dolomitization;
  • evaporative brines;
  • geochemistry;
  • Middle Devonian;
  • Winnipegosis carbonate

Abstract

The Middle Devonian Winnipegosis carbonate unit in south-central Saskatchewan is partially to completely dolomitized. Two major types of replacive dolomite are distinguished. Microcrystalline to finely crystalline dolomite (type 1) displays nonplanar-a to planar-s textures, mimetically replaces the precursor limestone, accounts for about four-fifths of dolomite phases volumetrically, and mainly occurs in the Winnipegosis mounds and the Lower Winnipegosis Member directly underlying the mounds. Medium crystalline dolomite (type 2) shows planar-s to planar-e textures, commonly occurs in the Lower Winnipegosis and Brightholme members, and decreases upward in abundance. The 87Sr/86Sr ratios of type 1 dolomite (0·70795 to 0·70807) fall within the estimated Sr-isotopic range for Middle Devonian marine carbonates. Stratigraphic, petrographic and geochemical data constrain the formation of type 1 dolomite to hypersaline sea water in a near-surface environment, after marine cementation and sub-aerial diagenesis and prior to precipitation of the Middle Devonian Leofnard salts. Movement of dolomitizing fluids could be driven by density differences and elevation head. The shift to lower δ18O values of type 1 dolomite [−7·4 to −5·1‰ Vienna Pee Dee Belemnite (VPDB)] is interpreted as the result of recrystallization at elevated temperatures during burial. Type 2 dolomite has higher 87Sr/86Sr ratios (0·70809–0·70928), suggesting that the dolomite probably formed from basinal fluids with an increased richness in the radiogenic Sr isotope. In type 2 dolomite, Sr2+ concentrations are lower, and Fe2+ and Mn2+ concentrations are higher, compared with the associated limestone and type 1 dolomite. Type 2 dolomite is interpreted as having been formed from upward-migrating basinal fluids during latest Devonian and Carboniferous period.