Dolomitization of the Middle Devonian Winnipegosis carbonates in south-central Saskatchewan, Canada

摘要

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 Sr-87/Sr-86 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 delta O-18 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 Sr-87/Sr-86 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.