EARLY DOLOMITIZATION AND RECRYSTALLIZATION IN SHALLOW MARINE CARBONATES, MISSISSIPPIAN ALIDA BEDS, WILLISTON BASIN (CANADA): EVIDENCE FROM PETROGRAPHY AND ISOTOPE GEOCHEMISTRY

Abstract: The Mississippian (Lower Carboniferous) Alida Beds of the northern Williston Basin, Canada, are carbonate rocks that contain a number of significant hydrocarbon reservoirs. These shallow marine carbonate platform deposits consist of abundant very finely to finely crystalline dolomite that occurs mainly in mud-dominated sediments. Dolomite crystals are ∼ 4 to 30 µm in length, subhedral to anhedral in shape, and typically replace the matrix, but also occur as thin lamellae within ooid cortices. Textural relations indicate that the dolomite predates all other diagenetic phases, including coarse calcite cement, anhydrite, and bitumen. Dolomite δ18O values (−9.3 to −4.5‰ PDB) are depleted relative to that expected for precipitation from Osagean (late Tournaisian) seawater (−0.5 to +1.5‰ PDB), while δ13C values (+2.3 to +3.4‰ PDB) are within the range of Osagean seawater values (+1.8 to +4.8‰ PDB). Dolomite 87Sr/86Sr values (0.70816 to 0.70921) are somewhat elevated with respect to Osagean seawater (0.70768 to 0.70817). A trend of increasing average dolomite crystal size with decreasing δ18O is observed in matrix-replacive dolomite. Based on petrographic relations, δ13C compositions, and the pervasive occurrence of finely crystalline dolomite, initial dolomitization is interpreted to have taken place in the presence of slightly evaporated seawater. The absence of depositional evaporites in dolomitized intervals suggests that dolomitization was driven by reflux of slightly evaporated seawater that had not reached gypsum saturation. Depleted δ18O values, elevated 87Sr/86Sr ratios, and correlation of dolomite crystal sizes and δ18O values suggest that the early-formed dolomite was variably recrystallized during burial.