Abstract

Dolomite in the Mississippian Mission Canyon Formation (Osagean to middle Meramecian) of central and southwestern Montana precipitated during multiple episodes of dolomitization and/or recrystallization. The earliest generation of dolomite most likely precipitated from evaporated sea water in near-surface diagenetic environments when the Mission Canyon platform intermittently aggraded to sea level. This dolomite has relatively positive δ18O values (-1.5‰ to +7.5‰) and typically is nonstoichiometric and trace-element enriched. A second generation of dolomite has very negative δ18O values (-11.2‰ to - 1.5‰) and always is stoichiometric and trace-element depleted. Dolomite with δ18O values <-6.8‰ is in the upper 100-180 m of the Mission Canyon Formation, beneath a regional post-Mission Canyon unconformity and karst surface that extends across the entire study area. Dolomite depleted in 18O may have (1) precipitated initially in a regional mixing zone during post-Mission Canyon subaerial exposure and karstification, (2) precipitated at elevated temperatures during later diagenesis, or (3) recrystallized from an earlier generation of dolomite during later shallow to deep burial diagenesis. Very negative δ18O values and low trace-element contents for the second generation of dolomite suggest that diagenetic solutions were dominated by meteoric water, whereas δ13C values indicate that diagenetic fluids were buffered by Osagean carbonate rocks. The Mission Canyon Formation is similar to many carbonate sequences in that multiple episodes of dolomitization may be responsible for the eventual stratigraphic distribution of dolomite and its petrographic and geochemical characteristics.

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