Alteration of early-formed dolomite during shallow to deep burial: Mississippian Mission Canyon Formation, central to southwestern Montana

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 [delta][sup 18]O values ([minus]1.5% to +7.5%) and typically is nonstoichiometric and trace-element enriched. A second generation of dolomite has very negative [delta][sup 18]O values ([minus]11.2% to [minus]1.5%) and always is stoichiometric and trace-element depleted. Dolomite with [delta][sup 18]O values < [minus]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 [sup 18]O 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 [delta][sup 18]O values and low trace-element contents for the second generation of dolomite suggest that diagenetic solutions were dominated by meteoric water, whereas [delta][sup 13]C values indicate that diageneticmore » fluids were buffered by Osagean carbonate rocks. 54 refs., 10 figs.« less