Diagenesis of shallow marine carbonate rocks : isotopic and trace element constraints from the Mississippian Mission Canyon Formation, central and southwestern Montana

Abstract

The Mississippian Mission Canyon Formation (Osagean to lower Meramecian) occurs in central and southwestern Montana as a thick sequence of interbedded shallow water limestones and dolostones. Most of the dolomite in the Mission Canyon Formation initially precipitated in near surface, sabkha-type environments when the platform intermittently aggraded to sea level. Geochemical data indicate that some of these early-formed dolomites may be relatively unaltered, although some may have partially recrystallized in hypersaline brines during syndepositional diagenesis. However, petrographic constraints and stratigraphic variation in dolomite δ[^18]O values indicate that most of the dolomite recrystallized during Meramecian meteoric diagenesis when the Mission Canyon platform was subaerially exposed and karstified. Mission Canyon dolomites which are relatively unaltered have 8 180 values which range from -1.5 to +7.5[parts per thousand] (PDB). These "unaltered" dolomites frequently are nonstoichiometric and trace element-enriched. Conversely, dolomites with δ[^18]O values between -11.2 to -1.5[parts per thousand] (PDB) probably recrystallized during meteoric diagenesis. These dolomites nearly always are stoichiometric and trace element-depleted. Meteoric recharge probably occurred intermittently during Osagean time along short-term (10^4 - 10^5 yr) subaerial exposure surfaces, and during Meramecian time along the regional unconformity and karst surface which overlies the Mission Canyon Formation (~10 my duration). Stable isotopic data from nonluminescent meteoric calcite cements which post-date dolomitization indicate that Osagean meteoric water may have had δ[^18]O values as low as -6[parts per thousand] SMOW, whereas δ[^18]O values for Meramecian meteoric water may have been as low as -12[parts per thousand] SMOW. The 6[parts per thousand] difference in δ[^18]O values between inferred Osagean and Meramecian meteoric waters is interpreted to be due to a change in local paleoclimatic conditions following deposition of the Mission Canyon Formation. Lower δ[^18]O values for Meramecian meteoric waters are consistent with a regional increase in relative humidity, lower atmospheric temperatures, and with cloud movement over large areas of exposed continental landmass following a significant fall in eustatic sea level...