Stable isotopic composition of meteoric calcites: Evidence for early mississippian climate change in the mission canyon formation, Montana

Abstract

The Lower Mississippian Mission Canyon Formation of central to southwestern Montana was deposited under dominantly semiarid to arid climatic conditions during Osagean to early Meramecian times. Following deposition, a pronounced climatic shift to more humid conditions occurred during middle Meramecian times. This climatic change is indicated by extensive, post-depositional karst fabrics and in the stable isotopic composition of early, meteoric calcite cements and diagenetically altered sediments. Early meteoric calcite cement in Mission Canyon limestones is generally nonluminescent and fills intergranular and fenestral porosity. Petrographic data indicate that this cement formed during intermittent subaerial exposure of the Mission Canyon platform during Osagean times. This initial generation of meteoric calcite cement has δ 18O values from −8.1 to −2.6%. PDB. These data, and the oxygen isotopic values from nonluminescent skeletal grains and micrite in host limestone indicate that Osagean meteoric water may have had δ 18O values as low as −6.0%. SMOW. A second generation of petrographicalty similar, but isotopically distinct, calcite cement fills biomolds and porosity within solution-collapse breccias in the Mission Canyon Formation. This cement generation postdates earlier nonluminescent Osagean calcite cement and is volumetrically most abundant near the top of the Mission Canyon Formation. δ 18O values from these cements and from nonluminescent lime mudstone clasts and matrix in solution collapse breccias range from − 13.8 to − 8.2%. PDB. These data indicate that Meramecian meteoric water may have had δ 18O values as low as − 12.0%. However, a higher-temperature burial overprint on the δ 18O values of the calcite cement cannot he ruled out. The more positive δ 18O values of the Osagean calcite components probably indicate warm and arid conditions during short-term [10 4(?) yr] subaerial exposure along intraformational sequence and parasequence boundaries. The more negative δ 18O values from Meramecian calcite components and the extensive karst associated with the post-Mission Canyon unconformity may have developed because of cooler and more humid climatic conditions and possible rain-out effects during middle Meramecian times. A dramatic shift towards cooler and more humid climatic conditions may be coincident with the onset of major continental glaciation in the Early Carboniferous. The post-Mission Canyon unconformity has been attributed to a major fall in sea level that may have glacio-eustatic origins. Growth of continental glaciers during a time of global cooling would have caused migration of polar fronts further toward the paleoequator. These polar fronts in turn, would have pushed moist, mid-latitude weather systems toward the paleoequator, resulting in cooler, more humid conditions in low-latitude settings during “icehouse” times.