STABLE ISOTOPIC COMPOSITION OF METEORIC CALCITES - EVIDENCE FOR EARLYMISSISSIPPIAN CLIMATE-CHANGE IN THE MISSION CANYON FORMATION, MONTANA

The Lower Mississippian Mission Canyon Formation of central to southwe stern Montana was deposited under dominantly semiarid to arid climatic conditions during Osagean to early Meramecian times. Following deposi tion, a pronounced climatic shift to more humid conditions occurred du ring middle Meramecian times. This climatic change is indicated by ext ensive, post-depositional karst fabrics and in the stable isotopic com position 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 por osity. Petrographic data indicate that this cement formed during inter mittent subaerial exposure of the Mission Canyon platform during Osage an times. This initial generation of meteoric calcite cement has delta O-18 values from -8.1 to -2.6 parts per thousand PDB. These data, and the oxygen isotopic values from nonluminescent skeletal grains and mic rite in host limestone indicate that Osagean meteoric water may have h ad deltaO-18 values as low as -6.0 parts per thousand SMOW. A second g eneration of petrographically similar, but isotopically distinct, calc ite cement fills biomolds and porosity within solution-collapse brecci as in the Mission Canyon Formation. This cement generation postdates e arlier nonluminescent Osagean calcite cement and is volumetrically mos t abundant near the top of the Mission Canyon Formation. DeltaO-18 val ues from these cements and from nonluminescent lime mudstone clasts an d matrix in solution collapse breccias range from -13.8 to -8.2 parts per thousand PDB. These data indicate that Meramecian meteoric water m ay have had deltaO-18 values as low as - 12.0 parts per thousand. Howe ver, a higher-temperature burial overprint on the deltaO-18 values of the calcite cement cannot be ruled out. The more positive deltaO-18 va lues of the Osagean calcite components probably indicate warm and arid conditions during short-term [10(4)(?) yr) subaerial exposure along i ntraformational sequence and parasequence boundaries. The more negativ e deltaO-18 values from Meramecian calcite components and the extensiv e karst associated with the post-Mission Canyon unconformity may have developed because of cooler and more humid climatic conditions and pos sible rain-out effects during middle Meramecian times. A dramatic shif t towards cooler and more humid climatic conditions may be coincident with the onset of major continental glaciation in the Early Carbonifer ous. The post-Mission Canyon unconformity has been attributed to a maj or 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 pola r fronts in turn, would have pushed moist, mid-latitude weather system s toward the paleoequator, resulting in cooler, more humid conditions in low-latitude settings during ''icehouse'' times.