Diagenesis of shallow marine carbonate rocks : isotopic and trace element constraints from the Mississippian Mission Canyon Formation, central and southwestern Montana /
| Main Author: | |
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| Other Authors: | , , , |
| Format: | Thesis Book |
| Language: | English |
| Published: |
1991.
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| Subjects: | |
| Online Access: | Link to ProQuest copy Link to OAKTrust copy |
| 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... |
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| Item Description: | "Major subject: Geology." Typescript (photocopy). Vita. |
| Physical Description: | x, 126 leaves : illustrations ; 29 cm |
| Bibliography: | Includes bibliographical references. |