Abstract

Post-depositional alteration of the dominantly fluvial Salt Wash Member of the Upper Jurassic Morrison Formation in southwestern Colorado was investigated by study of authigenic and detrital minerals. The abundance and isotopic, chemical, petrographic, and mineralogical characteristics of barite, calcite, chlorite, dolomite, hematite, illite-smectite, kaolinite, pyrite, and quartz overgrowths were evaluated. The results were used to define four sequential episodes of alteration: 1) diagenesis during shallow burial, 2) diagenesis during deep burial, 3) alteration by fault-related solutions, and 4) alteration by near-surface weathering.

During shallow burial (<400m), calcite, chlorite-smectite, ferric oxides, pyrite, and chalcedony precipitated. Common micritic calcite nodules and hematite are consistent with the semi-arid depositional climate. Pyrite formed locally from H₂S generated by sulfate-reducing bacteria near accumulations of detrital plant fragments. Reduction of dissolved uranium and vanadium by organic matter and sulfide created local ore-grade accumulations.

From mid-Cretaceous to mid-Tertiary, the Morrison Formation was progressively buried to an maximum depth between 2.5 and 4 km. During deep burial (1 to 4 km) changes in pore-water composition, pressure, and temperature formed quartz overgrowths, chlorite, and illite-smectite throughout the sandstones.

Deep burial alteration was followed by migration of brines derived partly from bedded salts in the Pennsylvanian Paradox Formation. These brines moved upward along faults and outward into permeable Morrison sandstones. Following the brine influx, dilute meteoric water circulated along the faults. Calcite, dolomite, and barite precipitated from the solutions that moved along faults. Late Tertiary erosion increased circulation of meteoric water on structurally elevated portions of the study area. Modern weathering by meteoric water has dissolved chlorite and plagioclase and precipitated kaolinite.