ABSTRACT

Sandstones of the Salt Wash Member, Late Jurassic Morrison Formation, contain abundant authigenic barite on the east side of the northern Colorado Plateau (mean = 0.5 g barite/kg rock) and systematically less barite (mean = 0.02 g barite/kg rock) in the western part. The area containing abundant barite coincides approximately with the extent of bedded evaporites in the underlying Pennsylvanian Hermosa Formation. A genetic link between the barite and solutes from the Hermosa Formation is supported by sulfur isotope data. Most barites from the abundant barite zone have ³⁴S values that range from +8 to +14 per mil, which is similar to the range of bedded gypsum and anhydrite in the Hermosa Formation. Sulfate derived from t e evaporites probably entered the Morrison Formation in solutions that moved along faults.

Measured ⁸⁷Sr/⁸⁶Sr ratios of barite in the abundant barite zone are indicative of fault-controlled solution flow. Strontium contents and ⁸⁷Sr/⁸⁶Sr ratios (0.7103 to 0.7084) in barite decrease with increasing distance from faults. These trends reflect mixing of radiogenic strontium in the saline waters that ascended faults (⁸⁷Sr/⁸⁶Sr = 0.7100), with strontium originally contained in early diagenetic carbonate cements within the sandstones (⁸⁷Sr/⁸⁶Sr = 0.7080). Because measured ⁸⁷Sr/⁸⁶Sr values exceed the ratios in Hermosa anhydrite, it is concluded that radiogenic strontium was added to the rising brines by reaction between the evaporite-derived saline solutions and arkosic san stones along the flow path.

Morrison sandstones within the abundant barite zone were altered by saline waters that added sulfate and strontium. Barite formation in this zone was limited by the availability of dispersed barium in the host rock. The zone of sparse barite on the western side of the Colorado Plateau was not affected by the same warm saline waters. Instead, intraformational redistribution of small amounts of barium, strontium, and sulfate formed the relatively sparse barite in this area.