Calcium and magnesium carbonates are perhaps the most widespread minerals in the oil-shale sequence of the Green River Formation. Crystallography and chemical composition of the principal carbonate minerals in the oil shale indicate a wide range in mineral composition that includes calcite, Mg-calcite, dolomite which is iron-free and has normal 1:1 Ca:Mg, dolomite with low iron and excess Ca, ankerites with excess Ca, siderite, magnesiosiderite with low Ca, and aragonite.

Recognition of the ubiquitous presence of Mg-calcite in these rocks seems important in reconstructing a plausible depositional model for the sedimentary environment and origin of the kerogen-rich shales. Recent studies show the ability of algae and many invertebrates to precipitate high Mg-calcite in aqueous saline environments. Biogenic precipitation of calcite in the hypersaline waters of the Green River lake may account for the higher Mg-content of laminae adjacent to and within high-kerogen zones than in low-kerogen zones.

Subsequent to the inferred biologic precipitation or accumulation of high Mg-calcite in the upper levels of the lake, the Mg-calcite sank to a lower zone of extreme salinity after death of the organism. Postdepositional processes in the lower zone of accumulation may have converted high Mg-calcite to low Mg-calcite and dolomite, as suggested by the presence of this mineral assemblage in many of the samples studied. Development of ankerite, siderite, and magnesiosiderite is believed to be authigenic or diagenetic. Metastable carbonate minerals such as Mg-calcite plus dolomite with excess Ca, or Mg-calcite plus ankerite with excess Ca have been identified in a single polymineralic grain. This finding makes untenable an interpretation of the oil-shale mineral assemblage based on conventi nal equilibrium conditions for the system CaCO₃-MgCO₃-MgCO₃-FeCO₃ at 25°C.

We find no compelling mineralogic or chemical evidence that indicates precipitation of calcite and protodolomite at the basin margin followed by reworking and transport of the carbonate sediments to the basin center, as other workers have recently proposed for their playa-lake model.

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