Trioctahedral smectite in the Green River Formation, Duchesne County, Utah

Abundant trioctahedral smectite and smaller amounts of illite comprise a simple assemblage of clay minerals in a basin-edge sequence of lacustrine claystones and oil shales in the Eocene Green River Formation, southwest Uinta Basin, Duchesne County, Utah. Chlorite and mixed-layer clay are present in a few samples. The smectite is well crystallized and has a high expansible-layer content; its chemical composition is uncertain for lack of pure material, but available data suggest a Mg- and Fe-bearing species in the compositional range between saponite and lembergite. Although no significant differences were noted in the clay-mineral content of the claystones versus the oil shales, it was found that calcite strongly prefers oil shale whereas dolomite strongly favors claystone, suggesting that interstitial waters of the claystones during deposition and lithification were more saline than those of the oil shales. The trioctahedral smectite may be part of a basin edge-to-center series of clay-mineral facies--kaolinite to smectite to mica (illite).