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Based on a study of 10 drill cores, a sequence of oil shale and associated nahcolite, nearly 2,000 feet thick, in the lacustrine Green River Formation (Eocene) in the Piceance Creek basin, Rio Blanco County, Colo., was divided in ascending order into zones 1 to 13, B-groove, Mahogany zone (with lower, middle, and upper parts), and A-groove at the top. The odd-numbered zones and the Mahogany zone are mappable subsurface units of relatively thick oil shale and are distinguished from the even-numbered zones and A-and B-grooves which are thinner units of oil shale of lower grade.Large amounts of nahcolite found in zones 5 to 12 occur in (1) coarsegrained crystalline aggregates scattered through oil shale, (2) laterally continuous units of fine-grained crystals disseminated in oil shale, (3) brown microcrystalline beds, and (4) white coarse-grained beds that grade laterally into halitic rocks toward basin center. The original upper limit of the nahcolite and halitic rocks is not yet completely known, but the present top is marked by a dissolution surface. Above this surface the rocks, extending from zones 11 or 12 upward into the Mahogany zone, form a water-saturated "leached zone," a geohydrologic unit in which large amounts of water-soluble minerals probably mostly nahcolite and halite, were removed by ground-water dissolution. Rocks' in the leached zone, mostly oil shale, are commonly broken and fractured and contain crystal cavities and solution breccias. Several solution breccias can be traced laterally into unleached beds of nahcolite and halite. Although evidence of salines is found in rocks above A-groove, the original saline facies that includes most of the bedded deposits extends from zone 5 upward into A-groove.Potentially minable beds of white nahcolite as much as 12 feet thick are found at depths of 1,560 or more feet below the surface. Some thicker beds of high-grade nahcolite are believed to be too close to the dissolution surface for safe room-and-pillar mining. Probably the most economical method of mining nahcolite would be as a coproduct of a shale-oil industry. Removal of nahcolite prior to retorting increases significantly the grade of oil shale by as much as 1.6 times. Several zones are more than 300 feet thick and average 30 or more weight percent nahcolite. Resources of nahcolite per square mile range as high as 489 million short tons. The total nahcolite resource in the basin is conservatively estimated at 32 billion short tons, which makes it the second , largest deposit of sodium carbonate known in the world.

Abundant trioctahedral smectite of probable authigenic origin and smaller amounts of discrete 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. Small amounts of chlorite and mixed-layer clay are present in a few samples; kaolinite is absent .The smectite is well crystallized and has a high expandable-layer content. Chemical analysis of partly purified material suggests an iron-bearing hectorite which contains notable quantities of zinc (740 ppm (parts per million)) , lithium (670 ppm), copper (175 ppm), and fluorine (0 .85 percent).No significant differences were noted in the clay-mineral content of the claystones and oil shales. However, of the associated carbonate minerals, calcite predominates in the oil shales, whereas dolomite strongly predominates in the claystones. This relationship suggests that the interstitial waters of the claystones during deposition and (or) lithification were more saline than those of the oil shales. The trioctahedral smectite evidently formed in a magnesium-rich lacustrine environment and appears to be part of a basin edge-to-center series of clay mineral facies-mixed detrital clay minerals to smectite to illite.

Marly lacustrine rocks of the Eocene Green River Formation, which occupy parts of several basins in Wyoming, Colorado, and Utah, contain world-class resources of oil shale and sodium carbonate minerals. The formation contains the world's largest known resource of natural sodium carbonate as bedded trona (Na 2 CO 3. NaHCO 3. 2H 2 O) in southwest Wyoming as well as the second largest known resource of sodium carbonate as bedded and nodular nahcolite (NaHCO 3) in northwest Colorado. Five companies currently mine three beds of trona in Wyoming and one company mines a bed of nahcolite in Colorado. Other sodium carbonate minerals in the Green River Formation that may have future economic potential include dawsonite (NaAl(OH) 2 CO 3), eitelite (Na 2 CO 3. MgCO 3), and shortite (Na 2 CO 3. 2CaCO 3). Several sodium carbonate brines in Utah and Wyoming may have economic value for soda ash and for water-soluble organic acids.

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