LITHOLOGY, MINERALOGY, AND DEPOSITIONAL ENVIRONMENTS OF THE UPPER PART OF THE GREEN RIVER FORMATION (EOCENE) IN INDIAN CANYON AND VICINITY, DUCHESNE COUNTY, UTAH

Lacustrine rocks comprising the uppermost 950 meters of the Green River Formation (Eocene) were measured and sampled in and near Indian Canyon in the southwestern part of the Uinta Basin, Duchesne County, Utah. The sequence extends from the top of the “delta facies” of Bradley (1931) up to the top of the “limestone and sandstone facies” of Dane (1955). Oil shale, marlstone, “mealstone”, dolomite, limestone, claystone, shale, siltstone, mudstone, and sandstone comprise most of the rocks in the measured section. Mealstone is defined as organic marlstone containing abundant calcite crystals that impart a mealy appearance to the rock. Many volcanic tuffs, a few millimeters to 0.9 meter in thickness, are scattered throughout the sequence but are sparse in the upper one-third. Bohor and others (1996) reported a radiometric date of 43.95 ±0.17 Ma for a 0.9-meter-thick tuff at 376 m above the base of the measured section.

Samples were analyzed for bulk mineralogy by X-ray diffraction and for organic content by pyrolysis fluorescence. Illite, smectite (stevensite), quartz, sodium and potassium feldspar, calcite, and dolomite are pervasive through most rocks with lesser amounts of kaolinite, talc, analcime, and gypsum restricted to certain parts of the measured section. The average organic content of the rocks, measured in gallons of shale oil per ton of rock (GPT), is about 5 or more GPT in about the middle one-third of the measured sequence, but decreases to an average of 0.4 GPT in the rocks above.

The Horse Bench Sandstone Bed of the Parachute Creek Member of the Green River Formation in the lower one-third of the measured section is laterally equivalent to the “birds nest” zone, a saline facies containing nahcolite and sodium carbonate brine in the subsurface northeast of Indian Canyon. Above the Horse Bench, another saline facies comprising the middle one-third of the measured section is recognized in outcrops by crystal cavities, cherty beds, and mealstone whose calcite crystals are probable pseudomorphs after shortite. Drill cores of this saline facies in the subsurface north of Indian Canyon contain shortite, nahcolite, trona, wegscheiderite, eitelite, northupite, searlesite, halite, and probably other evaporite minerals. Both saline facies may have economic value for their sodium carbonate salts and brine.