A TexasGulf composite core representing deposition during Wilkins Peak time of the Green River Formation (Eocene) is described. Two major facies, a mudstone facies and a trona facies, represent deposition within ephemeral alkaline saline Lake Gosiute. A laminated mud-stone subfacies indicates a distal saline mudflat subenvironment, whereas a crumbly, homogeneous mudstone formed within a proximal saline mudflat subenvironment. Oil shale signifies flooding and expansion of the lake with associated organic blooms. Drying of the lake permitted precipitation of trona in two forms; a) a displacive nodular type consisting of radiating crystals, and b) a granular or sugary type of detrital origin. A third, postdepositional trona type is described as a columnar crystalline vein type associated with hydraulic fracturing.