Origin of dolomite in the Green River Formation

The Green River Formation in the Green River, Uinta, and Piceance Creek basins was deposited in ancient Lake Gosiute and Lake Uinta. The Tipton Shale Member represents deposits of an early fresh water stage to a later saline, alkaline, and organically productive stage. The Wilkins Peak Member mostly accumulated under shallow, evaporative, saline, alkaline lake margin to mud flat settings. The lower Laney Shale Member represents a revival of the fresh water lake, and the upper Laney Shale Member an evaporative, saline, alkaline, and organically productive lake. Similar analyses have illustrated that Lake Uinta evolved from a fresh water lake to a progressively more saline and alkaline lake. In the Piceance Creek Basin, the Mahogany zone is the product of deposition in an organically productive lake.; Two distinctly different occurrences of dolomites were recognized. The oil shale-associated in situ precipitated dolomites are present in both the light-colored laminae/clots and dark-colored laminae. They generally are anhedral to euhedral dolomicrite and dolomicrosparite (4-60 mum in size) and are iron-rich, comprising ferroan-dolomite to ankerite. Bacteriomorphs are closely associated with this type of dolomite, and their high delta 13C values (up to +12.5%o) indicate that bacterial methanogenesis process played an important role. Enrichment of 34S in sulfides caused by bacterial sulfate reduction (BSR) processes was probably also associated with the dolomite precipitation in the Green River Formation. Thus, it is believed that bacterial methanogenesis was the dominant bacterial process in the formation of dolomites within the oil shales of the Green River Formation.; The dolomicrites from micrite beds are nearly stoichiometric in composition and commonly associated with evaporites. This type of dolomite may have formed penecontemporaneously by evaporative dolomitization of the preexisting micritic calcites in the mud flats.; Abrupt facies changes were recognized as a result of the close juxtaposition of two distinct lithofacies types, the micrite lithofacies (which represent shallow marginal lake to mud flat settings) and oil shale lithofacies (which represent open lake settings). A wide, low-relief, shallow lake, either chemically or thermally stratified, subject to dynamic fluctuation, is most likely to have developed such abrupt facies changes.