Geochemistry and paleolimnology of the trona deposits and associated authigenic minerals of the Green River Formation of Wyoming

The Green River Formation of Wyoming contains a rich record of events in the history of Gosiute Lake, which existed for roughly 4 million years during the early and middle parts of the Eocene Epoch. At its maximum extent, the area of the lake was nearly 0. 6 the area of Lake Victoria in Africa, the second largest lake in the world. Like Victoria, Gosiute Lake owed its existence and long life to progressive, gentle downwarp of the crust, a downwarp that made room for successive increments of sediment. Gosiute Lake passed through a complex and long series of changes of area and depth, but it had three major changes in size. During the first stage, which lasted roughly 1 million years, the lake was large and overflowed continuously, so that it was an entirely fresh-water lake. All during this stage it had a large central elliptical island made up of Upper Cretaceous rocks of the ancestral Rock Springs uplift. The climate then became considerably more arid, and the lake shrank to about half its former size and ceased to overflow. This second stage also lasted about a million years. As all closed lakes do, it fluctuated frequently in area and depth. Many times it became extremely saline, and extensive beds of trona and mixed trona and halite were deposited. The beds of marlstone, dolomitic mudstone, oil shale; and bedded salts deposited in this closed lake have a maximum thickness of about 365 meters and make up the Wilkins Peak Member of the Green River Formation. Within this member is a saline facies, with which this report is primarily concerned. The third, and last, major stage began when the climate became considerably moister so that Gosiute Lake again expanded, overflowed, and became a fresh-water lake with a large central island. In this stage, which lasted roughly 2 million years, the lake expanded to its maximum size of about 40,150 square kilometers (15,500 sq mi). The upper part of the sediments deposited in this large lake contains a great quantity of volcanic ash. The repeated influxes of volcanic ash, added to the normal limy sediment, may have filled the basin faster than the downwarp could accommodate them and so brought Gosiute Lake to its end. It is useful to think of the saline facies of the Wilkins Peak Member as a great lens of sediments within the Wilkins Peak Member, laid down during the lowest stages of the closed lake. After each of the lowest stages, when the lake deposited the trona and mixed trona-halite beds, the lake expanded, and each time it expanded it deposited rather thick units of more or less saline, dolomitic mudstone, marlstone, and oil shale. These units extend laterally far beyond the limits of the trona beds but, of course, not nearly so far as the nonsaline sediments of the Wilkins Peak Memberj In addition to trona and halite, the saline facies also contains the following saline minerals: Shortite, northupite, pirssonite, gaylussite, nahcolite, thermonatrite, wegscheiderite, and tychite. Of these, shortite, which is found only in the Green River Formation, occurs in tremendous quantities. N orthupite and wegscheiderite also occur in considerable quantities, but the others are rare. The saline facies also contains an array of at least 27 nonsaline authigenic minerals, six of which are new species. Several others, acmite, magnesioriebeckite, labuntsovite, and leucosphenite, and probably several others, are remarkable because they had previously been known as minerals that formed only at rather high temperatures. All the nonsaline authigenic minerals occur sparingly except analcite, searlesite, stevensite, adularia, albite, and loughlinite, which have been found in considerable abundance. The adularia and albite are especially plentiful in the altered tuffs and tuffaceous beds. Nearly all the authigenic minerals formed after the sediments had been fully compacted. Analcite and pyrite, however, also formed, in part, just after deposition of the sediments. New complete chemical analyses of seven typical rocks of the saline facies are given. From these analyses, and one analysis published by J. J. Fahey, and from microscopic and X-ray determinations, the mineralogy of the eight rocks has been calculated, or estimated. Limitations of the conclusions are discussed in the following paragraphs. Beds of trona and mixed trona and halite underlie an area of roughly 2, 600 km2 (square kilometers) in the central and southern parts of the Green River Basin. Within this area are at least 27 beds of trona or trona and halite that range from about 1 m (meter) to more than 11.5 m thick at their thickest places. Between these beds are locally few to many thin trona layers, ranging in thickness from mere films to 0.3 or 0.6 m. Most of the trona beds contain brownish organic matter, pyrite, microgranular dolomite, fine silt particles, and thin partings of normal sediment, all of which indicate that these beds are chemical deposits laid down on the lake floor during very low stages of the lake. The major trona beds range in area from less than 428 km2 to at least 1,870 km2. Very conservative estimates of the volumes of these beds indicate that 18 of them contain approximately