|GILSONITE VEIN NEAR BONANZA, UTAH|
One of the major subdivisions of the northern Colorado Plateaus is the Uinta Basin, a large basin associated with the Sevier/Laramide (Cretaceous & early Tertiary) Orogeny and located in eastern Utah south of the Uinta Mountains. The Basin is a structural basin, as opposed to a topographic basin, meaning that it is a very large syncline, and a compliment to the anticlinal Uinta Mountains. The structure is related to the Piceance Basin of northwestern Colorado, and the Fossil and Green River Basins north of the Mountains in Wyoming. The rocks in the Basin are an interesting sequence of latest Paleocene (~58 my) to early Oligocene (~28my) basin-fill sediments consisting of, in ascending order, the Colton Formation (stream and flood plain deposits), the Green River Formation (Lake Uinta, part of a large fresh water lake system), the Uinta Formation (lake edge and lake filling sediments), and the Duchesne River Formation (stream and flood plain sediments on top of the lake sediments.
The Basin is well-known for its production of oil and gas, as well as for interesting examples of solid hydrocarbons--kerogen-rich mudstone (oil shale) and bitumen-impregnated sandstone (tar sands). But perhaps the most interesting hydrocarbon in the Basin is one that a few years ago (pre-1995) would not be classified as a ”mineral” and that is Uintaite, referred to in this article as the trade name Gilsonite. In 1995 the International Mineralogical Association adopted a new definition of a mineral as “an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes” (Nickel, 1995). This definition opened the way for an organic class of substances that included hydrocarbons of which Gilsonite is a member. Gilsonite is a solid hydrocarbon that comes from the solidification of petroleum. It is usually a dull black in the field and resembles coal; however, some fresh surfaces are quite shiny with a conchoidal fracture and superficially looks a little like obsidian.
Gilsonite in the Uinta Basin occurs in long veins (measured in miles) from a few inches to several feet thick and hundreds of feet in a vertical direction. Most thick veins occur in the Green River and Uinta Formations, both units are Eocene in age. In fact, the veins seem “rooted” in the oil shales of the Green River Formation. Tripp (2004) believes the Gilsonite had its beginning in the large amounts of organic debris that accumulated in the sediments of tropical Lake Uinta. The burial of these sediments created heat and pressure and the Green River oil shales were formed. Again, burial of the oil shales created water and hydrogen and this explosive mixture was expelled and created fractures in the surrounding rock. These fractures were later filled with petroleum whose viscosity disappeared with desiccation. Gilsonite is essentially solid “oil”.
|ABANDONED ELEVATOR IN GILSONITE VEIN|
The Uintah also hauled passengers, freight, mail and livestock but was abandoned in 1939. Perhaps its best known cargo---the Dinosaur bones Earl Douglas quarried from what is now Dinosaur National Monument but were then shipped to the Carnegie Museum in Pittsburg. Today a few grades remain and I have done some hiking along the railway route. In the 1970’s spikes associated with the tracks were common; today, rare.
|TIMBERS SHORING UP OLD GILSONITE VEIN|
And, for a special treat, check out the video of the railroad at:
Burton, D. K. 1996, A History of Uintah County: Scratching the Surface in Utah Centennial County History Series: Salt Lake City: Utah State Historical Society, Salt Lake City.
Nickel, E. H., 1995, The Definition of a Mineral: The Canadian Mineralogist, v. 33, no. 3.
The Uintah Railway, 2009: http://home.bresnan.net/~bpratt15/
Tripp, B. T., 2004, Gilsonite, an Unusual Utah Resource: Utah Geological Survey Notes, v. 36, no. 3.