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EspañolPOWDER RIVER BASIN
The Powder River Basin
The 'Powder River Basin' is a region in southeast Montana and northeast Wyoming about 120 miles east to west and 200 miles north to south known for its coal deposits. It is both a topographic drainage and geologic structural basin. The basin is so named because it is drained by the Powder River, although it is also drained in part by the Cheyenne River, Tongue River, Bighorn River, Little Missouri River, Platte River, and their tributaries. It is the single largest source of coal mined in the United States, and contains one of the largest deposits of coal in the world. Most of the active coal mining in the Powder River Basin actually takes place in drainages of the Cheyenne River. Major cities in the area include Gillette and Sheridan, Wyoming and Miles City, Montana. The area is very sparsely populated and is known for its rolling grasslands and semiarid climate. Because of the Powder River Basin, Wyoming is now the top coal producing state in the United States, a distinction that formerly alternated between West Virginia and Kentucky prior to the 1990s.
| Contents |
| Geologic History |
| Cretaceous |
| Tertiary |
| Coal |
| Coal mining companies currently operating in the PRB |
| Power plants fueled from Powder River Basin coal |
| Coalbed Methane |
| Petroleum |
| Uranium |
| See also |
| External links |
Geologic History
The Powder River Basin contains a section of Phanerozoic rocks up to 17,000 feet thick, from Cambrian to Holocene.
Cretaceous
The thickest section of the Powder River Basin is composed of Cretaceous rocks, an overall regressive sequence of mostly marine shales and sandstones deposited in the Western Interior Seaway.
Tertiary
The coal beds of the region began to form about 60 million years ago when the land began rising from a shallow sea. The rise of the Black Hills uplift on the east and the Hartville uplift on the southeast side of the basin created the present outline of the Powder River Basin.
When the coal beds were forming the climate in the area was subtropical, averaging about 120 inches of rainfall a year. For some 25 million years, the basin floor was covered with lakes and swamps. Because of large area of the swamps, the organic material accumulated into peat bogs instead of being washed to the sea. Periodically the layers of peat were covered with sediments washed in from nearby mountains. Eventually the climate became drier and cooler. The basin filled with sediment and buried the peat under thousands of feet, compressing the layers of peat and forming coal. Over the last several million years, much of the overlying sediment has eroded away, leaving the coal seams near the surface.
Coal
Northeast view of the mile wide Deckers coal mine and the Tongue River in the Powder River Basin, southeastern Montana.
The majority of the coal mined in the PRB is part of the Fort Union Formation (Paleocene).
The low sulfur and ash content of the coal in the region makes it very desirable. Much of the output of the basin's mines is used to fire power plants east of the Rocky Mountains for generating electricity. In recent years over 350 million tons of coal have been mined annually, more than 25% of the total U.S. production. The coal mines are currently served by the BNSF Railway and the Union Pacific Railroad.
More than eighty train loads of coal, which vary in size from 125 to 150 cars, are shipped from Wyoming mines each day. A third railroad, the Dakota, Minnesota and Eastern Railroad, has faced strong resistance from an unusual array of parties for its attempts to extend its rail line into the coal mining area.
It has been estimated that Powder River Basin coal deposits contain over 800 billion tons of coal. The thickness of the coal seams in the region ranges up to 200 feet (60 m) and averages nearly 80 feet.
Coal mining companies currently operating in the PRB
Power plants fueled from Powder River Basin coal
Coalbed Methane
Recent controversy surrounds the extensive coalbed methane extraction in the region. In the last decade, nearly 7000 of these wells have been drilled. Extracting the gas requires water to be pumped to the surface in order to release the gas trapped in the coal seam. While most of the water is successfully utilized in agriculture production such as livestock water and crop irrigation, some waters are naturally high in salinity. There has been controversy on how to best manage these saline waters.
Petroleum
The Powder River Basin also contains major deposits of petroleum. The oil and gas are produced from rocks ranging from Pennsylvania to Tertiary, but most comes from sandstones in the thick section of Cretaceous rocks.
Uranium
The region also contains major deposits of uranium, contained in sandstones. (See uranium mining in Wyoming). The Wasatch Formation (Eocene) contains the uranium ore "roll front" type deposits found in the Pumpkin Buttes District. Cameco Corporation operates uranium mines in the basin.
See also
★ Thunder Basin National Grassland
External links
★ Wyoming coal mining
★ Geological history
★ USGS Open-File Report on impacts of CBM development in the region
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