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The coal-bearing Tongue River Member of the Fort Union Formation in the Powder River basin exemplifies fluvial deposits of Tertiary intermontane basins. The Tongue River Member coals are targets of extensive drilling exploration and development. About 200 sections, spaced an average of 0.5 mi (0.8 km) apart, were measured in a 60-mi (96 km) continuous outcrop along the Powder River in Wyoming and Montana to determine the environmental-stratigraphic framework of the coals in the 1,500-ft (450 km) thick Tongue River Member. Coal-bed distribution in this area may be typical of that in many parts of the basin.
The coals are distributed in two major facies: a lower (1,100 ft or 330 m thick) fluvial channel dominated facies, and an upper (400 ft or 120 m thick) lake-dominated, interfluvial and fluvial channel facies. Major coals, including the Anderson, Canyon, Cook, Wall, Pawnee, and Cache, were formed in the fluvial channel dominated facies, which contains numerous en echelon channel sandstones that range from 50 to 200 ft (15 to 60 m) thick and from 1 to 9.5 mi (1.6 to 15.2 km) in lateral extent. The offset arrangement of the sandstones suggests shifts of meandering channels among low-lying poorly drained interchannel backswamps which were filled by overbank-crevasse sandstone, siltstone, and shale. These backswamps, as well as poorly drained backswamps developed on abandoned channel ridge , were sites of coal deposition. Coal beds in this facies locally thicken from 1 to 30 ft (0.3 to 9 m) within 3 to 7 mi (4.8 to 11.2 km) and were traced in outcrops for 8 to 12 mi (13 to 19 km) as lenticular bodies. They split laterally, grade into carbonaceous shale, or are truncated by channel sandstones.
The lake-dominated interfluvial and fluvial channel facies consist of abundant crevasse-splay sandstone, siltstone, and shale, and lacustrine limestone and shale that contain abundant freshwater mollusks. A few channel sandstones are present; these range from 30 to 80 ft (9 to 24 m) thick and from 0.5 to 3 mi (0.8 to 4.8 km) across. The crevasse and channel deposits developed poorly drained to well-drained backswamp platforms where coals formed. Coal beds, including the Smith and Roland, average about 2.5 ft (0.7 m) thick and are laterally continuous in outcrops for as much as 5 mi (8 km). Crevasse splays dominated the interfluvial-lacustrine sedimentation and commonly interrupted lateral continuity by splitting the coal beds.
Thus, of the two major facies, the more coal productive is the fluvial channel dominated facies. The development of thick, lenticular coal beds in this facies was directly influenced by depositional settings of poorly drained backswamps which formed mainly on abandoned channel ridges and overbank areas.
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