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Coal Deposition in Fluvial Paleoenvironments of the Paleocene Tongue River Member of the Fort Union Formation, Powder River Area, Powder River Basin, Wyoming and Montana

Romeo M. Flores
Romeo M. Flores
U.S. Geological SurveyDenver, Colorado 80225
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January 01, 1981


The Tongue River Member of the Fort Union Formation in the Powder River area, Powder River Basin, Wyoming and Montana, is interpreted as a sequence of fluvial deposits that consists of two major facies: a fluvial-channel- dominated facies and a fluvial-lake-dominated facies. The fluvial-lake-dominated facies is differentiated into lake-lake splay, crevasse splay-crevasse splay channel, and fluvial channel-interchannel subfacies. Similar subfacies occur in the fluvial-channel-dominated facies except for the lake-lake splay subfacies, which is poorly developed. The coals in the fluvial-channel-dominated facies are thick, laterally persistent, and contain a modest amount of carbonaceous shale. In this facies, slowly subsiding, isolated, broad interchannel floodbasins marginal to abandoned meander bends of channels propagated by avulsion and abandoned channel ridges formed poorly drained backswamps. The backswamps accumulated thick peat-bog deposits that developed dome shapes accumulated and sustained by a gradual and continuous rise of the ground-water table. Occasional autocyclic shifts of major channels into topographically low backswamps and moderate amounts of overbank-crevasse splay sedimentation interrupted peat deposition, forming peat (coal) splits. Moderate incursion of sediment-laden flood waters into the backswamps formed carbonaceous shale.

The coals in the fluvial-lake-dominated facies are thin, laterally discontinuous, and contain abundant carbonaceous shale. In this facies low-lying backswamps along margins of lakes developed in rapidly subsiding interchannel floodbasins. The backswamps that were subjected to drowning and shoreline erosion, and were traversed by effective drainage systems, formed carbonaceous shale. Sedimented platforms formed by crevasse splay-crevasse splay channel and fluvial channel-interchannel systems provided limited peat-forming backswamps. In addition, frequent invasions by sediment-charged flood waters into these backswamps formed carbonaceous shales that are interbedded with the coals.

Coal exploration in the Tongue River Member should focus on the coals of the fluvial-channel-dominated facies. In the area studied, coals in this facies, as much as 30 ft (9.15 m) in thickness, can be traced as far as 12 miles (19.2 km) along outcrops, and they contain low to moderate amounts of carbonaceous shale. Finally, the coals show only a moderate amount of merging or splitting over distances of several miles, providing deposits of fairly uniform thickness for mining.

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SEPM Special Publication

Recent and Ancient Nonmarine Depositional Environments: Models for Exploration

Frank G. Ethridge
Frank G. Ethridge
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Romeo M. Flores
Romeo M. Flores
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SEPM Society for Sedimentary Geology
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Publication date:
January 01, 1981




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