Floodbasin Sequence of a Fine-Grained Meander Belt Subsystem: The Coal-Bearing Lower Wasatch and Upper Fort Union Formations, Southern Powder River Basin, Wyoming
Frank G. Ethridge, Timothy J. Jackson, Alv D. Youngberg, 1981. "Floodbasin Sequence of a Fine-Grained Meander Belt Subsystem: The Coal-Bearing Lower Wasatch and Upper Fort Union Formations, Southern Powder River Basin, Wyoming", Recent and Ancient Nonmarine Depositional Environments: Models for Exploration, Frank G. Ethridge, Romeo M. Flores
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Thick nonmarine coals and large epigenetic uranium deposits occur within the Wasatch (Eocene) and Upper Fort Union (late Paleocene) Formations in the Powder River Basin, Wyoming. The regional depositional framework of these formations as determined from published data is characterized by a northward-flowing intermountain basinal fluvial system. This system has three major components: a trunk stream along the basinal axis, a tributary subsystem along the basin margin and a paludal subsystem between the trunk stream and the tributary subsystems.Data from eighteen continuous cores in the lower Wasatch and upper Fort Union Formations from a four square mile area in southeastern Campbell County provide information for determining the depositional and stratigraphic characteristics of the tributary subsystem. The sedimentary rocks above and below the thick Anderson coal bed within the study area consist of fine-grained sandstones, siltstones, mudstones and claystones interbedded with thin discon-tinuous coal beds. The physical, lithological and biological characteristics of these sediments are similar to floodbasin sequences in the Holocene Atchafalaya Basin of Louisiana. Specific environments recognized include point bars, crevasse splays, lacustrine deltas, natural levees, lakes, abandoned channel fills, and well- to poorly-drained swamps. With the exception of the laterally continuous Anderson coal bed, lateral and vertical variability in lithologies is the rule rather than the exception within the study area.Thick, low ash, coal beds such as the Anderson probably originated as extensive peat swamps in the paludal subsystem. How the even thicker peats that formed these coals segregated themselves from adjacent alluvial sedimentation is still a matter of debate. Thin discontinuous coals with clay interbeds probably originated as small interchannel peat swamps that developed on slightly better drained crevasse splay platforms in the tributary subsystem.