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Montana Geological Society: Energy Resources of Montana
THE SENTINEL BUTTE MEMBER OF THE FORT UNION FORMATION, POWDER RIVER COUNTY, MONTANA
The Pumpkin Creek lignite deposit was mapped in 1965 by a photogeologic reconnaissance method developed by the Northern Pacific Railway Company. The techniques of the method are described elsewhere. The method is well suited to relatively flat-lying coal beds situated in areas where topographic control is limited and where terrace gravels and alluvium deposits do not severely interfere.
Ample fossil evidence exists for extending the Sentinel Butte Member of the Tertiary Fort Union Formation into the general area of the Pumpkin Creek lignite deposit of Southeastern Montana. A fossil assemblage was identified containing sixteen species of Mammalia, three of which are new. Also included in the assemblage are members of the Classes Chon-drichthyes, Teleostomi, Amphibia, Reptilia, and Gastropoda.
The fossils (Olive fauna) found in the Pumpkin Creek deposit, when used in conjunction with those obtained by others from several different sections of the Fort Union Formation, indicate that major coal beds and their time equivalents throughout the region may be used for broad correlation and structural interpretation throughout the Fort Union region. In turn, the correlation of fossil - dated coal beds from Southeastern Montana with coal beds in Northeastern Wyoming has provided considerable evidence for revising the stratigraphic column throughout the Powder River Basin. Evidence suggests that the Lebo Shale Member, as mapped in Northeastern Wyoming, may be correlative with the lower part of the Sentinel Butte Member in Eastern Montana. On the assumption that this is true, it is probable that the Tullock Member of Northeastern Wyoming correlates with the Tongue River Member of Eastern Montana and that the upper one-half or more of the Hell Creek Formation of Northeastern Wyoming correlates with the Lebo Shale and Tullock Members of the Fort Union Formation in Eastern Montana.
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