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Along the eastern margin of the Beartooth front a sequence of Laramide conglomerates, sandstones, siltstones, shales, and minor coals are exposed. These minor Paleocene-Eocene coals, ranging from lignite to subbituminous in rank, are the focus of this study.
Four coal exposures were sampled along a north-south strip (17 mi, 27 km), proximal to the Beartooth front. These coals range in thickness from less than a 1-in. (2.5 cm) lens to about 10 ft (3 m) and contain partings of carbonaceous shale, shale, siltstone and sandstone. Two of the seams, designated as Meeteetse Trail and Burgess Lignite occur in typical Fort Union sediments. The Meeteetse Trail locality is less than 0.25 mi (0.4 m) from vertical Madison Limestone with a westerly dip "under" the Madison. The other, the Burgess Lignite locality, is composed of alternating papery fusinitic lignite, siltstones, thin lenses of channel sandstones, carbonaceous shales, and carbonaceous mudstones. The coals here are high in organic matter, fossil stems, leaves, megascopic resins, randomly o iented petrified tree stumps, and gypsiferous deposits.
Two other localities, Gold Creek and Clarks Fork Canyon, are in Paleocene alluvial fan deposits. The Clarks Fork coals are found at the distal end of an alluvial fan system and dip rather gently basinward. The Gold Creek coals are found at the base of a fan interbedded with massive sandstones and conglomerates containing andesitic porphyry clasts up to 1.5 ft (0.4 m) in maximum dimension.
The Fort Union Formation in this area includes lacustrine, paludal, fluvial, and conglomeratic members. There is a wide diversity of megaflora in different sedimentary facies. Petrographic examination reveals well preserved cell structure, especially in the fusinoid macerals. Although the coals have undergone severe alteration and weathering, they still exhibit excellent fluorescence properties.
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