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The Amsden Formation, named by N. H. Darton in 1904 during mapping of the Bighorn Mountains, Wyoming, consists of red shales, limestones, sandstones, and cherty beds. The Amsden was later subdivided into four members (in ascending order): the Darwin Sandstone (E. Blackwelder, 1918), the Horseshoe Shale (W. W. Mallory, 1967), the Moffat Trail Limestone (W. J. Sando and others, 1975), and the Ranchester Limestone (W. W. Mallory, 1967). All members crop out in the study area.
The Darwin Sandstone Member is up to 88.4 m (290 ft) thick, disconformably overlies the Mississippian Mission Canyon Limestone, and is conformably overlain by the Horseshoe Shale Member. It is a quartzarenite with minor amounts of feldspar interpreted as being deposited in a nearshore or beach environment.
The Horseshoe Shale Member is 25.9 m (85 ft) thick and conformably overlies the Darwin Member. The Horseshoe Shale Member contains fine-grained terrigenous sediment interpreted to have accumulated in a lagoon or back-reef environment. The lower part is a white to red siltstone with localized areas of pisolitic hematite; the upper part is organic-rich black shale containing palynomorphs.
The Moffat Trail Limestone Member is 29.0 m (95 ft) thick and conformably overlies the Horseshoe Shale Member. The Moffat Trail consists of biomicrites, pelmicrites, and silty micrites. It is interpreted to represent open-marine, low-energy carbonate-shelf deposition.
The Ranchester Limestone Member is 9.4 m (31 ft) thick and conformably overlies the Moffat Trail Member. The Ranchester contains silty dolomite, carbonate, and red siltstone, representing the maximum transgression of the Pennsylvanian sea. The Pennsylvanian Wells Formation conformably overlies the Ranchester Member.
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