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The Upper Cretaceous Fox Hills Formation on the southeastern flank of the Powder River basin, Wyoming, was deposited along a regressive, northeast-trending, low-energy, tidally affected, wave-dominated coastline. The Fox Hills Formation consists of rocks deposited successively in marine transitional-lower shoreface, upper shoreface, foreshore, and tidal-flat environments.
The marine transitional-lower shoreface beds are characterized by generally highly bioturbated, very fine-grained sandstone and silty claystone with interbedded siltstone, silty shale, and shale. The amount of sandstone in them increases landward. Portions of all of these strata include hummocky cross-bedding, unidentified tracks and trails, traces of Rhizocorallium and small Ophiomorpha, and glauconite layers. The lower shoreface portion of this environment has a ledgy appearance due to the alternation of sandstone and finer grained sediments.
The upper shoreface deposits consist dominantly of sandstone and have minor amounts of siltstone, silty claystone, and shale. The sandstone generally becomes coarser and better sorted upward. Highly bioturbated beds are common throughout the upper shoreface strata. Low-angle planar cross-bedding and some parallel bedding are the major sedimentary structures. Trace fossils of Ophiomorpha and Arenicolites, and body fossils of pelecypods and gastropods appear in upper shoreface rocks.
The deposits of the foreshore (intertidal) consist of moderately sorted to well-sorted fine-grained sandstone that has seaward-dipping wedge-planar cross-bedding. Arenicolites and large, generally vertical, traces of Ophiomorpha are common in these rocks. Irregular polygonal weathering patterns are common on outcrop surfaces.
Tidal-flat deposits are common in the upper part of the section in the northern part of the study area. At one locality, these deposits consist of at least 75 ft (23 m) of intercalated fine-grained sandstone and claystone that have lenticular to flaser bedding. The majority of these deposits show a high degree of bioturbation and are considered to be intertidal. These sediments are overlain by approximately 15 ft (4.5 m) of fine-grained silty sandstone and claystone which have plant root casts and a high content of carbonaceous material, and which could represent supra-tidal deposits. Overlying these deposits is a 12-ft (3.7 m) sandstone bed having a scour base, lag of clay clasts and woody fragments, parallel bedding that grades upward into trough cross-bedding, and a 6-ft (1.8 m) be of oyster shells near its top. This sequence represents an ancient tidal-channel deposit.
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