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The Upper Cretaceous Fox Hills Sandstone exposed in outcrop on the east flank of the Rock Springs uplift is a regressive sequence of sandstone and siltstone which was deposited along barrier island coastlines in littoral, shallow-neritic, and tidal-river environments. The littoral and shallow-neritic sandstones are very fine- to fine-grained, and form coarsening upward sequences which are heavily burrowed by deposit-feeding organisms in the lower part and contain walled burrows of suspension-feeding organisms (principally Ophiomorpha) in the upper part.
The littoral and shallow-neritic sandstones are commonly overlain by fine- to medium-grained, channel sandstones interpreted to have been deposited in tidal-river environments. These sandstones exhibit a characteristic vertical sequence of stratification and sedimentary structures. The channel base is defined by a sharp scour surface at which there is a marked grain-size increase from that of the underlying sandstone. Lag deposits of oyster shells, wood imprints, and clay clasts are present above this surface. Individual sandstone units become finer upward and are commonly trough cross-stratified in the middle and lower parts and subparallel bedded in the upper part. The trace fossil Ophiomorpha is abundant, and a root zone is present at most places in the upper 1-3 ft. The tidal rive sandstones intertongue in a seaward direction with finer grained, littoral and shallow-neritic sandstones. In a landward direction they pass into lagoonal, marsh, and swamp deposits.
The origin of the widespread tidal-river sandstones is interpreted to be associated with the migration of tidal river and estuary channels on the landward and lateral sides of seaward prograding barrier islands.
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