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The Mesaverde formation of east-central Wyoming consists of sandstone, shale, and lignite beds. These units were deposited in near-shore, littoral, lagoonal, coastal swamp, and fluvial environments along the western margin of the Late Cretaceous interior sea. Eastward regression of the sea was interrupted by periods of stillstand and minor transgressions. The facies of the Mesaverde formation can be resolved in terms of regression, stillstand, and transgression.
During eastward regression the environments of deposition also migrated eastward, maintaining their same positions relative to the shoreline. The normal succession of facies in a regressive sequence includes, in ascending order, the facies of the near-shore, littoral, coastal swamp, and fluvial environments.
Stillstand occurred at times of maximum regression or transgression, and during pauses in regression or transgression. The shoreline and environments of deposition were essentially stationary during stillstand. Thickening of the facies of the near-shore, littoral, and coastal swamp environments is indicative of stillstand.
Westward transgressions of the sea inundated former lowland areas. The absence of persistent near-shore and littoral deposits at the base of transgressive sequences in east-central Wyoming may reflect rapid transgression.
The two major regressive sequences of the Mesaverde formation in east-central Wyoming are the Phayles Reef member (new name) of the Mesaverde formation in the Wind River Basin, and the Parkman sandstone member of the Mesaverde formation in the Powder River Basin. The Teapot sandstone member of the Mesaverde formation defies a simple regressive or transgressive interpretation. The Wallace Creek tongue (new name) of the Cody shale in the Wind River Basin, and the unnamed middle member of the Mesaverde formation in the Powder River Basin are transgressive marine tongues.
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