Published 1995 as part of Memoir 64
Copyright © 1995 The American Association of Petroleum Geologists.   All Rights Reserved.

Detailed stratal correlations of the Skull Creek, Muddy, Shell Cheek, and Mowry formations in the vicinity of Hilight field in the south-central Powder River basin, Wyoming, reveal that the Muddy Sandstone in this region is an unconformity-bounded stratigraphic unit (depositional sequence). This depositional sequence represents an erosional remnant of what were originally more widespread incised-valley and younger (highstand) shoreline deposits. 

The unconformity at the base of the Muddy Sandstone (green sequence boundary) is characterized by incision of underlying Skull Creek strata and onlap of overlying Muddy strata. In core, this boundary also corresponds to an abrupt basinward shift in facies (coastal plain on distal offshore deposits) and, locally, rooted horizons.

The unconformity at the top of the Muddy Sandstone (red sequence boundary) is characterized by truncation of and incision into underlying Muddy strata, onlap of overlying Shell Creek strata, and angular discordance. In core, this sequence boundary coincides with a marine flooding surface that is locally underlain by root structures. 

Within the "Muddy" depositional sequence (green to red sequence boundaries) three informal units, nonmarine (oldest), lower paralic, and upper paralic, can be defined. These units are separated by two regional well-log markers (M1 and M2) that, in core, correspond to bentonites that overlie interpreted marine flooding surfaces. The basal nonmarine unit of the Muddy: (1) onlaps topography carved into the underlying Skull Creek Shale by the green sequence boundary, (2) contains only nonmarine deposits in core, and (3) is overlain by a flooding surface and the M2 marker. This stratigraphic interval is interpreted as the lowstand systems tract (incised-valley fill) of the green sequence. The overlying lower paralic and upper paralic units of the Muddy contain shoreline and coastal-plain deposits in core and