Abstract

Recent interpretations of the lower Campanian Shannon Sandstone in the Powder River Basin, Wyoming have suggested that it was deposited in falling stage and lowstand incised shorefaces, rather than in stacked shelf ridges up to 160 km from shore. Despite these detailed studies, there is no published correlation of the outcrop deposits at Salt Creek Anticline with the subsurface deposits in the Hartzog Draw-Heldt Draw (HD-HD) area. A complete high-resolution sequence stratigraphic analysis of the Shannon requires that the outcrops at Salt Creek be correlated with the subsurface fields at HD-HD. There is little core control in the approximately 30 km between these areas, so correlations are based almost entirely on well logs. A template has been constructed in the HD-HD area, where there is good core and well log control, which embodies data from facies, facies successions, well log signatures, and bounding discontinuities. The template consists of a regressive surface of erosion (RSE) that forms the base of an incised shoreface sandbody, and a transgressive surface of erosion (TSE) that truncates the top of the sandbody and the RSE. Two cross sections have been constructed, and the template has been used to guide correlations between Salt Creek and the HD-HD area. From the cross sections, six progradational events and six transgressions have been defined during Shannon time. The positions of the RSEs and TSEs allow estimates of between a few meters and 60 m for the magnitude of sea level fluctuations that controlled Shannon deposition. The proposed correlations confirm and reinforce the interpretation of the Shannon as a series of incised shoreface sandbodies, separated by transgressive systems tract mudstone deposits.

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