HIGH-RESOLUTION SEQUENCE STRATIGRAPHIC ANALYSIS OF THE SHANNON SANDSTONE IN WYOMING, USING A TEMPLATE FOR REGIONAL CORRELATION

Recent interpretations of the lower Campanian Shannon Sandstone in the Powder River Basin, Wyoming have suggested that it was deposited in f alling-stage and lowstand incised shorefaces, rather than in stacked s helf ridges up to 160 km from shore. Despite these detailed studies, t here 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 analy sis of the Shannon requires that the outcrops at Salt Creek be correla ted with the subsurface fields at HD-HD. There is little core control in the approximately 30 km between these areas, so correlations are ba sed almost entirely on well logs. A template has been constructed in t he HD-HD area, where there is good core and well log control, which em bodies data from facies, facies successions, well log signatures, and bounding discontinuities. The template consists of a regressive surfac e of erosion (RSE) that forms the base of an incised shoreface sandbod y, 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 Cr eek and the HD-HD area. From the cross sections, six progradational ev ents and six transgressions have been defined during Shannon time. The positions of the RSEs and TSEs allow estimates of between a few meter s and 60 m for the magnitude of sea level fluctuations that controlled Shannon deposition. The proposed correlations confirm and reinforce t he interpretation of the Shannon as a series of incised shoreface sand bodies, separated by transgressive systems tract mudstone deposits.