ANALYSIS OF NEARSHORE STRATIGRAPHIC ARCHITECTURE OF MIDDLE SANTONIAN TO EARLY CAMPANIAN DEPOSITS OF THE WESTERN INTERIOR BASIN, UTAH, U.S.A

This study considers the relationship between Middle Santonian to Early Campanian deposits of the Western Interior Basin, Utah, and their basinward time-equivalent facies, the hemipelagic Niobrara Formation. The basinal deposits of the Niobrara Formation are characterized by rhythmic variations in carbonate, organic carbon, and clay content. All three of the major drivers of sedimentation (climate, tectonics, and eustasy) have been alternatively proposed as the cause of the rhythmicity in the Niobrara Formation. Specific hypotheses under investigation in this study concern the relationship between basin margin stratigraphic architecture and patterns of hemipelagic sedimentation in the center of the Western Interior Basin. Based on previous work it is hypothesized that periods of shoreface progradation are associated with higher terrigenous flux to the basin center, resulting in relatively carbonate-poor deposits. The opposite should be true of retrogradational intervals, when deposition of carbonate-rich sediments is expected. In order to test this hypothesis, a high-resolution sequence stratigraphic framework of the Middle Santonian to Early Campanian deposits (such as the Emery Sandstone and Garley Canyon Members of the Mancos Formation) is being developed for basinward correlation. Facies ranging from shoreface to coastal plain deposits are being evaluated for this purpose. The literature contains wide discrepancies in estimated ages of these units, as well as differences in their architectural interpretation. A high-resolution stratigraphic framework for the Middle Santonian to Early Campanian deposits is developed in order to improve the correlation of nearshore facies to basinward hemipelagic equivalents of the Niobrara Formation and thus evaluate the role of variations in siliciclastic dilution on the hemipelagic deposits.