THE RELATIONSHIP BETWEEN STRATIGRAPHIC ARCHITECTURE AND BASINAL SEDIMENT FLUX: EXAMPLES FROM THE SANTONIAN EMERY SANDSTONE MEMBER OF THE MANCOS SHALE AND THE NIOBRARA FORMATION, WESTERN INTERIOR BASIN

The relationship between clastic shoreline migrations and bedding patterns in coeval hemipelagic strata of Late Cretaceous basins like the Western Interior has been a topic of heightened interest lately. Earlier studies correlated shoreface transgressive surfaces to offshore carbonates suggesting that high frequency changes in relative sea level controlled the flux of mud to the basin (dilution model). Although a mechanism for this scale of Cretaceous sea level change remains poorly constrained, some recent studies have presented arguments in favor of Cretaceous glacioeustasy. In this study the stratigraphic architecture of the Emery Sandstone Member of the Mancos Shale in central Utah is compared with compositional trends in the basinward time-equivalent facies, the hemipelagic Niobrara Formation, in an effort to test the relationship between shoreline migration and hemipelagic dilution. Multiple scales of alternation between dominantly chalk and dominantly marl facies in the Niobrara provide the opportunity to evaluate a relationship to both parasequence-scale features as well as seaward and landward stepping parasequence sets. Biostratigraphic collections from the study constrain the Emery Sandstone to Lower-Middle Santonian time, indicating that it correlates basinward to the middle marl and middle chalk units of the Smoky Hill Member. The Emery consists of a series of upward coarsening shoreface deposits most of which oscillate from proximal offshore to middle shoreface. These are arranged into three main packages that are slightly basinward stepping (lower and upper units) and slightly landward stepping (middle unit). However, the overall architecture of the unit is aggradational, like the time equivalent John Henry Member of the Straight Cliffs Formation. Interestingly, most literature sources suggest ongoing eustatic rise and active subsidence during Emery deposition. Because sedimentologic observations are consistent with rapid deposition under high sediment supply, and there is no direct evidence of contemporaneous deltas in the study area, it appears that the Emery shoreline was supplied by longshore processes at a rate sufficient to keep pace with accommodation. This study illustrates that shoreline architecture may not be an accurate reflection of sediment flux to the basin.