LATE CRETACEOUS FAUNAL RESPONSES TO SEA-LEVEL CHANGE IN AN OFFSHORE DEPOSITIONAL SETTING OF THE WESTERN INTERIOR SEAWAY

Fluctuations in sea level and in turn water depth are well known to control the distribution of taxa in shallow-marine, cratonic settings. Despite being important for understanding faunal dynamics during critical intervals and during background times, relatively few studies have examined the degree of faunal change that can be associated with relative sea-level change in a single lithofacies that was deposited in an offshore marine setting. In this study, we examine the influence that sea-level change had on faunas in muddy, offshore depositional settings of the Cretaceous epicontinental Western Interior Seaway (WIS). To asses this issue, the concretionary fossil assemblages of the Baculites eliasi through B. clinolobatus biozones in a single clay-rich lithofacies of the Pierre Shale in eastern Wyoming were examined. This provided a detailed documentation of the structure of marine habitats in the WIS through a background interval spanning ~2.5 Ma during the late Campanian and early Maastrichtian. Changes in both the taxonomic composition of assemblages and the relative abundance of the various species are interpreted to reflect paleoecological and paleoenvironmental change through the study interval. This analysis shows that significant changes in richness, abundance, and ecological guild structure can arise in response to variations in sea level with no obvious changes in lithology. Our data also reveal that samples with lower diversity and abundances show a strong relationship with intervals when water conditions were deepest and the western paleoshoreline was furthest to the west, while higher diversity and abundances match periods when the paleoshoreline was the closest and shallow-water conditions prevailed in that part of the WIS. The decrease in diversity with depth can best be explained by the long-term presence of dysoxic/anoxic conditions that would have precluded benthic faunas. The distribution of taxa and diversity of the assemblages seen in the study interval most likely reflect migrating oxygen-controlled biofacies in the WIS that were responding to changes in depth and the proximity to the western shoreline that was in turn controlled by relative sea-level fluctuations likely dominated by tectonism associated with the Sevier Orogenic Belt or potentially early phases of Laramide activity.