TEMPORAL, SPATIAL, AND FAUNAL ANALYSIS OF METHANE SEEP DISTRIBUTION IN THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY (WIS), USA

Cold methane seep deposits in the Western Interior Seaway (WIS) of North America commonly occur within the upper Campanian to lower Maastrichtian sequence (Baculites scotti to Baculites baculus zones ~75-70 MYA, Landman et al., 2012). Constant flux of methane and sulfide was critical to the development of these unique and diverse ecosystems. Seep faunal assemblages include cephalopods, bivalves, crinoids, echinoids, gastropods, bryozoans, reptiles, crustaceans, scaphopods, and corals, with variation in species richness from one seep to the next. Some seep deposits are dominated by the bivalve Lucina occidentalis. Others show a more diverse faunal assemblage including ammonites Scaphites, Baculites, Didymoceras, Solenoceras, Placenticeras, and Nostoceras. Recent studies suggest that ammonites were a constant part of seep ecosystems despite their mobility. Methane seeps potentially acted as refugia for WIS marine life during periods of environmental stress. Patterns in the geographic distribution of methane seep deposits in the Late Cretaceous WIS have been linked to tectonic activity in the region. Seeps have also been correlated with fault lines.This study focuses on a representative sample of approximately 40 fauna-rich methane seep deposits from Colorado, Montana, Nebraska, South Dakota, and Wyoming. We compiled a series of digital geologic-GIS maps and information on faunal assemblages that when combined provide a spatio-temporal framework of seep distribution and diversity. Using this framework, we completed a detailed examination of species richness patterns at seeps to reconstruct the evolution of seep communities both geographically and over time.