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The Critical Role of Ichnology in Recognizing the True Last Big Marine Transgression of the Western Interior Sea: Paleocene Ferris Formation, Southern Wyoming

Abstract

Based on the presence of mineable coal beds, channelform sandstone bodies, freshwater mollusks, terrestrial pollen, and leaf fossils, the latest Cretaceous-Paleogene (~66-62 Ma) Ferris Formation (FF) in southern Wyoming’s Hanna Basin (HB) was interpreted to be entirely nonmarine since its initial description in 1918. In the 1990s, sedimentary structures common in tidal environments were described in sandy fluvial bars of the Paleocene section, but a lack of marine ichnofossils in these coarse-grained deposits, coupled with historical reconstructions of a completely nonmarine regional setting made interpretation of marine influence highly controversial. The first unambiguously marine ichnofossil, Bergaueria perata, was identified by George Pemberton in 1998 and led to more extensive ichnological investigation of the FF into the early 2000s. Abundant fossil leaves were systematically collected and documented as part of a separate study of the FF in the early 2000s and are best preserved in very fine-grained sand, silt, and mudstone lenses. A renewed, integrated and systematic investigation of fossil leaf and ichnofossil sites began in 2018 and focused on incorporating a total evidence approach to documenting changes in marine and continental depositional environments during the Paleocene. To date, this effort has revealed new insights, clarified some previously unresolved observations, and lead to the following conclusions: 1) over half of the entire Torrejonian section (370 m of 700 m) of the FF is composed of restricted marine deltaic, estuarine, bay, and lagoon deposits, 2) marine deposits of the Cannonball Sea can now be confidently mapped as far south and west as the central HB during the early Torrejonian (~63.5-64.5 Ma), 3) fossil leaves and logs are most abundantly and completely preserved in silty and sandy delta front and mouth bar deposits that also contain abundant marine ichnofossils (Rhizocorallium, Arenicolites, Cylindrichnus, Rosselia, Skolithos, Psilonichus, and Bergaueria), 4) cooler temperatures as indicated by leaf fossils in the FF relative to time-equivalent strata in Colorado are consistent with ameliorating effects of the Western Interior Sea, and 5) detailed ichnological analysis provides the critical key (more so even than sedimentology) to recognizing marine influence and flooding events in strata that are seemingly dominated by more easily identifiable features (e.g., coal beds, sandy barforms, and abundant terrestrial fossils).