Requiem for a Seaway: Tracking the Final Transgression of the Western Interior Sea in the Post-Apocalyptic World of the Paleocene

Abstract

Tremendous clastic influx to Laramide basins of the Western Interior during the latest Cretaceous-Paleocene (~70-58 Ma) coincided with a series of climatic, tectonic, extraterrestrial, and biological perturbations. The result is a complex record of extinction, radiation, shoreline migration, basin-filling, and rapid changes in local and regional depositional environments. Ammonites and inoceramids are the quintessential fossils of the Western Interior Sea (WIS), so their extinction at the K-PG boundary presents a paradox when interpreting fine-grained, Paleocene strata. Specifically, absence of marine body fossils from Paleocene strata has been cited as proof of continental origin. Abundant lignite, coal, plant fossils, and freshwater body fossils are consistent with this interpretation. Sedimentary structures common in tidally-influenced fluvial channels were recognized in the early Paleocene (~66-65 Ma) Ferris Formation (FF) in the mid-1990’s. Large lakes can experience wind tides, so mechanical tidal processes do not necessitate connection to a marine basin. In the late 1990’s to early 2000’s, discovery of unambiguously marine ichnofossil assemblages in the western Hanna Basin’s (HB) FF challenged the paradigm and necessitated major revision of local and regional reconstructions of the Paleocene WIS. Preserved within sandy estuarine bars and silty deltaic deposits similar the modern Trinity River and bayhead delta on the Texas coast, these assemblages include Bergaueria, Rhizocorallium, Rosselia, Arenicolites, Palaeophycus, Thalassinoides, Ophiomorpha, Skolithos, Psilonichnus, Planolites, and Siphonichnus. Mapping a ~325 m thick succession of early Paleocene strata (~65-63.5 Ma) around the western HB reveals a series of marine flooding events followed by coal accumulation. A similar succession of interfingering coals and marine strata occurs in the later Paleocene (~59-58 Ma) Hanna Formation, demonstrating that leaves and other freshwater fossils represent transported material into restricted, marine bays in at least some successions. Ichnofossils provide key insights into shoreline migration, sediment-routing to the Gulf Coast Wilcox sands, and the final Paleocene transgression of the WIS that has for so long remained obscured by the absence of open marine body fossils.

AAPG Datapages/Search and Discovery Article #90357 ©2019 AAPG Rocky Mountain Section Meeting, Cheyenne, Wyoming, September 15-18, 2019