RECONSTRUCTING SEDIMENTARY ENVIRONMENTS, BIOTA, AND PALEOCLIMATE WITHIN THE NANUSHUK FORMATION, KUKPOWRUK RIVER, WESTERN NORTH SLOPE, ALASKA

The Albian-Cenomanian Nanushuk Formation is discontinuously exposed along the Kukpowruk River in the foothills of the northwestern DeLong Mountains, North Slope, Alaska. On the western North Slope, the Nanushuk Formation comprises a basal interval of dominantly marine sandstone that is gradationally overlain by marginal marine to nonmarine conglomerate, sandstone, mudstone and coal. These rocks units characterize the Kukpowruk and Corwin formations of the former Nanushuk Group, respectively. We examined exposures from west of Igloo Mountain in the Coke Basin, to the Barabara Syncline approximately 80 kilometers to the north. Sections exposed along the Kukpowruk River contain a well-documented and well-preserved fossil flora recovered from marine, marginal marine and terrestrial deposits. Recent work focused on compiling detailed measured sections, interpreting sedimentary facies and facies associations, and documenting occurrences of fossil vertebrates. Facies associations include shallow marine deposits, tidal flats, distributary channels and interdistributary bays and bayfills, as well as fluvial channels, floodplains, and coal swamps. These facies associations indicate lower delta plain and upper delta plain environments. Plant fossils are abundant and some exposures contain standing tree trunks up to 58 cm in diameter. With approximately 75 newly discovered track sites, a rich fossil vertebrate ichnofauna is emerging. The ichnofaunal assemblage includes evidence of bony fishes, small and large predatory dinosaurs (including birds), as well as bipedal and quadrupedal herbivorous dinosaurs. Within the dinosaur ichnofauna, approximately 15% of the record is represented by fossil bird tracks. Woody fragments from the Nanushuk Fm. were analyzed for their carbon isotopic composition to relate δ¹³C to mean annual precipitation. Samples averaged -26.4‰ vs. VPDB, suggesting average MAP of 1412 mm/year. This pattern of increased precipitation in the Nanushuk Fm. during the mid-Cretaceous is regionally consistent with the global pattern associated with the Cretaceous Thermal Maximum. Our work provides a framework for further paleoecological and paleoclimatic analyses on the western North Slope during a period of peak Cretaceous greenhouse conditions.