Sedimentology and geochronology of Upper Cretaceous–Paleogene strata in the western Gobi Desert, Mongolia: resolving conflicting biostratigraphy and identifying the K–Pg and PETM boundaries

A newly discovered set of fossiliferous localities in the Naran Bulak Formation of Mongolia’s Eastern Gobi Desert (Daus I, IIa, IIb) has yielded the first early Paleogene mammal and invertebrate fossils from the Eastern Nemegt Basin (Yo et al. 2023). These fossils are biostratigraphically incongruent with the current Asian Land Mammal Age (ALMA) zonation system based on mammalian index fossils (Dashzeveg & Russell 1988), highlighting the limitations of biostratigraphy in establishing temporal control. The ALMA system, while still commonly used (Missiaen 2011), relies heavily on outdated correlations and lacks an independent chronostratigraphic framework, particularly for the Gobi basins. This project seeks to establish a high-resolution chronostratigraphic framework for the uppermost Cretaceous strata and Naran Bulak and Gashato formations through integrated magnetostratigraphy, U/Pb dating, C/O isotope analysis (chemostratigraphy), and Ar/Ar geochronology of interbedded lava flows. This integrated approach assesses whether these lava flows represent primary inter-sedimentary features, and thus precise depositional tie points, or intrusive bodies unrelated to depositional timing. We will test whether the measured sections span the Cretaceous–Paleogene boundary and the Paleocene–Eocene Thermal Maximum (PETM), the latter of which has been tentatively hypothesized near the Naran–Bumban contact but has not been validated through independent dating and chemostratigraphy. Refining the age model for these formations will improve the global correlation of Paleogene terrestrial records and allow direct comparison with better-studied sections in North America and China (Bowen et al. 2002, Ting et al. 2003, Clyde et al. 2008). Moreover, this work has regional significance for understanding the stratigraphic development of intermontane basins in Central Asia during the early Cenozoic, a crucial period of tectonic reorganization preceding the Himalayan uplift. Despite over 75 years of fossil discovery in these units, they remain poorly constrained in time. This study will address this critical gap.