Reconstructing Late Cretaceous Environment in the Gobi Desert, Mongolia

Saneyoshi, M., Mainbayar, B., Buyantegsh, B., Nyamjargal, Ja., Badamkhatan, Z.

GIMAR2006

Geological Society of Mongolia

ウランバートル市、モンゴル

Long-term geological collaborations between Okayama institutions and Mongolia, initiated by the Hayashibara Museum of Natural Sciences (1993–2013) and sustained by Okayama University of Science (OUS) in partnership with the Institute of Paleontology, Mongolian Academy of Sciences (IPMAS) since 2014, have produced comprehensive stratigraphic datasets across the Gobi Desert. Recent joint field surveys have targeted Upper Cretaceous terrestrial successions at key localities, including Bayn Shire, Shar Tsav, Udyn Sayr, Yagaan Khoovil, and Bugin Tsav. Joint expeditions during the 1990s identified extensive, high-density dinosaur trackways at Shar Tsav, contributing to its designation as Mongolia’s first national geopark and its ongoing development toward UNESCO Global Geopark status. Integrated lithofacies mapping and sedimentological analyses demonstrate that Late Cretaceous depositional systems constituted spatially complex environmental mosaics, where eolian dunes, interdune depressions, active fluvial channels, and floodplains varied dynamically over short distances. Localized differences in vertebrate fossil assemblages thus reflect spatial depositional partitioning rather than purely temporal or paleobiogeographic turnover. Establishing robust chronological frameworks for these terrestrial deposits has historically been challenging because datable volcanic ash beds are scarce, necessitating reliance on indirect K–Ar basalt ages or regional biostratigraphic correlations. To provide absolute temporal constraints, recent OUS–IPMAS geochemical investigations evaluated non-traditional U–Pb geochronometers, yielding calcite U–Pb ages of 95.9 ± 6.0 and 89.6 ± 4.0 Ma from Khongil Tsav pedogenic caliches (Kurumada et al., 2020), alongside a minimum apatite U–Pb age of 66.7 ± 2.5 Ma from a Tarbosaurus tooth (Tanabe et al., 2023). Integrating high-resolution facies architecture with direct numerical ages establishes a refined chronostratigraphic baseline for terrestrial Cretaceous successions and paleoenvironmental evolution in East Asia. These fossil sites span Upper Cretaceous Gobi sequences, underscoring the necessity for continued joint field investigations and clearer delineation of their academic and global conservation significance.