In Situ U–Pb Dating of Caliche to Constrain the Depositional Ages of Vertebrate Fossil-Bearing Strata in the Gobi Desert, Mongolia

Fujii, Y., Takeya, Y., Aoki, K., Chiba, K., Tsogtbaatar, K., Mainbayar, B., Buyantegsh, B., Saneyoshi, M.

The 6th International Symposium on Asian Dinosaurs in Japan 2025

Asian Dinoaur Association

Fukui, Japan

The Gobi Desert of Mongolia is one of the most prolific regions globally for vertebrate fossils, particularly those of dinosaurs. Numerous novel specimens of terrestrial vertebrates have been reported, unveiling previously unrecognized terrestrial biodiversity during the Late Cretaceous. These fossil assemblages constitute indispensable archives for elucidating patterns of adaptive radiation and ecosystem evolution in inland Asia during this period. However, the Upper Cretaceous strata in the Gobi Desert are broadly and discontinuously distributed, and rarely yield microfossils suitable for biostratigraphic correlation. In addition, they generally lack extensive, datable volcanic ash beds. These factors create significant challenges to the regional stratigraphic correlation of vertebrate fossil-bearing horizons. To overcome these issues, we conducted U–Pb dating of calcite within pedogenic carbonates (caliches) collected from three Upper Cretaceous formations (Baynshire, Javkhlant and Nemegt formations) extensively distributed across the Gobi Desert. Six in-situ caliche samples, verified through field investigations by the joint expedition of Okayama University of Science and the Institute of Paleontology, Mongolian Academy of Sciences, were selected for analysis.  Microscopic observation revealed that calcites infilled pore spaces among detrital grains in all samples. The detrital assemblages were composed predominantly of quartz and feldspar, with accessory rutile and garnet. The calcite exhibited micritic, nodular/concretionary, and sparry textures. Electron Probe Microanalysis (EPMA) and Raman spectroscopy confirmed that the calcite was nearly pure, with MgO concentrations less than 1 wt%. For U–Pb dating, inclusion-free domains of calcite were meticulously selected under polarized and reflected light microscopy and analyzed via Laser Ablation–Inductively Coupled Plasma–Mass Spectrometer (LA-ICP-MS). Statistically robust U–Pb ages were obtained from three of the six samples. In this session, we would like to discuss the obtained ages and deposition ages of the target formations.