Quartz ESR Characteristics to Establish Lithostratigraphic Frameworks in the Upper Cretaceous and Paleogene in the Gobi Desert of Mongolia

Kawatsuki, Y., Amimoto, M., Nitta, Y., Toyoda, S., Chiba, K., Buyantegsh, B., Mainbayar, B., Tsogtbaatar, K., Saneyoshi, M.

The 6th International Symposium on Asian Dinosaurs in Japan 2025

Asian Dinoaur Association

Fukui, Japan

For nearly a century, the Upper Cretaceous and Paleogene sedimentary successions of the Mongolian Gobi Desert have been a world-renowned source of exceptionally preserved dinosaur and mammal fossils. However, these strata are predominantly composed of fluvial deposits and generally lack stratigraphic marker beds or datable minerals, and thus, the correlation between fossil-bearing successions remained challenging, necessitating the reconstruction of a more robust stratigraphic framework essential for accurately reconstructing the evolutionary and ecological history of these fossil assemblages. This study aims to provide a new basis for a regional correlational framework by using Electron Spin Resonance (ESR) to determine the geological provenance of quartz grains from these deposits. While ESR is an established tool for provenance studies, its application to sediments as old as the Cretaceous and Paleogene is novel. Our methodology utilizes three different ESR signals. The E1′center signal measured allows for the quantitative evaluation of the number of oxygen vacancies in quartz, which reflects the formation age of the crystalline rocks in the sediment’s source region. In contrast, signals from aluminum (Al) and titanium-lithium (Ti-Li) centres are linked to the specific geochemical environment (e.g., temperature, chemical composition) in which the quartz formed initially. We sampled sediments from localities across the Gobi Desert, which supposedly covers the area where four Upper Cretaceous and one Paleogene formations are distributed: the uppermost Cretaceous Nemegt Formation in the southwestern Gobi, the underlying Djadokhta Formation in the Central Gobi, and the lowermost Cretaceous Baynshiree and Javkhlant formations, and the Paleogene Ergilin Dzo Formation in the southeastern Gobi. The number of oxygen vacancies shows a distinct spatial pattern, with lower values in the southwestern and central Gobi and higher values in the eastern Gobi. This suggests that the source rocks for the eastern Gobi are fundamentally older than those supplying sediment to the other regions. In contrast, the Al and Ti-Li signals exhibit that the sample from the Paleogene Ergilin Dzo Formation is significantly different compared to the Upper Cretaceous formations, indicating a shift in the geochemical signature of the source rocks. A cluster analysis based on the number of oxygen vacancies, Al, and Ti-Li signals quantitatively supports the geographic and temporal trend. The samples were primarily divided into the southwest-central and the southeastern groups, and the southeastern samples were further separated into the Baynshiree and Ergilin Dzo samples. It is important to note that the term "group" as used here refers to an informal, provenance-based unit, not a formal lithostratigraphic rank. The results reported here demonstrate that a multi-signal ESR approach can successfully differentiate the sedimentary provenance of each formation and locality and thus highlight the potential of ESR analysis as a meaningful tool for the future refinement of the complex stratigraphic framework of important fossil-bearing successions in Mongolia.