The basis for the age of the Bayantumen formation sediments distributed along the border of Mongolia (Choibalsan Sum area of Eastern province)

Bayantumen Formation sediments occupy 18.2 per cent of the area of the Ovoot Tolgoi-50 project, which covers the Choibalsan, Chuluun Kohot, and Gurvanzagal districts of Dornod Province, or 523.8 km2. This sediment is widely distributed in the Eastern Mongolia region. The Ovoot Tolgoi-50 project area borders with China in the southeast. Previous researchers mapped the sediments of the Lower Cretaceous Bayantumen Formation along the border of Mongolia, and Jurassic volcanogenic and volcanic sedimentary rocks adjacent to it in Chinese territory. As part of the research work of the Ovoot Tolgoi-50 project, we have taken absolute age samples from the volcanics of the lower member of the Bayantumen Formation and determined by the uranium-lead method. The age was determined to be 141 ± 1.7 million years, corresponding to the Valanginian-Gotian period (Kt). Lower Cretaceous Valanginian-Goterian shells were found in the acid metasands of the lower member of the Bayantumen Formation, which confirmed the basis for the age of the sediments.