Litho- and Biostratigraphy of the East Mesa in Shara Murun Region of the Erlian Basin, Inner Mongolia, China, and the subdivision of the Ulangochuian Asian Land Mammal Age

The Paleogene deposits in the Erlian Basin are nearly continuous from the upper Paleocene to the lower Oligocene, and most Eocene Asian Land Mammal Ages were proposed on the basis of the corresponding mammal faunas from different formations in the basin. However, the correlation and extension of some lithologic units are complex and controversial in this region due to the spatial variation of the terrestrial sediments and a complex research history for more than a century. In the past decades, we have clarified the definition and correlation of the late Paleocene through early middle Eocene deposits in Huheboerhe area, but those of the middle to late Eocene deposits in Shara Murun region still remain disputable. The Ulan Gochu Formation, which was initially referred to a set of red clay at the typical Ulan Gochu locality, was considered to extend for a much wider range at East Mesa, Erden Obo, and Nom Khong. Here we provide the detailed litho- and biostratigraphy at the Bayan Obo (= Twin Oboes) and Jhama Obo sections with description of new materials of Eomoropus major and Brachyhyops neimengolensis. Deposits exposed at the Bayan Obo section can be subdivided into the Tukhum, Shara Murun, Ulan Gochu, and Baron Sog formations, totally about 70 m in thickness. The exposed strata at the Jhama Obo section can be subdivided into partial Shara Murun, Ulan Gochu, and Baron Sog formations, about 50 m in total thickness. The lithostratigraphic correlation and faunal compositions of the East Mesa are discussed based on our recent fieldworks and CAE collection records. Furthermore, the late middle Eocene Ulangochuian ALMA can be divided into Ug1 and Ug2, based on the mammal faunas from the upper member of the Shara Murun Formation and the Ulan Gochu Formation, respectively. The Ulangochuian is roughly correlated to the Duchesnean NALMA.