High-resolution paleomagnetic constraint on the oldest hominoid- fossil-bearing sequence in the Xiaolongtan Basin, southeast margin of the Tibetan Plateau and its geologic implications

Sediments in the Xiaolongtan Basin at the southeastern margin of the Tibetan Plateau (SEMTP) not only contain an abundance of the oldest Chinese hominoid fossils, but also a record of the evolution of the Indian monsoon and the uplift of the southeastern Tibetan Plateau. However, these Middle–Upper Miocene sedimentary sequences have lacked a precise chronology, hampering our ability to reconstruct the regional tectonic uplift and the paleoecological environment. We carried out high-resolution paleomagnetic dating throughout the fluviolacustrine sequence in the Xiaolongtan Basin. The 487.5 m-thick sequence here revealed 11 reversed and 11 normal zones that span from chron 5n.2n to chron 5ACr, yielding magnetostratigraphic ages of ~14.1–10.0 Ma for the section, and 12.5 Ma and 11.7 Ma for the two hominoid-fossil bearing beds. Combined with lithological analyses and the evidence for ecological characteristics, we suggest that four notable ecological environmental phases had emerged in the Xiaolongtan Basin after it was formed at around 14.1 Ma. These were as follows: Phase I (14.1–12.6 Ma) with humid-hot forest steppe, Phase II (12.6–11.6 Ma) with subtropical evergreen broad-leaved forest, Phase III (11.6–10.0 Ma) with a hot-dry and evaporative shallow lake and Phase IV (after 10.0 Ma) with a subtropical semiarid steppe environment. After ~10.0 Ma deposition gradually ceased, albeit recurring sporadically until the Early Pleistocene. •Depositional ages of the Xiaolongtan Basin range from 14.1 Ma and 10.0 Ma.•The oldest Chinese hominoid fauna is estimated at 12.5 Ma.•Four distinct ecologic environments had emerged in Xiaolongtan Basin.