New early Oligocene age for the Mouding Basin, Southwestern China: Source and paleoenvironment

Sedimentary basins in the southeastern margin of the Tibetan Plateau yield valuable geophysical and fossil records, documenting the paleoenvironmental and biodiversity changes in the Tibetan Plateau and the Hengduan Mountains. Constraining the age of sediment deposition is fundamental in understanding the evolutionary history of biodiversity, climate (e.g. development of the monsoon), and regional landscape changes. Many basins in this area were originally considered as Miocene basins based on regional lithostratigraphic comparison and biostratigraphy, although this has been challenged recently for several basins (e.g. Luhe, Markma, Jianchuan, Wenshan and Relu basins). Here we report a new age for the Mouding Basin, Central Yunnan, China. We arrive at an early Oligocene (33 +/- 1 Ma) instead of a Miocene age for the fossil-bearing strata of this basin. U-Pb and 40Ar/39Ar dating of a volcanic ash layer from the Mouding Basin and one from the Luhe Basin in combination with zircon Hf isotope data suggest that the early Oligocene Laojiezi eruption is a likely source for ash layers in both basins. Fossilized dicot leaves were used to reconstruct the paleoclimate and the paleo-elevation of the Mouding Basin. This paleoclimate reconstruction indicates that the paleoclimate in the Mouding Basin closely resembles that of the Luhe Basin. The paleoclimate between the two floras is in fact more similar than their respective modern climates. During the early Oligocene, the Mouding Basin experienced colder and more humid winters with a milder monsoonal climate compared to today. Despite this, the Mouding flora exhibits high floristic similarities with its contemporary Luhe and Markam floras, as well as with the modern vegetation in southwestern China, supporting that the modernization of the biodiversity in the SE margin of the Tibetan Plateau has initiated since the Paleogene.