Recent Advances in the Small Mammal Biostratigraphy and Magnetostratigraphy of Lanzhou Basin

The consensus view among geologists is that the Lanzhou Basin sequence of sediments contains the Oligocene/Miocene boundary, but where this occurs is unknown. Indeed, this is the crux of the issues concerning mid-Tertiary paleontology in Asia: what fauna corresponds with this time? What turnover events or distinctive taxa signal the beginning of the Miocene epoch? Once known, this will be of broad usefulness because vertebrate-bearing deposits are widespread in Asia and can be used to date basin sediments. Lanzhou and other basins permit a means of correlating to the time scale by using fauna and paleomagnetic data locally, and ultimately radiometric data from distant localities, to develop a precise biochronology. Herein, we summarize faunal constraints, primarily those of small mammal taxa, on correlation of the Lanzhou magnetozone sequence to the magnetic time scale. We conclude that the lower part of the Xianshuihe Formation contains the Oligocene/Miocene boundary, and that the top of the formation, both in the south and north parts of the basin, is middle Miocene in age. Rodents from lower white sand units of the Xianshuihe Formation, correlate to the Xiejia assemblage of the Xining Basin, Qinghai Province, and are early, but not earliest, Miocene age. The implication for rodent faunas of the epoch boundary is that they retained a mainly "Oligocene" composition, and that other presumed late Oligocene assemblages may be, in fact, early Miocene in age. The Lanzhou appearance of Proboscidea is 19-18 Ma, as expected.