The consensus view among geologists is that the Lanzhou Basin sequence of sedimentscontains the Oligocene/Miocene boundary, but where this occurs is unknown. Indeed, this is thecrux of the issues concerning mid-Tertia...The consensus view among geologists is that the Lanzhou Basin sequence of sedimentscontains the Oligocene/Miocene boundary, but where this occurs is unknown. Indeed, this is thecrux of the issues concerning mid-Tertiary paleontology in Asia: what fauna corresponds with thistime? What turnover events or distinctive taxa signal the beginning of the Miocene epoch? Onceknown, this will be of broad usefulness because vertebrate-bearing deposits are widespread in Asiaand can be used to date basin sediments. Lanzhou and other basins permit a means of correlating tothe time scale by using fauna and paleomagnetic data locally, and ultimately radiometric data fromdistant 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 themagnetic time scale. We conclude that the lower part of the Xianshuihe Formation contains theOligocene/Miocene boundary, and that the top of the formation, both in the south and north partsof the basin, is middle Miocene in age. Rodents from lower white sand units of the XianshuiheFormation, 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 theyretained a mainly "Oligocene" composition, and that other presumed late Oligocene assemblagesmay be, in fact, early Miocene in age. The Lanzhou appearance of Proboscidea is 19-18 Ma, asexpected.展开更多
文摘The consensus view among geologists is that the Lanzhou Basin sequence of sedimentscontains the Oligocene/Miocene boundary, but where this occurs is unknown. Indeed, this is thecrux of the issues concerning mid-Tertiary paleontology in Asia: what fauna corresponds with thistime? What turnover events or distinctive taxa signal the beginning of the Miocene epoch? Onceknown, this will be of broad usefulness because vertebrate-bearing deposits are widespread in Asiaand can be used to date basin sediments. Lanzhou and other basins permit a means of correlating tothe time scale by using fauna and paleomagnetic data locally, and ultimately radiometric data fromdistant 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 themagnetic time scale. We conclude that the lower part of the Xianshuihe Formation contains theOligocene/Miocene boundary, and that the top of the formation, both in the south and north partsof the basin, is middle Miocene in age. Rodents from lower white sand units of the XianshuiheFormation, 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 theyretained a mainly "Oligocene" composition, and that other presumed late Oligocene assemblagesmay be, in fact, early Miocene in age. The Lanzhou appearance of Proboscidea is 19-18 Ma, asexpected.
