宁夏同心丁家二沟哺乳动物群生物地层学研究进展及青藏高原隆升背景

宁夏同心丁家二沟哺乳动物群是中国中中新世早期最具代表性的地方动物群，但长期以来，对这一动物群产出层位的认识处于模糊和混乱的状态。本文通过对丁家二沟一带新生界地层的划分和对比，将出产丁家二沟动物群的彰恩堡组划分为下、中、上3个层位，其中下部和中部的层位分别产出两个时代不同的动物群：下部的印子岭动物群以Alloptox gobiensis-Protanancus tobieni-Caementodon tongxinensis-Turcocerus sp.1组合为代表，时代对应于欧洲陆相哺乳动物分期的MN5；中部的马二嘴子沟动物群则以Alloptox gobiensis-Platybelodon tongxinensis-Hispanotherium matritense-Turcocerus sp.2组合为代表，时代对应于欧洲陆相哺乳动物分期的MN6；而上部的地层则需要进一步研究划分。同时，丁家二沟地区彰恩堡组的下部层位的时代约为17~15Ma，跨越了中中新统的下界，可以考虑在这一地区重新建立中国陆相通古尔阶下界，使其与中中新统的下界相一致。此外，丁家二沟地区北部彰恩堡组与下伏清水营组成不整合接触，南部干河沟组中发育了生长断层，可能代表了这一地区对约20Ma和10Ma青藏高原两次强烈构造事件在东北部的远程响应。

Biostratigraphic subdividing of the Neogene Dingjia'ergou mammalian fauna,Tongxin County,Ningxia Province,and its background for the uplift of the Tibetan Plateau

The Dingjia'ergou mammalian fauna from Tongxin County, Ningxia Hui Autonomous Region, is one of the most typical early Middle Miocene local fauna in China. The published taxa of the fauna contains the lagomorph Alloptox gobiensis, the primate Pliopithecus zhangxiangi, the carnivore Tongxinictis primordialis, the proboscidean Protanancus tobieni and Platybelodon tongxinensis, as well as the suid Kubanochoerus lantienensis and Bunolistriodon intermedius. Other recorded taxa include Atlantoxerus, Heteroxerus, Megacricetodon, Democricetodon, Cricetodon, Protalactaga, Heterosminthus, Sayimys, Tachyoryctoides, Prodistylomys, Dryomys, Steneofiber, Amphicyon, Gobicyon, Percrocuta, Sansanosmilus, Amebelodon, Serbelodon, Gomphotherium, Miomastodon, Caementodon, Huaqingtherium, Chalicotherium, Stephanocemas, Hispanomeryx, Eotragus, and Turcocerus. The number and the preservation state of the Dingjia'ergou fauna are better than the other contemporary faunas in China. However, the fossil site in this area are spread across a large area within 15km EW and 10km NS that is east to the Tonxin County and near the Dingjia'ergou Village, pointedly distributed in the fluviolacustrine deposits. Statigraphic correlation in this area is of severe difficulty due to discreteness and similarity of the semdimentation. Therefore, the fossiliferous layers are poorly correlated that leads to much confusion. Based on our study, the fossil sites of the Dingjia'ergou fauna are all existed in the Miocene Zhang'enbao Formation(the origin name is Hongliugou Formation and renamed because of synonymy), overlying the Oligocene Qingshuiying Formation(in the north region)or the Eocene Sikouzi Formation(in the south region), and underlying the Miocene Ganhegou Formation or Quaternary deposits. The Zhang'enbao Formation is dominated by channel and overbank sandstones and siltstones punctuated by mudstones. It is composed of four sedimentary cycles of sandstones-silt/mudstones. At least two asynchronous fossil assemblages have been identified by us. The fossil assemblage from the first(lowermost)sedimentary cycle is characterized by the combination of Alloptox gobiensis-Protanancus tobieni-Caementodon tongxinensis-Turcocerus sp.1, corresponding to the European Land Mammal Age MN 5(from late Early to the earliest Middle Miocene). Here we defined it as the Yinziling sub-fauna. The fossil assemblage from the second and the third(middle)sedimentary cycles is characterized by the combination of Alloptox gobiensis-Platybelodon tongxinensis-Hispanotherium matritense-Turcocerus sp.2, corresponding to the European Land Mammal Age MN 6(early Middle Miocene). Here we defined it as the Maerzuizigou sub-fauna. However, the fourth(uppermost)sedimentary cycle yields little fossils(Alloptox sp.)and the age of this cycle should be further studied. The lower horizon of the Zhang'enbao Formation spans almost 17~15Ma and step across the lower boundary of the Middle Miocene. Thus it is suitable for re-establishing the lower stratotype of the terrestrial Tunggurian Stage in East Asia, being in accord with the lower boundary of the Middle Miocene. Furthermore, in the north region of this area, the Zhang'enbao Formation and the underlying Qingshuiying Formation show unconformable contact, and in the south region, a growth fault was developed in the Ganhegou Formation. These two phenomena may represent two northeastward tele-responses for two tectonic events in the inner Tibetan Plateau at about 20Ma and 10Ma, respectively.