西宁盆地晚中新世沉积序列与环境演变研究

The lacustrine strata of Xining Basin occupies an important role on studying paleoclimate environment and the uplift of the Qinghai-Tibetan Plateau during the late Miocene. The key problem is that we lack high resolution and long lacustrine sedimentary strata records, because of previously paleomagnetic ages between 52Ma and 13.5Ma, from Eocene to Miocene. There is about 250m long fluvial-lacustrine facies stratum, found Hipparion dongxiangense, Chilotherium sp., Parelasmotherium sp., Stephanocemas sp., Kubanochoerus sp. and other mammal fossils, near Hongyazi village in northern Xining Basin belongs to the late Miocene. The project will investigate the high resolution magnetostratigraphy with the macro age of mammalian fossils of the Hongyazi section during late Miocene to establish the high resolution time series in Xining Basin, China. Morever, by using sedimentology, sequence stratigraphy, biostratigraphy, environmental proxies, such as carbonate, magnetic susceptibility, grain size, pollen and rate of Sr/Ba, and other analytical methods to get the multi index record of the climate and environmental changes, the project will establish the high resolution sedimentary sequence and environmental evolution sequence in the late Miocene in Xining Basin. Eventually, it will compare with the available research results of other basins on Qinghai-Tibetan Plateau and the global environmental change records. In the end, this research will provide scientific basis for the studies of the lacustrine strata and the sedimentary sequence and environmental evolution sequence in the late Miocene in Xining Basin.

西宁盆地河湖相地层在研究古气候环境、青藏高原隆升等重大地学科学问题方面具有重要地位。目前，前人给出盆地河湖相地层的古地磁年龄介于52~13.5Ma之间的始新世-中中新世，缺少晚中新世高精度年代控制的河湖相沉积地层研究。红崖子乡附近出露一套长约250m的河湖相地层，申请人在地层中发现大型哺乳动物化石东乡三趾马、大唇犀、副板齿犀、皇冠鹿和库班猪等化石，宏观地质时代属于晚中新世。本项目通过对西宁盆地晚中新世红崖子剖面详细的磁性地层年代学研究，结合剖面中哺乳动物化石时代，建立西宁盆地晚中新世精细的地层年代序列。然后，采用沉积学、层序地层学、生物地层学、环境代用指标提取等分析方法（碳酸盐含量、磁化率、粒度、孢粉、Sr/Ba比值），获取西宁盆地晚中新世气候环境变化的多指标记录，建立西宁盆地晚中新世高分辨率的沉积序列和环境演变序列，可填补这一时段的研究空白。

西宁盆地位于青藏高原东北缘，处于东亚季风区和西北干旱区的交界处，盆地中保存了厚达上千米的河湖相沉积物，地层沉积连续。沉积地层记录了高原东北缘的隆升特征和环境变化，因而在研究西北地区古气候环境、青藏高原隆升等重大地学科学问题方面具有重要地位。前人给出盆地河湖相地层的古地磁年龄介于52~13.5Ma之间的始新世-中中新世，缺少晚中新世河湖相沉积地层研究。本项目选择西宁盆地北部红崖子乡和班彦村附近出露的2条河湖相地层剖面为研究对象，结合剖面中哺乳动物化石时代，通过磁性地层年代学研究，建立了西宁盆地晚中新世地层年代序列。采用沉积学、层序地层学、生物地层学、环境代用指标提取等分析方法（粒度、碳酸盐、磁化率、Sr/Ba比值），获取西宁盆地晚中新世气候环境变化的多指标记录，建立了西宁盆地晚中新世环境演变序列。.研究结果认为，西宁盆地发现的大型哺乳动物化石东乡三趾马、大唇犀、副板齿犀、皇冠鹿和库班猪和两栖动物化石三趾马陆龟，类型上与临夏盆地三趾马动物群存在较强的相似性，时代上晚于西宁盆地吊沟哺乳动物群，与兰州盆地邢家湾动物群的时代相一致，应属于晚中新世，古地磁年龄为8.3Ma。古地磁年代学研究表明，班彦剖面的古地磁年代为7.25~8.4Ma，红崖子剖面古地磁年代为6.92~12.15Ma。班彦剖面环境代用指标研究显示，7.5Ma左右，西宁盆地区域内环境发生了突变，从相对湿润的气候环境转向冷干，之后亚洲内陆更加干旱化。