柴达木盆地鄂博梁地区中新世–上新世介形虫组合及其地质意义

Miocene-Pliocene Ostracoda Assemblage and Its Geological Significance in Eboliang Area, Qaidam Basin

摘要: 通过对柴达木盆地鄂博梁地区天然剖面所采样品的介形虫化石的研究，发现介形虫5科15属38种，并自下而上建立了3个介形虫组合带：1) Mediocypris-Candona-Cyprinotus组合带；2) Cyprideis- Prionocypris-Cypricercus组合带；3) Eucypris-Cyprideis-Leucocythere组合带。根据组合带将鄂博梁采样剖面的时代厘定为中新世中晚期-晚上新世。依据介形虫动物群在剖面上的分布规律，建立了5个介形虫群落，自下而上为：1) Advenocypris eboliangensis-Ilyocypris群落；2) Candona arcina-Cyprideis群落；3) Candoniella-Candona群落；4) Eucypris-Cyprinotus-Cypridopsis群落；5) Cyprideis littoralis- Leucocythere群落。通过对所建立的介形虫群落进行详细的生态特征分析，将鄂博梁剖面的古环境划分为3个期次：1) 中新世中晚期–早上新世为温湿浅水期；2) 中上新世为凉湿深水期，沉积环境更为咸化；3) 晚上新世为冷干浅水期。

Abstract: Well-preserved, more than 2500 ostracod specimens were recovered from the Miocene and Plio- cene profile at Eboliang area of Qaidam Basin, northwestern China. At least 38 taxa have so far been iden- tified belonging to Cyprididae, Ilyocyprididae, Eucandonidae, Limnocytheridae and Cytherideidae. Quali- tative and quantitative analyses indicate the presence of three main assemblage zones. In ascending order, they are: 1) Mediocypris-Candona-Cyprinotus Assemblage Zone from the Lower Youshashan Formation, is dominated by Mediocypris, Candona and Cyprinotus, and is dated as Middle-Late Miocene; 2) Cyprideis- Prionocypris-Cypricercus Assemblage Zone from the Upper Youshashan Formation (Early Pliocene); 3) Eucypris-Cyprideis-Leucocythere Assemblage Zone from the Sizigou Formation (Late Pliocene). This study also aims to analyse the ostracod community, so as to establish its ecological status as a basis for future paleoecological studies using ostracods to reconstruct the evolution of the Tibetan Plateau. Based on the distribution of ostracod fauna in the section, five ostracod communities are proposed in ascending order: 1) Advenocypris eboliangensis-Ilyocypris Community; 2) Candona arcina-Cyprideis Community; 3) Cando- niella-Candona Community; 4) Eucypris-Cyprinotus-Cypridopsis Community; 5) Cyprideis littoralis-Leuco- cythere Community. According to the ecological characteristics of ostracod communities, the late Cenozoic paleoenvironmental evolution at Eboliang area can be inferred as following: 1) a warm and humid climate with shallow lake during the period from Middle Miocene to Early Pliocene; 2) the climate became cool but wet, and the lake was deeper and salty in the Middle Pliocene; 3) it was cold and dry in the Pliocene, and the lake became shallow again.