青藏高原昆仑山垭口地区第四纪介形虫及环境变迁的探讨

Quaternary Ostracoda in the Pass Area of the Kunlun Mountains,Northern Qinghai-Tibet Plateau,with a Discussion on the Environmental Change

青藏高原昆仑山垭口地区发育一套第四纪地层,自下而上有:惊仙组(惊仙冰碛层)、羌塘组、纳赤台沟组、三岔河组、小南川组和热水组。在羌塘组和三岔河组发现丰富的介形虫化石,可明显地划分两个介形虫组合,即:①Ilyocypris-Candona-Leucocythere-Li mnocytherellina组合和②Li mnocythere-Prionocypris-Eucypris-Ilyocypris组合。第1组合的化石包含8属25种,分布于羌塘组,以其中段化石最为丰富,该组合显示了西藏色林湖-班戈湖夏穷错组、青海阿乙亥组、陕西-河南三门组和华北地区泥河湾组早更新世的特征,故含第1组合及所含地层---羌塘组的时代无疑应归属为早更新世,但不排斥其上段上部有延至中更新世的可能。第2组合由9属12种组成,其中有9个现生种,该组合发现于三岔河组上部,显示了青海中-上更新统二郎尖组,西藏色林湖-班戈湖地区上更新统同旧藏布组的特征,故含第2组合及其地层---三河岔组应归属晚更新世,但其中-下部有属中更新世的可能。根据介形虫化石古生态特征和生态组合的变化及沉积特征,羌塘组为河湖相沉积,下段为冲积扇的粗碎屑沉积,中段为湖相细碎屑沉积,上段为扇三角洲粗碎屑沉积。三岔河组的中-下部为辫状河流相的沉积,上部为湖相沉积。青藏高原在整体上升的同时,于早-晚更新世高原的边缘地区受到河流的切割,而在断陷盆地则仍接受沉积。

Quaternary system developed at the pass area of the Kunlun Mountains, Northern Qinghai--Tibet Plateau. They are composed of in ascending order as follows ： Jingxian Formation （Jingxian glacial series）, Qiangtang Formatiom, Nachitaigou Formation, Sanchahe Formation, Xiaonanchuan Formation and Reshui Formation. The abundant ostracoda fossils are found in the Qiangtang Formation and the Sanehahe Formation and may be subdivided into 2 assemblages, i. e. （1） Ilyocypris -CandonaLeucocythere-Limnocytherellina assemblage and （2） Limnocythere- Prionocypris-Eucypris-Ilyocypris assemblage. The fossils of the first assemblage contain 8 genera and 25 species and they are distributed in the Qiangtang Formation, of which the fossils are most abundant in the Middle Member of the Qiangtang Formation. This assemblage reveals the Early Pleistocene characteristics of the Xiaqiongcuo Formation in the Qilin--Bange Lake of Tibet, the Ayihai Formation in the Qinghai, Sanmen Formation in Shaanxi- Henan and the Nihewan Formation in Northern China. Therefore the first assemblage and its strata in the Qiantan Formation undoubtedly belong to the Early Pleistocene, but its upper section of the upper member may contain the Middle Pleistocene. The fossils of the second assemblage consist of 9 genera and 12 species, in which 9 species are the recent speies. This assemblage is found at the upper section of the Sanchahe Formation, and they reveal the characteristics of the Middle Upper Pleistocene Erlangjian Formation in Qinghai and the upper Pleistocene Tongjiuzangbu Formation in the Qilin-Bange Lake area, Tibet. So the second assemblage and its strata （Sanchahe Formation） should belong to the late Pleistocene, but the Mid-Lower part of the Sanchahe Formation may belong to the Middle Pleistocene. Based on the paleoecological characters of ostracod fossils, together with changes of ecoassemblages and sedimentary features, the Qiangtang Formation is fluviolacustrine facies deposits; its lower member is coarse-grained alluvial fan deposits, the middle member is fine-grained lacustrine dopdsits and the upper member is coarse-grained fan-delta deposits. The mid-lower parts of the Sanchahe formation is braided river deposits, the upper part of the Sanchaihe Formation is Lacustrine deposits. During the uplift of the Qinghai-Tibet plaleau, the margin of the plateau was cut by rivers and the fault basin still received deposits in the Early-Late Pleistocene.