西藏西南部札达盆地新近纪的孢粉组合

摘要

The Zanda Basin is a Neogene rifted basin in Ngari Prefecture, southwest Tibet. It was formed in Late Miocene, at about 9.2Ma. The Duojidong section is located about 4km south of Zanda County. It has a well-developed set of lacustrine sedimentary strata. Palaeo-current measurement and provenance analysis show that the age interval of the Tuolin Formation is about 9.2-2.6Ma, and the Xiangzi Formation is dated at about 2.6-1.7Ma. As the sporopllen data can direcdy reflect paleoclimate changes, sporopllen data have been always regarded as one of the best environmental indicators. Through the palynological analysis, 6 palynological assemblages and vegetable types were recognized, and 3 phases of paleoclimatic changes were revealed: ① from 9.2Ma to 5.6Ma, correlated with assemblages 1-2, the environment was cold shrub steppe at about 9Ma, and then became warm and humid subtropical-temperate deciduous coniferous and broad-leaved mixed forests; ② from 5.6Ma to 3.1Ma, correlated with assemblages 3-5, the environment was mild and humid evergreen-deciduous broad-leaved mixed forests, and changed to the warm and a litde dry deciduous conifer mixed forest at about 3.6Ma. This stage had a climate fluctuation; ③ from 8Ma to 1.7 Ma, correlatied with assemblage 6, the environment became cold and dry deciduous coniferous and broad-leaved mixed forests and subalpine coniferous forests.%札达盆地位于西藏自治区西南部阿里地区的喜马拉雅山区,是形成于晚中新世的断陷盆地.研究剖面位于札达县城南4km的多几东,发育一套良好的河湖相沉积地层.结合前人得出的古地磁结论和地层综合对比,托林组的时代为9.2～2.6Ma,香孜组的时代为2.6～1.7Ma.沿剖面自下而上可划分出6个孢粉组合及对应的植被类型:①孢粉组合1～2(9.2～约5.6 Ma),为亚热带—温带落叶针阔叶混交林,其中9Ma左右气候较寒冷干旱,经历了一次由寒冷干旱向温暖湿润的转变过程;②孢粉组合3～5(5.6～约3.1Ma),由亚热带—温带常绿—落叶针阔叶混交林,期间存在一次暖湿气候的波动;③孢粉组合6(2.8～约1.7Ma),为寒冷干旱气候下生长的温带落叶针阔叶混交林—亚高山针叶林.