柴达木盆地一里坪凹陷新近纪孢粉特征及其古环境意义

Characteristics of Neogene palynology and palaeoenvironment significance in Yiliping Sag,Qaidam Basin

柴达木盆地位于青藏高原北部,受高原隆升以及全球气候影响,新近纪盆地内古环境发生显著变化。通过对新近纪柴达木盆地一里坪凹陷孢粉特征进行分析,恢复盆地植被类型和环境演化过程,进而探讨高原构造隆升和全球气候变化对盆地古环境的影响。结果表明,一里坪凹陷新近纪孢粉组合以裸子植物繁盛,被子植物较为常见,旱生植物发育为主要特征。孢粉特征指示柴达木盆地一里坪凹陷整体新近纪属于半干旱—干旱气候,古环境演化受青藏高原隆升强度以及全球气候影响显著:下油砂山组环境为亚热带森林,上油砂山组环境为温带森林—草原,狮子沟组环境转变为寒温带草原—半荒漠。该时期环境背景,不利于一里坪凹陷在新近纪发育大规模优质烃源岩。

Objectives: Qaidam Basin is located in the northern part of Qinghai—Xizang( Tibet) Plateau. Due to the influence of Qinghai—Xizang Plateau uplift and global climate, the paleoenvironment in the basin changed significantly during Neogene Period.Methods: Based on the analysis of the characteristics of palynology in the Yiliping Sag of Qaidam Basin,the Neogene vegetation types and environmental evolutionary process of the basin were restored,and the effects of plateau tectonic uplift and global climate change on the paleoenvironment of the basin were discussed.Results: The results show that the Neogene palynological assemblages in the Yiliping Sag is characterized by abundant gymnosperms,a lot of angiosperms and many xerophytes.Conclusions: The palynological characteristics indicate that the climate in the Yiliping Sag of Qaidam Basin belongs to semiarid or arid climate during the whole Neogene period,and the Neogene palaeo-environmental evolution was significantly affected by the uplift intensity of the Qinghai—Xizang Plateau and the global climate:the environments during the deposition of the Lower Youshashan Formation,Upper Youshashan Formation and Shizigou Formation are belonging to Subtropical forest,temperate forest—grassland and cold temperate grassland—semidesert,respectively. This environmental background of this period is not beneficial for the development of large-scale high-quality source rocks in the Yiliping Sag during Neogene Period.