西宁盆地晚始新世石膏-红色泥岩旋回的古环境指示

始新世时期东亚环境在轨道时间尺度上变化特征的研究对于认识青藏高原规模较小、两极无冰盖、高大气CO2浓度背景下天文因素对东亚环境演化的驱动有重要意义。文章通过对西宁盆地塔山剖面石膏-红色泥岩互层沉积的粒度、颜色和孢粉进行分析,初步探讨了晚始新世时期该区石膏-红色泥岩旋回的古环境指示意义及其与轨道变化的关系。结果表明: 红色泥岩层粒度较粗,其中孢粉含量较少,以蒿粉和藜粉为主,指示草原或疏林草原景观; 石膏层粒度较细,其中孢粉含量丰富,以栎属、楝属、麻黄属、松科植物为主,指示相对温暖湿润的灌丛-森林景观。西宁盆地规律的石膏-红色泥岩旋回具有广泛的空间可对比性,符合轨道尺度上的气候变化特征。晚始新世时期,西宁地区石膏-红色泥岩旋回体现了地球轨道对该区沉积环境和植被变迁的驱动。

SEDIMENTARY CHARACTERISTICS AND PALEOENVIRONMENTAL SIGNIFICANCE OF LATE EOCENE GYPSUM-MUDSTONE CYCLES IN THE XINING BASIN,NORTH EASTERN TIBETAN PLATEAU

It is important to understand the response of East Asian climate to orbital cycles during the Paleocene-Eocene when the Earths climate system was under such a background: high atmospheric CO2 concentration,no bipolar continental ice sheet,and a smaller Tibetan Plateau.In this paper,we studied the Tashan section by pollen analysis,grain-size analysis,and color reflectance.This section is located in the Xining Basin on the northeastern edge of the Tibetan Plateau,providing a continuous continental sedimentary record of latest Eocene to earliest Oligocene.It can be litho-stratigraphically correlated to the Xiejia section and Shuiwan section in the Xining Basin,all showing clear gypsum-mudstone cycles about.3.5m thick.Our analyzing results show that the red mudstone layers are coarser in grain size and less in pollen content,dominated by Artemisia,indicating a relatively dry grassland environment,whereas the gypsum layers are finer in grain size and much in pollen content,dominated by Quercus,Melia,Pinus,and Picea,indicating a relatively wetter forest environment.We argued that an astronomical force to be the most likely allogenic mechanism to result in such regular mudstone-gypsum cycles. Our grain size, color reflectance(a *), and spore-pollen data thus corroborate the previous interpretation that the gypsum layers formed at greater water supply intervals, and further suggest that the orbital-scale changes in sedimentary environment and the advance/retreat of vegetation in the Xining Basin during Late Eocene were controlled by orbital-driven oscillations in climate.