NATURAL ENVIRONMENT AS REFLECTED IN SEDIMENTARY FACIES AND SPORO-POLLEN ASSEMBLAGES IN DONGTING BASIN IN QUATERNARY
Cai Shuming, Guan Zihe, (Institute of Hydrobiology, Academia Sinica, Wuhan)Kong Zhaochen Du Naiqiu(Institute of Botany, Academia Sinica, Beijing)
Based on the formation of sedimentary facies and the sporopollen assemblages derived from drilling cores of Huatian and Anyou in the Dongting basin and comparison with other boring profiles of the region, the natural environmental changes of the basin during the Quaternary period can be summarized as follows:There are eighteen rhythmic successions of grain size in the Huatian boring profile (Drill Hole Tian 11) and seven rhythmic successions in the Anyou boring profile (Drill Hole CK. 10). They represent four different sedimentary periods (Q1; Q2, Q3, Q4). The series of sediments belongs to cyclothem of fluvial facies and reflects that brooklets and lakes interwove here and tliere on the basin and these water bodies were often wandering in nature. This denotes that the supposed existence of a great lake embracing the entire Dongting basin during the Quaternary period is inconceivable.The Huatian boring profile can be subdivided into six vegetational zones in the following ascending order: evergreen broad and deciduous broad-leaved forest, herb-shrubby fern and aquatic plants, needle-leaved and deciduous broad-leaved mixed forest, deciduous broad-leaved forest, deciduous broad and evergreen broad-leaved mixed forest, and herb-shrubby fern (with deciduous broad-leaved trees) and aquatic plants (Tab. 1), corresponding climatic changes of wet-hot, temperate, cold-temperate, warm, temperate-hot and wet-hot respectively.From the above-mentioned conditions, it is clear that the world-wide alternate cold and hot climate in the Quaternary period did not affect the climate of this region as much as of the northern part of China. We hold that this has been due mainly to the continuous uplift of the Qinling Mountains since the Quaternary, preventing the arid-cold flows of Siberian anticyclone from passing southward. In the meantime, owing to the effects of southeastern monsoon, the climate of southern China (taking Qinling Mountains as the dividing line), remained humid. In other words, during the glacial and interglacial periods, the climate was less influenced in southern China than in northern. Therefore the opinion about the presence of ancient glacier in the east part of China during the Quaternary is worth reconsidering.