| Abstrakt: |
The palaeoenvironment has an important influence on the formation and evolution of karst geomorphology, and the planation surface at the edge of the karst plateau is one of the important recorders. The Yunnan-Guizhou Plateau is an important karst plateau in China, on the southeast edge of which is located Xiangxi in Hunan Province-a deep cut platform and gorge development area. Xiangxi is also a combination zone of low mountains and middle mountains at the edge of the Yunnan-Guizhou Plateau, in which the high plain transits to the alluvial plain, with rich records of planation surfaces. These planation surfaces are characterized by rapid altitude changes, different weathering crust materials and different types of bedrock lithology. However, the weathering crust properties and palaeoclimate characteristics of these planation surfaces are still unclear. Therefore, we choose two weathering crusts representing the elevation of 1,400 m and 600-700 m of planation surfaces in Luota Period (Luota section) and Zhaoshi Period (Jiaba section) as study objects. In this study, conducting the inversion of palaeoclimate indicated by weathering crusts of planation surfaces through grain size, magnetic susceptibility and sporopollen analyses, we hope to provide more evidence for the development characteristics of planation surfaces and palaeoclimate evolution on the edge of the Yunnan-Guizhou Plateau. Results show that the weathering crust of Luota section is 5.0 m thick and can be divided into 5 layers. The lithology is mainly composed of humus, gravel layer, silt, clay, etc. The content of silt with sizes ranging from 5 to 50 μm is the highest, accounting for 57.6%; the average content of clay is 41.9%; the sand content is the lowest, at 0.56%, all of which are fine sand. The weathering crust in Jiaba section is 3.5 m thick and can be divided into two layers. The lithology is mainly composed of humus, silty, clay and a small amount of gravel. The content of silt with sizes ranging from 5 to 50 μm is also the highest, accounting for 54%; the average content of clay is 39%. In addition, the analyses of Md, Mz, σ, Sk, Kg, SC/D values, magnetic susceptibility and other indicators show that there appear multiple index fluctuations in both Luota and Jiaba sections, but the fluctuation range of Jiaba section is relatively limited. The types of sporopollen in the two sections are different. Luota section is mainly composed of Polypodium and Pinus, with a small number of Abies, Podcarpus and Chenopodium. The main sporopollen components of Jiaba section are Polypodium and Pinus, and a few Gramineae and Carpinipites are found. The types of sporopollen in Jiaba section are fewer than those in Luota section, and the concentrations are also lower. Based on the development of Polypodium in both sections, it is supposed that the climate during the sedimentary period of weathering crust was warm and hot. However, the climate during the sedimentary period of Luota section may have been a warm climate with high humidity, while the climate of Jiaba section may have been a cool and humid climate with low humidity. In addition, according to the analysis of sporopollen and the previous data, it is speculated that the weathering crust of Luota section may have been formed in the early Neogene, and the planation surface may have been formed in the Miocene. The weathering crust of Jiaba section may have been formed in the early Quaternary, and the planation surface may have been formed in the Pleistocene. [ABSTRACT FROM AUTHOR] |
