Clay Mineral Characteristics Of The Jianzha Basin In Northeastern Margin Of The Tibet Plateau, And Its Environment Significance

The Jianzha Basin is located in the northeast margin of Tibetan Plateau, and its formation and evolution must have been affected by the uplift of the Tibetan Plateau.Meanwhile, it is also located in the intersection zone of the East Asian monsoon, the India monsoon and the westerly jet, and it very sensitive to global climate change. The sedimentary sequence(11.8~5.8 Ma) of Jiarang section in the Jianzha Basin provided a good material for studying the regional paleoenvironmental evolution of the northeastern margin of the Qinghai Tibet Plateau at the Late Miocene, and it also provide clues and evidence for exploring relation between the uplift of the Tibet Plateau, the change of polar ice volume and the evolution of East Asian monsoon and Asian inland aridification.Based on the field lithology observation and sediment grain size analysis, we could determine the Jiarang profile was mainly formed by the aeolian red clay deposits,but it underwent sedimentary subfacies changes, including lake and river delta subfacies deposition. It is generally believed that the change of sedimentary facies and subfacies directly reflected the climate evolution during the sedimentary period. However, much research work about the reconstruction of paleoclimate paid more attention to the physical,chemical, biological and other indicators,while the comparison between the climatic proxy and the sedimentary facies transition was neglected. In this study,we attempted to make a comparative analysis on clay minerals proxies and the sedimentary facies in the sediments of the Jiarang section in the Jianzha basin, and found the degree of fitting between the mixed-layer ratio change of the disorder chlorite and smectite mixed layers minerals and the evolution of the paleoclimate reflected by the sedimentary facies change was very high. The change of Illite chemical index, clay mineral content, ratio, crystallinity and other conventional indicators could also consistent with it to a certain extent, but the degree of fitting was not too high. The main results of this study as follows:(1) The mixed-layer ratio of the disorder chlorite and smectite mixed layers minerals and Illite chemical index were sensitive to the climate change. This index was less affected by the tectonic activity and so it can as the proxy of the paleoclimate in the tectonic zone.When the mixed-layer ratio of the disorder chlorite and smectite mixed layers minerals decreased, the Illite chemical index increased and the climate was wetter. While the mixed-layer ratio of the disorder chlorite and smectite mixed layers minerals increased, the illite chemical index decreased and the climate was drier.(2) Compared to the illite chemical index, the degree of fitting between the mixed-layer ratio change of the disordered chlorite and smectite mixed layers minerals and the evolution of the paleoclimate reflected by the sedimentary facies change was higher.And this is because the disorder chlorite and smectite mixed layers minerals was more autogenous. The mixed-layer ratio can indicate directly the paleoclimate condition in the sedimentary area. However, the other unit minerals including in illite were formed dominantly by the detrital origin. The illite chemical index more or less contained the information about the parent rock area and the area where the mineral went through. On the whole, the mixed-layer ratio of the disorder chlorite and smectite mixed layers minerals was not only as the paleoclimate index of the tectonic activity area, but also could as the index of the area where the clay mineral of detrital origin developed.(3) The sediment of Jiarang section in the Jianzha Basin can be divided into nine climatic evolution stages according to the change of mixed-layer ratio, the section lithology and the variation of sedimentary facies. It has experienced three stages of cold-dry climate,including A(11.8~11.0 Ma), D(9.5~9.4 Ma) and H(7.7~7.5 Ma), three stages of wet climate that were B(11.0~10.1 Ma), C(10.1~9.5 Ma), E(9.4~9.05 Ma) and three stages of warm-moist and cold-dry alternated which including F(9.05~8.5 Ma), G(8.5~7.7 Ma) and I(7.5~5.8 Ma).(4) The paleoclimate evolution processes reflected by the index of the content, ratio and the crystallinity of the clay mineral had difference at some stages. And they were not correspond with the climate evolution process reflected by the sediment facies change at some times. This is because the content, ratio and the crystallinity of the clay mineral and some other conventional indexes affected by the comprehensive influence of tectonic activity and sedimentary facies change. Based on the analysis,we concluded that the Tibetplateau may have been experienced a tectonic uplift event at ca. 10.1Ma and ca. 7.7Ma,respectively. Due to the change of sedimentary facies, the content and ratio of clay mineral presented the sudden changes at ca. 9.78 Ma, 9.5Ma and 9.4Ma. The clay minerals of two stages of ca. 9.78~9.5Ma and ca. 9.4~9.05 Ma were affected by the parent rockexcept the climate factors.(5)The relative content and ratio of clay mineral could only as the auxiliary indexes for reconstructing paleoclimate in the tectoniczones. And the information of the paleoclimate revealed by this index was worth to be verified further. In a sedimentary sequence, if the sedimentary facies have been changed, the change of the clay mineral index in every sedimentary facies represents the different climate significance. But it was difficult to interpret the climatic information using the clay mineral index if the clay minerals in the alternate sedimentary facies, because the difference of clay mineral assemblage and abundance was much greater in the different sedimentary facies.