| The uplift of the Tibet Plateau has a profound effect on global climate change. Cenozoic basins of in the central Tibet Plateau mainly distributed in the Bangonghu-Nujiang fault zone and the north area. Due to its special geographical location and drought-desertification environment, Cenozoic basins record the clues of climatic and environmental changes due to the uplift of the Tibet Plateau.Gerze Basin is located in central Tibet Plateau, which is one of the typical basins formed due to the uplift of the Tibet Plateau during the Cenozoic time. This paper is aimed to study the early Oligocene-late Miocene typical section of the Kangtuo Formation in Gerze Basin, established the chronology, sedimentology and petrology background frame of the sediments. Study of clay mineralogical characteristics, their relative content, morphological characteristics, geochemical characteristics of the Gerze sediments were undertaken using X-ray diffraction (XRD), scanning electron microscopy (SEM), and chemical analyses, paleoclimate and environment evolutions of in Gerze Basin in the Cenozoic time were deciphered, Based on study of clay mineralogy, palaeoclimate change in the study area and its relation with global climate events and plateau uplift events were reconstructured. Meanwhile, based on geochemical analyses results of whole rock chemical composition and trace element abundances, the weathering index of the sediments were calculated, and fluctuation features along the profile were obtained, which indicated climate and environment conditions in Gerze Basin during the early Oligocene-late Miocene age. our results may provide basic data to understand the Tibetan plateau uplift and the effect of climate change.The Gerze Basin evolved from Pan-Africa continental crust crystalline basement since Pre-Ordovician, located between the Qiangtang continent and the Gangdese continent, has experienced different tectonic environment and depositional evolution. And the sedimentary sequences have different development from Pre-Ordovician to Quaternary. Its development has experienced the Ancient Tethys Ocean to Neo-Tethys Ocean, then to collision orogeny and intracontinental development process, and the Gerze sediments are mainly continental deposit. Base on clay mineralogical and geochemical characteristics of the Gerze sediments, palaeoclimate evolution can be divided into the following three stages:(1) Early Oligocene to late Oligocene (0-691.78m), clay mineral assemblage zone of montmorillonite, illite, and kaolinite. Characteristic clay minerals are montmorillonite, illite, and kaolinite. Illite is the dominant clay mineral spcies, its content varied in 63%-79%, averaging 73%, and chlorite was in 9%-21%, averaging 13%. Kaolinite content was in 5%-14%, averaging 9%; Smectite content was relatively smaller, averaging 5%, but the content changed dramatically and frequently, between2%-13%. CIA ratio varied in 51.6-74.6, averaging 63.5; w(Al2O3)/w(Fe2O3) ratio was 2.7-4.0, averaging 3.2; w (SiO2)/w(Al2O3) ratio was 7.9-19.0, averaging 12.4; w (MgO)/w (Al2O3) ratio was 0.14-0.29, averaging 0.18; w(TiO2)/w(Al2O3) ratio was 0.04-0.06, averaging 0.05. During this period, (I+Ch)/(K+S) ratio is small overall and relatively stable, indicating that the climate was relatively warm and wet; The CIA value was relatively small in some intervals, illustrating that the climatic condition was cold and dry in the certain time. In general, this period was characterized by seasonal warm and dry climate. In addition, in this stage there are two small warm humid period and a cold arid period, experienced two cycles from cold and arid to warm and humid climate, this indicated that the Tibetan Plateau has rise to a certain height, leading to the seasonal monsoon of mainly cold and dry climate.(2) Late Oligocene to Early Miocene (691.78-896.15m), clay mineral assemblage zone of illite and chlorite. Illite content ranged from 61% to 77%, with an average of 70%, and chlorite content was 18%-31%, averaging 26%. Montmorillonite content was relatively lower, the main content is between 3% and 9%, averaging 5%. Kaolinite was almost absent. CIA ratio varied in 51.9-74.1, averaging 64.9; w(Al2O3)/w(Fe2O3) ratio was 2.9-5.5, averaging 3.7; w (SiO2) /w(Al2O3) ratio was 2.6-4.1, averaging 3.2; w(SiO2) /w(Fe2O3) ratio was 8.8-14.6, averaging 10.8; w (MgO)/w (Al2O3) ratio was 0.16-0.31, averaging 0.20; w(TiO2)/w(Al2O3) ratio was 0.04-0.06, averaging 0.05. During this period, (I+Ch)/(K+S) ratio is higher and fluctuate drastically, indicating a cold and dry climate condition. However, the CIA values were generally small, suggesting that relatively cold and dry climate condition prevailed over this period. Furthermore, several climate evolution cycles occurred during the period, and the M-il event was observed during the the Oligocene/Miocene time. Tibetan Plateau uplift has affected the warm air from Indian Ocean to Asia inland, and form the downdraft in the north of Tibetan Plateau, affects the process of Asian inland aridification, climate changed from cool moist gradually to cold and arid.(3) Early Miocene to middle Miocene (896.15-1090.48m), clay mineral assemblage zone of illite, chlorite, and kaolinite. The content of illite was 66%-77%, averaging 71%, and the content of chlorite was slightly higher than that of the bottom,10%-29%, averaging 19%. Smectite content kept stable and changed in 2%-8%, averaging 4%.It is lower compared with the middle portion. Kaolinite occurred occasionally in some layers and varied in 0-13%, averaging 6%. CIA ratio varied in 39.1-71.7, averaging 61.5; w(Al2O3)/w(Fe2O3) ratio was 2.7-5.8, averaging 3.4; w (SiO2)/w(Al2O3) ratio was 2.9-4.8, averaging 3.5; w(SiO2)/w(Fe2O3) ratio was 8.2-21.1, averaging 11.9; w (MgO)/w (Al2O3) ratio was 0.16-0.31, averaging 0.19; w(TiO2)/w(Al2O3) ratio was 0.03-0.06, averaging 0.05. At this stage (I+Ch)/(K+S) ratio also presents certain volatility change, its value is higher than the bottom but lower than middle section, indicating that the climate condition was between those of the early and the middle onces. CIA value displays obvious fluctuation. Hence, it was clear that the Gerze Basin was dominated by cold and dry climate during this period, but presented more moisture compared to the middle stage.The morphological characteristics of clay minerals in sediments is directly controlled by the weathering intensity at the time of formation, physical weathering is dominant under cold and dry climate, while chemical weathering is dominant under warm and wet climate. Hence, clay minerals often have a smooth surface and clear edge formed in the cold and dry climate; on the contrary, clay minerals often have obvious corrosion appearance formed in the warm and wet climate. It is clear to see that sediments have the same material source during the above three periods, based on the stable ratio of w (TiO2)/w (Al2O3) along the profile. In addition, Gerze sediments have Rb/Sr values of 0.07 to 1.19, averaging 0.51. According to the curve of Rb/Sr against depth, Rb/Sr ratio appeared two minimum values at 978.68 m and 575.36 m. This result is consistent with the CIA index, which suggested that two dry climate events recorded during this period. Gerze sediments have Sr/Ba value of 0.12 to 3.18, averaging 0.86, with a wide range. This indicated that the paleosalinity of the Gerze basin also has experienced a great change. Gross amount of rare earth elements in Gerze sediments is 118-226 u g/g, averaging 182 u g/g. Comparing with the average amount of the Earth’s crust in the continent (146.4 u g/g), the average content of rare earth elements of Gerze sediments is relatively higher, as well as higher than the shale content of rare earth in North America (170.7 u g/g). From the picture of rare earth elements chondrite normalized distribution pattern, it can be seen that there is an obvious Eu negative anomaly. Basin materials mainly come from errigenous weathering, and it indicates a drought climate condition.In conclusion, major elements of the Oligocene-Miocene sediments in Gerze Basin reflect that the surface sedimentary environment is continuously alkaline, low degree of weathering, and short-distance transport. The averaged CIA value is 53.07, which also implies that the source area experienced weak weathering. The value is generally comparable to the chemical weathering degree under cold dry climate conditions of modern sediments (CIA value 50～65). This suggests that Gerze Basin formed under generally cold-drought climate conditions though there were relative warm and humid period. In addition, trace element characteristics show that Gerze Basin recorded two dry climate events, which is consistent with the CIA index. From the diagram of rare earth elements chondrite normalized distribution pattern, it is referred that there is an obvious Eu negative anomaly, and the basin deposits were derived from terrigenous weathering, and thus indicates a drought climate condition. In Oligocene to Early Miocene Gerze Basin had experienced volatile climate change and several climate evolution cycles, climate condition changed from cool moist to cold and arid. In Early Miocene to middle Miocene it was a cold climate period, climate fluctuation was not obvious, and on the whole it was partialy dry and wet climate condition. These regional climate characteristics may be the results of the plateau uplift and global climate cooling. Central Tibetan Plateau uplift blocked the moisture from the Indian Ocean into the plateau, which led to change of vegetation of the hinterland and northern Tibetan Plateau from a broad-leaved forest to coniferous forest, suggested that Gerze Basin during the period has reached a certain height, but also affected by the global climate change. |
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