| The northeastern margin of Tibetan Plateau is one of the best area to reveal the uplift and environmental change, which lies a series of Cenozoic basins from different ages. These basins formed as the uplift of the plateau and range of the strong integrity of the natural section exposed, which stratigraphic records are very clearly for the full reveal Cenozoic uplift of the northeastern Tibetan Plateau and environmental changes by the material provides a good foundation. Xunhua Basin is one of them.This paper selects the the southern margin of Xunhua Basin Yangjuangongbai-Xigoushangzhuang continuous stratigraphic section (section No. S20), the times throughout the Late Oligocene to Miocene, can be discussed in the context of this scale evolution of ancient vegetation and climate and environment fluctuations.Detailed profile measurement system, the analysis of sedimentary strata in the polynological pollen species of plants, systematic classification, the establishment of pollen assemblages, and polynological zones through Research on the measured section section in northern margin of the Xunhua Basin from Qinghai-Tibet Plateau. The dominant species through pollen fossils to inverse the ancient vegetation types, and then speculated that the face of the ancient vegetation, reconstruction of the environmental characteristics of ancient geological times.By analyzing the constant element geochemistry, trace element variations, combined with sedimentary environments sedimentary facies analysis, and deep-sea oxygen isotope curve, the global comparison of major climatic events such as the purpose of reconstruction of the northern Tibetan Plateau during the Neogene climate evolution.General of 1 the Xunhua 1 Basin late Oligocene- Miocene sedimentary areas typically Palynological Yangjuangongbai-Xigoushangzhuang system analysis section, laboratory, and pollen fossils are classified by species, combined with geochemical results of elemental analysis and clay minerals and Magnetostratigraphic study, restoration of the ancient geological history of vegetation types, palaeoclimate reconstruction of the evolution of the study area, the following conclusions:1. The measured section lying in the southern margin of Xunhua Basin, and located at Yangjuangongbai-Xigoushangzhuang village, Jishi Town. Xunhua County,Qinghai Province, consists of Miocene to Pliocene strata, named as Tala Fm., Xianshuihe Fm.(consists of Zhongzhuang Mem., Shangzhuang Mem. and Dongxiang Mem.) and Linxia Fm.(consists of Liushu Mem. and Hewangjia Mem.). Based on the detailed research of sedimentary facies, the depositional system of the section consists of alluvial fan facies (Tala Fm), salt water lacustrine facies and delta front facies (Xianshuihe Fm.),lacustrine facies and delta front facies with underwater fan facies (Linxia Fm.) from bottom to top.2. Paleocurrent measurements obtained from the imbricated structures and cross-beddings show that the of paleocurrent direction of section were SE-NW, except the Linxia Fm.,.Based on the methods of sedimentary origin analysis, it indicated that the northern margin of the western Qinling thrust belt and the south margin of the Lajishan thrust belt had uplifted as denudation areas and provided deposits to the Xunhua Basin. On the contrary, during the depositional stage of the Linxia Fm., the paleocurrent was NWW, indicating that the Jishi Mountain of the eastern edge of Xunhua Basin had uplifted as a main denudation area during the late Miocene-Pliocene. The results of the grain size probability accumulative curves and C-M patterns accord with the division of sedimentary facies mentioned above, and indtcate that some tractive current developed in delta front facies and underwater fan facies.3. consists of late Miocene to Pliocene strata, there were 4 Palynological Zones and 7 Palynological Assemblages:Palynological ZoneⅠ:Pinaceae- Chenopodiaceae-Betulaepollenites- Ephedripites Zone (consists of Palynological Assemblage 1:Compositae-Artemisiaepollenites- Chenopodiaceae- Betulaepollenites- Ephedripites Assemblage and Palynological Assemblage 2: Pinaceae- Chenopodiaceae- Nitrariadites- Betulaepollenites-Ephedripites Assemblage); Palynological ZoneⅡ:Faguspollenites- Nitrariadites-Chenopodiaceae- Qinghaipollis Zone (consists of Palynological Assemblage 3:Faguspollenites-Chenopodiaceae- Ephedripites- Gramineae- Betulaepollenites Assemblage and Palynological Assemblage 4:Faguspollenites- Nitrariadites- Chenopodiaceae- Qinghaipollis- Betulaepollenites Assemblage); Palynological ZoneⅢ:Gramineae- Juglanspollenites- Chenopodiaceae-Artemisiaepollenites Zone (consists of Palynological Assemblage 5:Pinaceae- Gramineae-Juglanspollenites- Chenopodiaceae- Artemisiaepollenites Assemblage and Palynological Assemblage 6:Juglanspollenites- Betulaepollenites- Compositae- Chenopodiaceae- Gramineae Assemblage); Palynological Zone IV:Pinaceae- Nitrariadites- Chenopodiaceae- Ephedripites Zone (Palynological Assemblage 7:Pinaceae- Chenopodiaceae- Ephedripites- Nitrariadites-Betulaepollenites Assemblage)4. Evolution trend can be found by Xunhua section pollen studies:compared with mid-Oligocene, the slight recovery in the process of temperature and humidity; the late Oligo-cene, content of evergreen broad-leaved trees down again, slightly higher content of evergreen and deciduous plants Indicating the temperature and humidity was again a slight decrease.Early Miocene-Interim study 1 area, similar 1 to other 1 parts 1 of China, thermophilic aquatic plants take deciduous broad-leaved species are well developed, mass propagation of coniferous species, advantage 1 of the waterfront,1 the representative 1 of the 1 typical flora of temperate coniferous, deciduous Leaves mixed vegetation types. Warm 1 and 1 humid 1 climate, 1 there are many dry fluctuations. Late Miocene period,1 the 1 vegetation 1 in temperate coniferous forest gradual transition with the coexistence of desert grassland vegetation in arid landscapes, the temperature again dropped significantly.5. Xunhua Basin section of pollen records reveal in the late Oligocene-Miocene in general represent a northeastern warm, temperate deciduous broad-leaved coniferous forest -temperate mixed forest-coniferous forest and arid mountain desert steppe vegetation landscape evolution. During cold and warm dry climate there are multiple changes, the overall trend reflects the warm and humid to cold dry climate trends.6. The palaeoclimate of the Xunhua section in 23.1-5.0Ma can be divided into four stages, StageⅠ:23.1-16.6Ma, it characterized by cool and half-dry paleoclimate; substageⅠ2 (21.8～17Ma) became cold and dry. StageⅡ:16.6-12.5Ma, the characterization was warm and wet. StageⅢ:12.5-7.3Ma, it was cooler than stageⅡbut also wet. StageⅣ:7.3-5.0Ma, it changed cold and dry again with two substages. |
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