The Sedmentary And Environmental Evolution On Yellow River Source Area During Late Cenozoic Era

The Qinghai-Tibet Plateau, the world's highest unique geographical unit formed during India-Eurasia collision since～65 Ma, is a hot spot of global change study--an international forefront of current research topics. It is considered to be one of the still active regions in the continental collision zone, and then an ideal field lab to study the formation and evolution of the lithosphere, and the mechanism of crust movements. The uplift of the plateau since the late Cenozoic has influenced the evolution and differentiation of natural environment for its own and adjacent areas profoundly, and has been also a sensitive response of and strong influence on global change. Lacustrine sediments from several phase ancient Great Lake in the Qinghai-Tibet Plateau since the Late Cenozoic, containing abundant information on palaeoclimate change, are good carriers for the study of plateau environmental change.The Yellow River Basin located at the northeast of the plateau, is the Yellow River source region sensitive to the plateau uplift and environmental change. The lacustrine strata and environmental change since the Late Cenozoic in this region has not been investigated in detail due to the lack of strengthened actual data on plateau uplift and lake evolution. In this dissertation, the lacustrine sedimentary strata of Yellow River Basin are selected to be the main target of palaeoclimate and environmental study.The sedimentary sequence of lacustrine strata, the palaeoenvironment and palaeoclimate of Yellow River Basin, and the relationship among lacustrine evolution, plateau uplift and environmental change since the Late Cenozoic, are summed up and restored in this dissertation, through the combined study of sedimentology, isotopic geochronology, environmental geochemistry, pollen analyses and micro-palaeontology, tectonic geomorphology, and structural geology of the basin system. The main progresses and achievements made by the research here are as follows:1. The Late Cenozoic lacustrine strata in the Yellow River source region, surrounded by Bayankala Mt. in the south, Buqing Mt. in the north, Yaladaze Hill in the west, and the Tehetu area of Dari County in the east, composed by the Holocene Heihexiang Group (Qh1h), the Upper Pleistocene Dayemaling Group (Qp31d), the Middle Pleistocene Eling Lake Group (Qp21e) and the Lower Pleistocene Yeniugou Group (QpI1y),is suggested to be formed since 2,239 kaB.P..2. The Yellow River Basin is a sag basin in the Late Cenozoic, controlled by the two compressional margin faults, i.e., the Kunnan fault in the north and the Bayankala mountain-front fault in the south, and several intrabasinal faults such as the Buqingshan mountain-front fault, the Maduo fault, the Bayan River fault, and the Maduo-Yeniugou fault. The formation and evolution of the basin can be divided into two phases:the initial phase in the Oligocene between 35.4 Ma to 23.3 Ma, and the finally formation phase in the Miocene.3. Two ductile shear zones, which are controlling and corresponding to the distribution of two lake zones, are detected in the Yellow River source region through magnetic fabric analyses of the basement rocks. 4. The structural landform analysis shows that the basin area is regionally high with elevation of 4500～5000 m in the gentle dipping water shedding area, and higher in the catchment area of>4500 m. The drainage system of the basin is charactered by the significant asymmetry in growth, with the Yellow River as its axis and clearly differentiated northern and southern valley landforms. Three-dimension DEM analysis suggests that the present elliptical basin with a long axis in E-W is a structural basin resulting from combined tectonic and surface processes.5. The Yellow River Basin has suffered from four stage changes in palaeovegetation and palaeoenvironment since the early Pleistocene (2,239 kaB.P.). The first stage (2,239～901 kaB.P.) was dominated by the forest-steppe environment with a warm and humid climate. The second stage (901-350 kaB.P.) was the shrub steppe environment with a cold and dry climate. The third stage (350 kaB.P.～28 kaB.P.) was the meadow steppe environment with gradually cooling climate. The fourth Stage (28kaB.P.-8 kaB.P.) was the desert steppe with dry and cold weather in the early time and later drought.6. The development of the ancient great lakes in the Yellow River source region had went through three expanding and three shrinking processes since the Early Pleistocene (2,239 kaB.P.). The first expansion and shrinking period as a limited lake was in the early Pleistocene during 2,239 kaB.P.～764 kaB.P., with sediments mainly of coarse-grained sands and gravel layers. The second period was in the Middle Pleistocene during 211 kaB.P.～70.7 kaB.P.when the lake had reached the maximum extent in the Quaternary and then shrank. The third period was during approximately 205kaB.P.～81 kaB.P, with the quickly expanding first, and then the beginning of shrinking of the lake. The lake was shrinking in the Holocene with several fluctuations.Finally, the relationship among the formation and evolution of the lakes, the climate change and the plateau uplift is discussed, and the overall evolutional law of the intra-plateau environment is considered to be strongly controlled by the uplifting of the plateau with the superposing of global climate change.