The Sedimentary Studies Of Cenozoic Paleoclimatic Events In The Northeastern Qaidam Basin

In order to understand the Cenozoic paleoclimate evolution of Qaidam Basin, we focus on the Cenozoic sedimentology of Dahonggou Section in Qaidam Basin, combining the grain-size analysis, susceptibility analysis and electriciting scaning analysis in this paper. Meanwhile, gypsum halite and pollen are also taken into consideration. Compared with the same period sedimentary records from other sections and the global oxygen trends of isotopic variation, the following implications have been achieved.(1)The Cenozoic exposed strata with a total thickness of about 5271m of Dahonggou section in Qaidam basin can be divided into six formations:Lulehe formation, Lower Ganchaigou formation, Upper Ganchaigou formation, Lower Youshashan formation, Upper Youshashan formation and Shizigou formation, ranging old from new. It recorded the sedimentary environment evolution from lake delta facies, lake facies, fan delta facies to braided river facies. It reflects the lake basin became rapidly expanding large lakes sedimentary deposition from alluvial fan delta in the early stages. At last, the lake basin turned into river deposition after that the lakes gradually narrowing dried up.A detailed record of complete flooding and lake retreat process since early Eocene of the northern Qaidam Basin reflects sedimentary palaeogeographic evolution of the basin.(2) Based on the results of the environmental sensitive groups extracted from the grain-size, susceptibility trend, gypsum halite records, pollen data, global oxygen isotope records and the sedimentary records from surrounding basin, the pleaoclimatic and environmental evolution of Dahonggou section in Qaidam Basin during 43-2.5Ma can be divided into 6 stages:Stage 1 (0-563m,2.5-5.4Ma)is cold and dry climate, with a higher degree than stage 2; stage 2 (564～2040m,5.4～13.5Ma)is cold and dry climate; stage 3 (2041m-2435,13.5-16Ma)showed warm and humid climate, with a higher degree than stage 4; 4 (2436～402lm,16-27Ma)for the warm and humid climate environment; stage 5 (4022～4747m,27～37Ma) is the cold and humid climate; stage 6 (4948～5271m,3～-43Ma) is a cold and dry climate environment.(3)During 27-16Ma, gypsum halite suddenly disappears, grain-size decreases and magnetic susceptibility increases and both of them changes smoothly, all of them reflects the phase of Qaidam Basin in warm humid climate environment. We noticed 27 Ma was in the late Oligocene warm period, after this period, although it has experienced the early Miocene cooling events in short time, it shows a relatively stable and warm period in the global oxygen isotope curve during 16-27 Ma.What’s more, the changes of grain-size, susceotibility, gypsum halite of Dahonggou section in Qaidam Basin are earlier than Tibet plateau uplift events around 25 Ma.(4)Grain-size reaches the minimum value in 14Ma while the maximum value of magnetic susceptibility in 13.5Ma, grain-size showed an increasing trend during 14-13.5Ma. We believe that the issue of monsoon enhance event happened 13.5 Ma, the wind carries outside material into the basin, aeolian sand in the soil process with the formation of small particles of magnetite make magnetic susceptibility appear a peak in Dahonggou section. We find that there are some grain size frequency distribution curves in Dahongogu section are similar with the Aeolian sand and dust in Tengger Desert.Aeolian components outbreak in 2000-2500 meters, that is, about 13.5 Ma. SEM results showed that the particles appear round or nearly round, the surface of dish shaped crater. Eolian sediments coarse particle content is higher, its the numerical distribution between 100～200 microns, is generally based on fine sand containing a small amount of powder sand and medium sand. The outbreak of Aeolian sand make grain-size increase around 13.5 Ma, the experimental results support previous speculation.(5) After Cenozoic climate transition, in the true sense of drought time of Qaidam Basin begin to 13.5 Ma. We compare the grain-size and magnetic susceptibility data of Dahonggou section with other area data, it supports global cooling driver model.this modle means that global cooling enhances the temperature and pressure between low latitude and high altitude, inhibits monsoon which carries water vapour flowing northward. The development of polar ice makes winter monsoon strong, weaken the strength of the summer monsoon, then the drought degree increased.(6) Quality susceptibility curve suddenly appeared a peak value in 5.4Ma, however, corresponding grain-size curve, global oxygen isotope curve and gypsum halite curve without mutations. We consider that global cooling has no direct relationship with sudden increase of magnetic susceptibility. It is speculated that this may represent a monsoon enhancement event. SEM results showed that the particles in 5.4 Ma appear round or nearly round, the surface of dish shaped crater. The experimental results support our inference, the changes of magnetic properties of rocks may be associated with aeolian material outside basins.