Magnetostratigraphy And Palaeoenvironment Records Of Cenozoic Sedimentary Sequence In Lanzhou Area

The formation and development of the drying in the Asian interior during the Cenozoic was closely related to the global climate change,the uplift of the Tibetan Plateau and the retreat of the Tethys.Previous work has shown that the aeolian red clay appeared around 8 Ma in the main Loess Plateau and 22-25 Ma in the western Loess Plateau,but the aeolian record from the North Pacific suggests that aeolian accumulation exists throughout the Cenozoic with its flux changed in stages.Thus,the basal age of the Chinese aeolian deposits,the evolution history and its driving mechanisms are present key scientific issues.As an important aeolian area in the northern Hemisphere to connect the northwestern deserts and the main Loess Plateau,Lanzhou area is just located in the western margin of the Loess Plateau and northeastern edge of the Tibetan Plateau,which was one of the regions affected directly by the Tibetan uplift as well.Several kilometers of fluvial,lacustrine and aeolian deposits have developed in the area during the Cenozoic,composed of the Xiliugou formation,the Yehucheng formation,the Xianshuihe Formation,the Linxia formation and the Quaternary loess-paleosol sequence from old to young.In this study,after detailed field investigation of the Cenozoic sediments,three sections,the Fenghuangshan section including the Xiliugou formation,the Yehucheng Formation and the Xianshuihe formation,the Gaolanshan section composed of the Linxia formation and the Jiuzhoutai section insisting of the Quaternary loess,have been selected to carry out the research of magnetostratigraphy and sedimentologic indicators as particle size,magnetic susceptibility,chroma and so forth,in order to reconstruct the drying history of the Asian interior and understand the tectonic history of the northeastern Tibet during the Cenozoic.Several conclusions have been obtained in this study as following:1)Supported by the rock magnetic results,after systemic thermal demagnetization and analysis of the characteristic remanence,the detailed measured polarity sequences of the Gaolanshan section and the Fenghuangshan section have been obtained.In consideration of the ages of the fossils and sedimentary rates and comparisons with the Geomagnetic Polarity Time Scale(GPTS,1995),the magnetostratigraphic sequences of the two sections have been established.The results show that the magnetostratigraphic sequence of the Gaolanshan section is well correlated to the C2An.3n-C4r.1r of the GPTS,while that of the Fenghuangshan section corresponds to the C4Ar-C8n.2n of the GPTS.By extrapolating the ages according to the sedimentary rates and correlating the lithology to the similar ones on the northeastern Tibet,the ages of the Gaolanshan section and Fenghuangshan section are finally dated at?8.3-?3.5 Ma and-54.0-?9.1 Ma,respectively.2)The results of the field observations and sedimentological measurements show that the Xiliugou formation(?54-?33 Ma),mainly consisting of red and brick red sandstone,are major fluvial sediments.The lower part of the Yehucheng formation,composed of red sandstone and aubergine mudstone accompanied by gypsum occasionally,are major fluvio-lacustrine sediments but with high aeolian content revealed by grain-size data as well.The most red clay of the upper part of the Yehucheng formation and the whole Xianshuihe formation(26.0-?9.1 Ma),having uniform structure and no horizontal bedding,are typical aeolian sediments,supported by the analysis results of grain-size,element geochemical composition and scanning electron microscope,while the red or greyish-white sandstone in those sedimentary units are mainly fluvial or alluvial sediments.The Linxia formation are alluvial-diluvial mixed layers of mud,sand and gravel in the bottom(?8.3-7.2 Ma),thick layers of aeolian red clay with thin layers of fluvial sediments in the middle(7.2-5.3 Ma)and fluvial Wuquan graval in the top(<3.5 Ma).3)The results of the sedimentary facies analysis and magnetostratigraphy show that the typical aeolian sediments began to deposit at 26.0 Ma in Lanzhou area,which is the oldest aeolian sediments found in Lanzhou as far,implying that the arid environment of the Asian interior had been formed at least since the late Oligocene.Moreover,aeolian sedimentary sequence,established based on the technique of component separation and origin analysis of measured grain-size data,reveal that?33,?22.0,?14.0,?8 and?2.6 Ma were also important phases of drying development and enhancement besides 26.0 Ma.The Cenozoic environmental evolution of Lanzhou and even the Asian interior was probably driven by the global climate changes and the Tibetan uplift.4)The early Cenozoic significant deformation,indicted by the unconformity between the Eocene Xiliugou formation and the Cretaceous Hekou group,marked the formation of the Lanzhou Tertiary basin,which was likely the response to the India-Eurasian collision.The unconformity between the Xianshuihe formation and the Linxia formation(9.1-?8.3 Ma)indicated a strong tectonic uplift in the northeastern margin of the Tibetan Plateau.The appearance of the Wuquan graval around 3.5 Ma indicated another strong surface uplift which forced the Lanzhou Tertiary basin to break up,probably the response to the Qingzang movement A.The Quaternary tectonism of Lanzhou area was active,characterized by the reciprocating processes of regressive erosion of Yellow River and loess accumulation on terraces.