| Xining Basin in the northeast of Tibetan Plateau(TP),which lies in the transition zone of Tibetan Plateau(TP)and Chinese Loess Plateau(CLP).The unique regional geographical position and continuous Cenozoic sedimentary records(about 55?17Ma)make the Xining Basin favored by scholars,as an ideal site for the study of the interior tectonic evolution of Asia and the evolution of Cenozoic global environmental change.Multi-cycle of mudstone-gypsum deposits was formed during late Eocene here which well-exposed in shuiwan village,shuiwan village is an ideal' site for the study of the climate evolution of late Eocene.There are several different opinions about the depositional environment,especially about the cause of formation of mudstone depositions,some scholars believe they are typical of a distal alluvial fan environment,another part of scholars believe that they are formed in the salt and deep lake,recent years some consider that they are Aeolian deposits,researchers still not have the same conclusion about the forming reason of mudstone stratigraphy.In addition,for the strata lithology classification,it is generally divided into two types of gypsum deposits and mudstone deposition,overlooked widely existing in the strata of gypsum-mudstone transition the lithologic combination,no more giving explanations to the cause of formation for this type of lithology combination,so giving systemic reasonable explanation for the late Eocene multi-cycle of mudstone-gypsum strata has important significance.In this study,we plan to explain the formation of the environment from the lithology combination and the chromaticity parameters.Based on the analysis of the above,combining with field survey and indoor data analysis,we believe that the mudstone formation should be the aeolian origin,instead of diluvial fan in the remote and salty deep lacustrine deposits.From the perspectives of lithology combination,the formation of the strata environment at least can be divided into four categories,gypsum deposits,pewter argillaceous gypsum deposits,red clay gypsum deposits,and red mudstone deposit,if we regard gypsum deposit and red mudstone deposition as relatively moist and relative drought,respectively,so grey gypsum deposit and red mudstone deposit corresponding climate dry and wet conditions is between the two kinds of extreme cases,respectively corresponding to the relative moisture level of high and low,chromaticity parameters for this kind of fluctuations of dry and wet conditions have very good response.In order to reveal the depositional environment of the mudstone-depositions,we collected several samples from Shuiwan section,systematically and detailed grain size measurements and chromaticity parameters measurement were conducted,some other experimental methods like Rare Earth Elements(REE)?diffraction of x-rays(XRD)?Diffuse reflection spectrum(DRS)and rock magnetism were also conducted.Experimental result indicate that:grain structure is similar to typical loess-palaeosol of Chinese Loess Plateau(CLP);distribution patterns of REE is similar to Luochuan loess;Quartz surface morphology also indicate aeolian characteristics.High temperature magnetism?XRD and DRS results indicate that hematite is the only magnetic minerals,it means mudstone formations are more likely in a relatively dry environment,should not be salty water deep lake deposits.so we think that the mudstone should be Aeolian deposits,is not belonging to distal alluvial fan environment or salt and deep lake environment.Lithology combination and chromaticity parameters have a good response to the geological period climate fluctuation,Chromaticity parameters L*is sensitive to the environmental evolution,can be used as a good environmental proxy.The conclusion of this study in conformity with the Eocene climate simulations,means that the Asian inland drying time should be much earlier,provides the reference to the research for Xining Basin and the early Cenozoic Asian inland climate change. |
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