Evolution Of Sedimentary Environment And Paleoclimate In Yardang Landform Area Of Dunhuang,gansu Province During Middle Pleistocene

The Yardang landform is a typical erosion landform,which is distributed in extreme drought and partial drought areas.Based on the fluvial and lacustrine sediments which is not fully consolidated and forming in Neogene or Quaternary,it is a landform give priority to mound-shaped,tower-shaped and columnar shape forming by the tectonic uplift,wind,water,gravity and other effect.In this paper,taking the Yardang landform area of Dunhuang,Gansu province as the study object.Actual measurement,sampling,grain size and susceptibility were used to analyze material composition,sedimentary environment and paleoclimate evolution in Yardang landform.The study could deepen the degree of Yardang landform,and the results could compared with the research achievements of the Lop Nur,which is of important significance.By analyzing the lithology of sedimentary,sedimentary structure and grain size characteristics such as grain size composition,grain size parameters,grain frequency curves,grain cumulative curves,divided the Yardang profile into 6 parts,named them as???.And their sedimentary environment is:?formed in fluvial sedimentary environment,?formed in lacustrine sedimentary environment,?formed in fluvial sedimentary environment,?formed in fluvial sedimentary environment,?formed in fluvial sedimentary environment,and?formed in fluvial sedimentary environment.This study combined grain size characteristics of the sediments and change characteristics of susceptibility,to comprehensively analyzed paleoclimate of Yardang profile.The susceptibility of Yardang profile is between 2.9×10^?5 and57.4×10^?5,which is low as a whole but great fluctuation partial.Grain size curves have a good correspondence to susceptibility curve.?formed in fluvial sedimentary environment which has a characteristic of thin particle and low susceptibility and reflects a cold and dry climate period.?is lacustrine sedimentary environment which also has thin particle and low susceptibility,and the climate became warm and wet.?to?is fluvial sedimentary environment,generally,which has thin particle and low susceptibility but also has two strong fluctuation.The climate was still in a cold and dry period when prevailing winter monsoon,but there were two large-scale and transitory warmer phenomenon.In conclusion,the Yardang profile has experienced the evolution from fluvial sedimentary environment to lacustrine sedimentary environment and to fluvial sedimentary environment again during Middle Pleistocene.The paleoclimatic characteristic sandwiched a warm and wet period between two cold and dry periods.The cold periods have fluvial sediments while the warm period is lacustrine sedimentary environment.