Formation Of Yardang Landform In Dunhuang,Gansu Province

As a representative of Yardang Landform in China,the Yardang Landform in Dunhuang,Gansu Province has become a key area for research on this kind of landform.The previous studies are most on the classification of Yardangs,the exogenic geological processes and environment or climatic conditions of it,short of research on the formation and evolution of Yardang Landform in geology.The dissertation analized the depositional environment,paleovegetation and paleoclimate during the sedimentation of Yardang materials,restored the tectonic characteristics since Late Pleistocene,and proposed the evolution of Yardang Landform through the study on sedimentary characteristics,pollen assemblage,geochemical features and regional neotectonic movements etc..Two stratigraphic sections were selected to measure,sample and test,with the following results got: the Yardang strata are composed of sandy and clayey sediments,and the sedimentary facies are recognized as riverway subfacies,floodplain subfacies,lakeshore subfacies and shallow lacustrine subfacies;pollen assemblage shows the paleovegetations varied as desert grassland-grassland-desert grassland-desert grassland-grassland-grassland-desert grassland;and the corresponding paleoclimate evolved as warm and drought-relatively moist-cold and drought-relatively warm and moist-cold and moist-relatively warm and moist;the formation time of Yardang strata ranges from 306.0±30.0ka BP to 131.4±13.0 ka BP,during the middle to late periods of Middle Pleistocene.Analysis according to the tests of macroelements,trace elements and rare earth elements shows that the sedimentary environment was dominated by mild oxidizing environments and oxygen-free to oxidizing environment;the paleoclimate reconstructed according to the geochemistry showed warm and drought-relatively warm and moist-warm and drought-cold and drought,coinciding with that concluded from the pollen assemblage.The dissertation emphasized on the role of endogenic geological processes in the formation and evolution of Yardang Landform.The Yardang materials were uplifted to be exposed to the earth surface,and eroded by fluvial and wind erosion,leading to the formation of Yardangs with various morphologies.According to the results of ESR,the Yardang Landform experienced 3 periods of evolution,with the starting time since109.0±10.9?98.5±10.0ka BP,72.9±6.0?66.84±7.35 ka BP,and 53.2±5.4?38.0±3.0ka BP respectively,forming the current distribution of Yardang Landform.The dissertation preliminarily divides the Late Pleistocene tectonic movement into three periods,which started since 100,000,70,000 and 40,000 years before present respectively.