The Age And Significance Of The Wuquan Conglomerate And Geomorphic Evolution Of The Lanzhou Basin Since The Late Pliocene

The formation and evolution of the geomorphic system are closely related to the tectonic activity and climate change.Geomorphic surface and sedimentary records developed on the geomorphic system are important geological archives for deciphering the interaction and driving mechanism of stratified landforms,climate change,and tectonic activities.The collision,extrusion and deformation of the India and Eurasian plate have caused the uplift of the Tibetan Plateau,which to be one of the most important geological tectonic events since the Cenozoic.The formation of the Tibetan Plateau has greatly influenced the tectonic and distribution pattern of the geomorphology in the Asian continent.The evolution of the terrain caused by the uplift of the Tibetan Plateau is closely related to the formation and evolution of the Asian river systems and loess landform.The Yellow River is the cradle of the Chinese nation,and its formation and evolution is the result of the uplift of the Tibetan Plateau.The study of the formation and development of the Yellow River plays an important role in revealing the history of plateau uplift and geomorphic evolution that has always been a scientific issue concerned by domestic and international scholars.The Lanzhou basin locates in the tectonic and seismic activity zone of the northeastern margin of the Tibetan Plateau.It is also located in the northwest inland arid area,the eastern Asian monsoon region and the unique high cold region of the Tibetan Plateau,the intersection hinge of three nature geography and climate zone,which is sensitive to tectonic activity and climate change.The thick Cenozoic sediments are preserved in the Lanzhou basin.The regional tectonic geomorphic types and paleoclimate information since the late Cenozoic have been archived in detail in the Lanzhou Basin.Consequently,the thick Cenozoic strata are the best complete and ideal geological materials to study the interaction and coupling process mechanism of the tectonic,geomorphic and climate in this area.A great deal of research has been done on the Cenozoic strata of the Lanzhou Basin,including biostratigraphy,chronology,paleoenvironment and geomorphic evolution and many important achievements have been acquired.However,the age and genetic significance of the famous Wuquan conglomerate and the age of the Gaolanshan Red Clay-Loess sequences have not been studied in detail.In this paper,we use the high resolution magnetostratigraphy(thermal magnetic curve,hysteresis loop and paleomagnetism),loess stratigraphy(calcium carbonate and grain size),absolute dating(cosmogenic nuclide burial age),regional stratigraphy correlation and combined with the research of the Xijin loess-paleosol borehole and Wuquanshan loess section to determine the age of the Fanjiaping and Wuquanshan conglomerate section and Gaolanshan Red Clay-Loess borehole in the Lanzhou Basin.Furthermore,we use the sedimentary facies,lithology,provenance(U-Pb chronology of detrital zircon)and paleocurrent analysis to ascertain the genetic significance of the Wuquan conglomerate.Moreover,we combine previous research results to reconstructing the geomorphic development stage of the Lanzhou basin since the late Pliocene,which provides the basis for the study of the uplift of the northeastern Tibetan Plateau,the formation of the Yellow River and its coupling process with the loess landform.The preliminary conclusions are listed as below:1.The results of the high resolution magnetostratigraphy(thermal and alternating demagnetization),loess stratigraphy(calcium carbonate and grain size)and regional stratigraphy contrast indicate that the basal age of the 265.7 m-long Gaolanshan Red Clay-Loess sequences in the Lanzhou basin is about 3 Ma.The bottom age of the Gaolanshan loess is 2.8 Ma.Therefore,the Gaolanshan loess is the oldest eolian loess in the Lanzhou Basin.2.Through the study of the magnetostratigraphy,cosmogenic nuclide burial dating,fossil record and regional stratigraphy contrast show that the sedimentary age of the Wuquan conglomerate in the Lanzhou basin is approximately 3.6-2.2 Ma.As well,the boundary age of the upper and lower Wuquan conglomerate is about 3 Ma.3.The detailed analysis and comparison of the sedimentary facies,gravel composition,paleocurrent and U-Pb chronology of detrital zircon manifest that the source area,paleocurrent and sedimentary environment of the river sequence represented by the Wuquan conglomerate is the same as those of the Yellow River.Therefore,we speculate that the establishment time of the proto-Yellow River system was approximately 3.6 Ma in the Lanzhou Basin.Moreover,we combined with previous research results and conclude that the development and evolution of the river system in the Lanzhou Basin may be influenced by the uplift of the Tibetan Plateau and the Asian monsoon.4.According to the previous research results,we conclude that the Lanzhou region has experienced several tectonic movement since the late Cenozoic,which were 3.6,3,2.2,1.8-1.7,1.2-1.1 and 0.9-0.8 Ma,respectively.The formation of the proto-Yellow River system in the northeastern Tibetan Plateau was triggered by the huge terrain difference and the strengthening of the Asian summer monsoon in 3.6 Ma,which indicate the Tibetan Plateau had strongly uplift at the beginning of the Qingzang Movement.In addition,the boundary age between the upper and lower Wuquan conglomerate was about 3 Ma.The unconformity between the Red Clay and bedrock in the Gaolanshan region was dated to 3 Ma.It was proved that a tectonic movement was occurred around 3 Ma,which is an important geological curtain of the Qingzang Movement.Moreover,the tectonic uplift resulted in the termination of the Gansu period pediment surface and the formation of the modern Yellow River after2.2-1.8 Ma,which is equivalent to the third phase of the Qingzang Movement.It implys that 2.2-1.8 Ma was the period of the strong tectonic activity.After that,the 9grade typical Yellow River terraces and loess landform system in the Lanzhou Basin were gradually formed,which controlled by the Kunhuang Movement,Gonghe Movement and Quaternary glacial-interglacial climate.