Rock Magnetism And Paleoenvironment Of The Neogene Red Clay Deposits In The Liulin Area

The red clay,as the most typical sediments of the Neogene,records the change information of paleoclimate,paleoenvironment and the evolution process of development at The Loess Plateau of China.The eastern Loess Plateau is located at the west of Lv-liang Mountain.The uplift of the Lv-liang Mountain has formed the gravel bed in the Piedmont.The gravel layer is characterized by thick red clay.Direct accumulation of red clay is overlain by Mesozoic sand-stones.Red clay deposits often mixed with sand and gravel,hydrogenous material and other typical fossil.These compositions are good indicators of the evolution of the Lv-liang Mountain uplift and the dust accumulation in the Loess Plateau since the Neogene.The Liu-lin District is located at the eastern the Loess Plateau and the western Lv-liang Mountain,which formed the thick red clay sedimentary sequence.Being based on the literrature review of red clay,we chose Weijiawa red clay section to make detailed petrographic researches at Liu-lin County.By means of Paleomagnetic,Environmental Magnetism and AMS,taking a further study on the evolution of the red clay dust accumulation in the west of the Lv-liang Mountain and the east of the Loess Plateau.The Weijiawa red clay section is located at west of the Lv-liang Mountain and the east of the Yellow River.Modern landform belongs to loess hilly and Gully Region.The region has experienced strong denudation,and the terrain is characterized by the loess,red clay gully and valley on the basement.The red clay exposed to the surface.The depth of the red clay is about 112m,which is covered with loess-paleosol sequence.The Neogene strata can be divided into three segments according to the lithologic characteristics of the field.The upside(0m-30m)consists of red brown clay and calcareous nodules,which shows stronger weathering than Quaternary loess-paleosol.The bottom(81m-112m)is the red clay or sandy clay mixed with gray white sand,which is composed of two alluvial fan sedimentary sequences from top to bottom.The middle(30m-81m)has the same character with the upside and the bottom,some of the lithology is complex.Firstly,the K-T curves and hysteresis loops show that the magnetic minerals of the red clay are the same as that of the loess,which guarantees the feasibility anisotropy of magnetic susceptibility and the accuracy of the results of the paleomagnetic study.Then,by means of paleomagnetic experiments,it is concluded that the polarity of the geomagnetic polarity pole recorded by the Liu-Lin red clay profile corresponds to the geomagnetic polarity C3n.1n-C4n.2r.The starting time of red clay can be traced back to around 8.1Ma.The sediment transport distance,transport medium and dynamic energy stability are analyzed by age division and sedimentation rate calculation of the layer.Finally,by using the anisotropy of magnetic susceptibility,it is concluded that there are some differences in the dominant orientation of the red clay particles and the dynamic direction of the sediment dynamic direction in different depths.According to the formation of magnetic polarity,the results show the sedimentary environment is divided into 3 stages influenced by the uplift of Lv-liang mountains.The 8.1Ma-7.3Ma red clay is under water deposition transported from east to west.The 7.3Ma-5.5Ma is mainly formed by eolian dust deposition,some parts of the stage are transported to west by the Lv-liang mountains eastern flow.Since 5.5Ma,the anisotropy of magnetic susceptibility has begun to show the characteristics of aeolian deposition.