The Magnetostratigraphy And Palaeoenvironme Ntal Change Of The Honghe Formation(Eocene) In Lantian,Weihe Basin,Central China

The semi-continuous fluvial-lacustrine deposit in Weinan-Lantian area,Weihe Basin,central China,which preserves richpaleoclimate and paleoenvironment information,containing a large number of mammal fossils,provides an excellent record for understanding Asian monsoon evolution and the environmental changes in the Cenozoic era in East Asia.Apreliminary magnetostratigraphy,sedimento logy and environmental proxies analysis ofthe typical Honghe Formation at Zhijiagou site(330 m thick),which is located between the Lishan mountain and Hengling tableland,Lantian County,Shaanxi Province,is carried out.We discuss the sedimentary environment ofthe Honghe Formation and the Eocene climate changes in central China,combining with the mammal fossils and sedimentary characteristics.The conclusions are as followings.1.The Honghe Formation was formed from 45.6 to 33 Ma including the Xiahancun section from 36.0 to 33.1 Ma and the Wangpingcun section from 45.6 to 36.0 Ma,corresponding to the middle and late Eocene,and/or early Oligocene,under a relatively continuous fluvial-lacustrine depositional environment with sediment source from the Qinling orogeny and the Lishan Mountains.2.The anisotropy of magnetic susceptibility ellipsoid(AMS)analysis indicates that the paleocurrent direction during the Honghe Formation is from south to north,lasting the entire depositional time from the top to bottom,showing a stable rivier system and depositional environment.3.Thermo demagnetion curves show the hematite and magnetite are the domiant magnetic minerals,and the isothermal remanent magnetism(IRM)results also indicate the dominance of incomplete antiferromagnetic minerals with high coercive force.Thus,the dominant magnetic carrier of the Honghe Formation is hematite,perhaps contains a small part of magnetite.4.We conclude that the climate was more humid and warmer in the middle and late Eocene in Weihe Basin,central China,when there were abundant plants,lakes,and swamps in Lantian area,but it may turn to relatively cold and dry in early Oligocene,which is consistent with the climate transition of Eocene-Oligocene around the world.Our preliminary results from the Honghe Formation suggest that the Asian monsoon-like atmospheric circulation may exist by the middle Eocene,when central China was relatively humid.However,our finding needs further tested.The results provide new records for understanding paleoclimate change in central China,the global warning and the driving mechanism of atmospheric CO2 in Eocene.