Study On The Magnetic Stratigraphic Age And Paleoenvironment Of The Third Terrace Of The Hanjiang River

Loess is a good carrier for recording the reversal of geomagnetic polarity.Applying magnetic stratigraphy to loess research can not only obtain the age of sedimentary strata,but also can deeply study paleoclimate and palaeoenvironmental information combined with magnetism and other magnetic research methods.The intermountain basins and valley terraces in the upper reaches of the Han River in the southern part of the Qinling Mountains are covered with clear eolian accumulations of loess-palaeosolic cycles.These eolian loess is an important information bank for the study of paleoclimate and geomorphological evolution,?but it has always been The paleoenvironment of the district,especially the high-level river terraces,has very few dating data,and only a few ages have large differences in the results.This has caused the formation mechanism of the third terraces to remain unclear,and has caused difficulties in studying the long-term climate and environmental evolution rules.Based on detailed field investigations,two sections of the third terraces of the Yunxian section(HPC)and the Hanzhong section(YSZC)of the Upper Hanjiang River were selected for comprehensive research.The ancient paleomagnetism dating,combined with the OSL age and paleontology,were used to dating the third terraces.At the time of its formation,a complete stratigraphic sequence was established,and the magnetic properties of loess in this area were determined using rock magnetism.In addition,paleo-climate evolution information was obtained by combining common climatic proxies such as magnetic susceptibility,grain size,and geochemical elements.As well as the palaeo-climate characteristics of the area.combining the above several research methods,the main conclusions are as follows:(1)The B/M polarity reversal limits and Jaramillo polar subbands were recorded at both locations.The HPC profile also recorded two short-term polarity shifts for CR1 and BMP.Paleomagnetic dating results show that the bottom of the third terrace of the Hanjiang River is within the negative phase of Songshan,Contrast to the GPTS,and extrapolate at the deposition rate that the age of the third terrace in the upper reaches of the Hanjiang River was 1.05-1.10 Ma B.P.?which belongs to the late early pleistocene.it was the glacial period,corresponding to the loess L10 accumulation period.(2)The main magnetic bearing minerals of the loess in the third terrace of Hanjiang River are magnetite and maghemite,and some of them contain antiferromagnetic minerals such as hematite,which are basically the same as the typical profile of the Loess Plateau,but the overall magnetic properties are weak,There is no obvious difference and regularity between loess and paleosol,which may be due to the high degree of pedogenesis and the source of material in the area.(3)The granularity in L10 at the bottom of the third terrace of Hanjiang River is relatively coarse,but the grain size above S 9 is fine,and the change is not obvious,indicating that the climate in the late Pleistocene was relatively cold,and the climate was relatively stable and warm and humid since the Middle Pleistocene;In addition,The weathering index indicate the weathering and pedogenesis is strong in this area.(4)Combining the age and the climatic characteristics of the terraces at formation stage of this study with predecessors' studies,it was concluded that the formation mechanism of the third terraces in the upper reaches of the Hanjiang River is dominated by the C-Screen of the Qinghai-Tibet movement and the uplift of the Qinling since Q1.The Qinling area was supplemented by the transition from cold to warm during the late stage of the Early Pleistocene,indicating the uplift of the Qinghai-Tibet Plateau and the Qinling Mountains.