Comparison Of Early Pleistocene Loess-palaeosol Sequence And Fluviolacustrine Sediments In Weihe Basin

Different sedimentary facies are developed in Weihe Graben:the eolian loess-palaeosol and fluviolacustrine sediments,which provide favorable materials for the study of early Pleistocene monsoon evolutions and environmental changes at Guanzhong Basin.Typical Wucheng Loess was deposited on high terrace?T5?whilst Sanmen Paleolake sediments deposited on lower terrace?T4?.The similarities and differences between climatic and environmental changes revealed from different sedimentary facies is still unclear.In this paper,the significance of proxies recorded in synchronous heteropic facies sediments was explored by analysis of magnetic susceptibility and grain size of Yanyu loess-palaeosol and Wujiabu fluviolacustrine depositional sequences,then preliminarily discussed the sensitive differences response to the monsoon and hydrological changes in those two sedimentary facies.The main findings of our study are as follows:Yanyu section is 154 m long and developed from L₃₃/S₃₂2 to L₁/S₀ loess-palaeosol interbeds?33 sedimentary cycles?.Magnetic susceptibility and grain size changes are highly related and can be regarded as perfect summer-winter monsoon proxies.Wujiabu section,the upper part of it deposited a 26 loess-palaeosol cycles(L₂₆/S₂₅-L₁/S₀)and the lower 40 m is early Pleistocene lacustrine deposition,aged from 2.6 to 1.95 Ma,containing 6 sand-silt sands-silt-clay cycles.According to the characteristics of grain size and magnetic susceptibility,the whole lacustrine facies are divided into two stages:deep-lake to shallow-lake cycle?2.6-2.17 Ma?and shallow-lake to lakeshore cycle?2.17-1.95 Ma?.The susceptibility and grain size of the lacustrine sedimentary phase are generally positively correlated.Comparison of the susceptibility of early Pleistocene loess-palaeosol and lacustrine sediments shows,when it is relatively dry,susceptibility of the fluvial-lacustrine sediments can sensitively record the climate signals,indicating that magnetic mineral input changes affected by precipitation intensity changes.While it is warm and humid,the susceptibility of the lacustrine sediments is not sensitive to climate signals.Mean grain size?frequency curves and the C-M plots show that eolian particles have a single-peaked grain size structure,which mainly record the wind transport effect.However,the sedimentary structure of fluviolacustrine is complex,there are two motion pattern for hydrodynamic action-suspension and jumping components.The end-member model analysis?EMMA?shows that grain size of loess-palaeosol is controlled by airflow proximal deposition,while the coarse-partial in fluviolacustrine sediments was a reflection of the enhanced hydrodynamic effect resulting from the deep-to-shallow variations of lake level.Reconstruction of the Quaternary climate changes in Weinan,synchronous heteropic facies depositional sequences record different sedimentary evolutionary information.During the period from 2.6 to 1.95 Ma,loess-palaeosol and fluviolacustrine facies existed simultaneously in Weinan,and fluvial-lacustrine sedimentary sequences are the reflection of the lake level changes,the paleolake have changed from deep-shallow lake stage to shallow-lakeshore lake stage.Relative to loess-palaeosol,fluviolacustrine sediment documented faint 41-ka obliquity period and23-ka precession period,which reflected the precipitation's climate signal to some extent.After¹.95 Ma the lake gradually disappeared and loess-palaeosol accumulated.