Development Characteristics And Paleoenvironmental Significance Of Paleoweathering Crust In Southern Liaodong Peninsula

The discovery of Wolong weathering crust in southern Liaodong Peninsula provides important geological information for the study of Cenozoic geomorphic evolution in Liaodong Peninsula and even the whole northern China.As an important part of the palaeo-planation surface,the palaeoweathering crust can not only serve as an important basis to identify and reconstruct the palaeo-planation surface,but also an important carrier to reconstruct the environmental information of the palaeo-planation surface.The formation of the paleoweathering crust is closely related to the climatic conditions at that time,so the study of it is helpful to understand the formation and evolution history of the paleoweathering crust in Liaodong Peninsula,and to restore and reconstruct the palaeoclimate environment at the time of formation.Therefore this study selected the Wolong paleoweathering crust section,through the test of chemical elements,particle size and magnetic susceptibility index and combined with the feature of sporopollen and quartz grain surface such as a comprehensive analysis,to the Wolong systemic research on the paleoweathering crust,study the developmental features of paleoweathering crust in southern Liaodong peninsula and the paleoenvironment significance,achieve the following:(1)The chemical composition of Wolong weathering crust profile is dominated by SiO₂,Al₂O₃and Fe₂O₃,and the total content of the three oxides is 86.09%.The significant depletion of K,Na and Ca elements occurs in the profile,and almost all Na elements are leached out.The average value of chemical alteration index(CIA)is 88.25,indicating that the weathering crust was formed in a relatively hot and humid climate environment,which belongs to the high weathering level;Silicon aluminum coefficient(S/A),ferrosilicon coefficient(S/F),silicon aluminum iron coefficient(S/R)and saturation(A/NK,A/CNK),chemical alteration index(CIA),eluvial coefficient(FA/CNM)and weathered eluvial coefficient(Ba)geochemical indicators as the change of depth in the section have good corresponding relationships,and has A certain regularity,indicate that the weathering crust is the product of the rocks on the in situ weathering residual;The A-CN-K triangle diagram shows that the montmorillonite is gradually exhausted and has entered the middle de-K stage characterized by the decomposition of potassium feldspar and illite,which belongs to the strong weathering stage.(2)Silt is the main component in Wolong weathering crust profile,followed by sand and clay;The grain size component triangulation shows that the samples are concentrated at the silt-sand line end;The average particle size is 120.11?m,and the average kurtosis is 1.08,which belongs to the medium kurtosis.The average skewness is 0.26,showing positive skewness.The mean standard deviation and sorting coefficient are higher,indicating that the sorting of this weathering crust is poor.(3)The?_lf of Wolong weathering crust section is only 13.5×10^-8m³/kg,which is 100times higher than other sections.It is speculated that the low mass susceptibility of Wolong weathering crust may be caused by excessive water and high temperature oxidation into low magnetic hematite,or reoxidation into lower magnetic goethite,or fibronite and bog iron.?_fd(%)trends from the top section to the bottom showed a trend of decrease the fluctuation in the whole,the same with the CIA and the change trend of powder stick than,shows that the quality of the weathering crust of magnetic susceptibility to a certain extent,can reflect the development degree of weathering crust and the formation conditions of climate change,but can objectively reflect the formation of the ancient climate debate.(4)Geochemical,particle size and magnetic susceptibility index as the change of depth has a good corresponding trends,show that Wolong paleoweathering crust of weathering degree has experienced strong-weathering abate-strong-weathering weaker,the change process of sporopollen analysis results that formed roughly experienced the warm climate change,humidity,recovery,the change trend of relative dry;By analyzing the palynological assemblage characteristics of the profile,it is speculated that the weathering crust profile was formed in Oligocene.(5)From the top of the weathering crust to the weathering front,the weathering degree of the weathering crust gradually weakens,and the developmental maturity gradually deteriorates,which is in line with the weathering crust development front theory;In the analysis of the surface characteristics of quartz particles,it is found that the ancient weathering crust of Wolong has a poor roundness degree and a short transport distance.The microscopic morphology shows that the weathering crust has experienced strong chemical weathering.The above analysis shows that the paleoweathering crust of Wolong is formed by in-situ weathering of bedrock.(6)The comparison between the weathering crust in Wolong and other weathering crust in the south of Liaodong Peninsula shows that the weathering degree in the study area gradually increases from east to west,and there are two possible reasons:(1)The loess overlying the weathering crust becomes thinner and thinner in the east,until the Wolong paleoweathering crust profile is exposed to the surface directly,which leads to the infiltration of surface water;(2)The eastern and southern parts of the Liaodong Peninsula are the Yellow Sea,the largest marginal sea in the western Pacific Ocean,which is closely related to the water vapor of the Pacific Ocean.The eastern and southern parts of the Liaodong Peninsula are the Bohai Sea,an inland sea,and the atmospheric dynamic conditions on the east and west sides are obviously different.In the process of weathering crust formation,the influence of groundwater level on the grain size composition of the profile is very complicated.In addition,the change of the surrounding micro-geographical environment makes the grain size and magnetic susceptibility of weathering crust more complicated.The comparison between the paleoweathering crust in Liaodong Peninsula and the red weathering crust in the south shows that the weathering crust in the north is not as mature as the weathering crust in the south even under better hydrothermal conditions.