The Characteristics Of Paleoweathering Crust Of Siming Mountain In Eastern China And Its Paleoenvironmental Significance

The discovery of the ancient planation surface at the summit of Siming Mountain in eastern Zhejiang provides important geological information for studying the evolution of Cenozoic geomorphological in eastern Zhejiang and even eastern China.The paleoweathering crust as a part of the ancient planation surface is not only an important basis for identifying and reconstructing the ancient planation surface,but also an important carrier carrying the environmental information of the ancient planation surface.The formation of the paleoweathering crust is strongly controlled by the climate and environmental changes at that time,and the study of its physical,chemical and mineral characteristics may help to understand the history of the formation and evolution of the ancient plane in Siming Mountain,Zhejiang,and the restoration and reconstruction Characteristics of the paleoclimate and environment.Therefore,this article selected the paleoweathering crust profiles of Zhucao,Nipingao and Xinlan in Siming Mountain in eastern Zhejiang Province.The characteristics of the paleoweathering crusts developed on the ancient planation surface were systematically studied,and the development characteristics of the paleoweathering crusts in the local area and their paleoclimatic and environmental significance were discussed.?1?In the typical paleoweathering crust of Siming Mountain,the Zhucao profile and the Nipingao profile are mainly composed of silt,followed by clay particles,with the least content of sand components;the Xinlan profile is mainly composed of clay particles,followed by silt,The content of sand component is the least.The triangle diagram of particle size composition shows that the three samples of ancient weathered crust profiles are concentrated at the clay-silt connection line.The particle size parameters generally have the characteristics of small average particle size,poor sorting,and moderate to wide peak state.The particle size components and parameter characteristics generally indicate that the maturity of paleoweathering crust development is relatively high,and the degree of weathering development is relatively good.The chemical weathering intensity experienced by the profile is consistent.?2?The chemical composition of the three paleoweathering crust profiles is mainly SiO₂,Al₂O₃ and Fe₂O₃.In the three profiles,Ca,Mg and Na elements show strong leaching,and almost all of the Na element is leached.Silicon-aluminum coefficient?S/A?,ferrosilicon coefficient?S/F?,ferrosilicon coefficient?S/R?,aluminum saturation?A/NK,A/CNK?,chemical alteration index?CIA?,and weathering The vertical variation characteristics of weathering earth indicators such as leaching coefficient?ba?in the profile indicate that the weathering of the paleoweathering crust is under strong chemical weathering intensity under hot and humid conditions.A series of geochemical index parameters of the three paleoweathering crust profiles have a good correspondence with the change of depth,showing a certain degree of regularity.These characteristics also indicate that the weathered crust is in situ weathered residue on the basis of the parent rock product.?3?The clay minerals in the three paleoweathering crust profiles mainly include illite,kaolinite and 1.4nm clay minerals,with a small amount of montmorillonite.The clay mineral assemblages are illite-kaolinite-1.4 nm clay minerals?mainly vermiculite/1.4nm transition minerals,chlorite/1.4nm transition minerals,etc.?-montmorillonite.The 1.4nm transitional minerals and vermiculite in paleoweathering crust may be related to the humid leaching climate.The drastic change of montmorillonite content in Nipingao profile and Xinlan profile developed on the same parent rock may be related to the stagnant water environment in paleoweathering crust.?4?The types and combination characteristics of clay minerals and chemical weathering index CIA indicate that the three paleoweathering crust profiles studied in this study experienced hot and humid climate during their formation The difference of clay mineral assemblage between Baiyuping paleoweathering crust and the three paleoweathering crust profiles studied in this study may be affected by the difference of formation time of paleoweathering crust by comparing the kaolinite content between paleoweathering crust and modern weathering crust in Siming Mountain area,it is considered that the degree of damp heat in the formation period of paleo-weathering crust is higher than that of modern weathering crust.?5?Among the three paleo-weathering crust profiles,the chemical weathering index of Zhucao paleo-weathering crust profile whose parent rock is granite is the lowest,but its low-frequency magnetic susceptibility is the highest,which indicates that the high magnetic susceptibility of Zhucao profile is less likely to be affected by parent material and climate factors.According to the special geological conditions in Simingshan area,it is considered that the high magnetic susceptibility of Zhucao profile whose parent rock is granite may be affected by the late basalt coverage.The pH value of Zhucao profile,Nipingao profile and Xinlan profile gradually increases from weathering front down,while the low frequency magnetic susceptibility has a decreasing trend,which indicates that the weathering degree of paleoweathering crust is gradually decreasing with the increase of profile depth,which accords with the general law of weathering crust development.There is no significant correlation between the low-frequency magnetic susceptibility of three paleoweathering crust profiles and the content of grain-size components and representative weathering parameters,so it is necessary to be cautious when using the magnetic susceptibility index to retrieve paleoclimate.