Physical Source,genetic Characteristics And Systematic Classification Of Soil Overlying The Outcrops Of The Shaximiao Formation

The Shaximiao Formation is widely exposed in Sichuan,and there are differences in the color of the rocks in different profiles,which is beneficial to explore the relationship between the exposed rocks of different colors and the overlying soils.The samples were collected from Weiyuan County,Neijiang City,Sichuan Province,with 21 profiles.The color of the underlying bedrock was divided into two categories:yellow and red,including S1,S3,S4,S6,S7,S8,S9,S12,S13,S14,S16,S17,S18 and S20 for the yellow bedrock profiles and S2,S5,S10 and S11 for the red bedrock profiles.There are S2,S5,S10,S11,S15,S19,S21,and a comprehensive analysis of the material origin of the rocks and overlying soils of the Shaximiao Formation from several aspects,including major elements,trace elements,rare earth elements,and mineral composition,and to explore the genetic characteristics of the soils and their position in the Chinese Soil Taxonomy（CST）,leading to the following conclusions:（1）The results of major,trace,rare earth elements and mineral composition of soils and rocks in representative yellow bedrock profiles S4,S9 and representative red bedrock profile S15 show that the sediments on the profiles are inherited,indicating that the soils overlying S4,S9 and S15 were developed from the underlying rocks.The chemical index of alteration（CIA）and Ba/Sr of the red bedrock profile S19 indicate that the underlying rocks are"reverse weathering",while the fractionation of rare earth elements is not consistent,and the XRD of the overlying soil and the underlying rocks are significantly different,indicating that the overlying soil of S19 may not be formed by the underlying rocks,because the profile is in the rock rhythm layer,the parent lithology of the overlying soil of S19 is different from that of the underlying rocks.（2）The ratio of primary minerals to secondary minerals in the soil overlying yellow bedrock is higher than that in the soil overlying red bedrock,and the distribution points of the soil overlying red bedrock in the A-CN-K are closer to the minerals containing Al₂O₃,indicating that the weathering development of the red soil is relatively deeper.After excluding the elemental anomalies,the CIA（67.47）and Ba/Sr（5.87）of the red bedrock overlying soil are higher than those of the yellow soil（67.26,5.79）,indicating that the red bedrock overlying soil experienced more obvious weathering.（3）According to the Keys to Chinese Soil taxonomy（3rd edition）,the overlying soils of the Shaximiao Formation can be classified into two Order:Cambosols and Primosols;two Suborder:Udic Cambosols and Orthic Primosols;three Group:Ferri-Udic Cambosols,Hapli-Udic Cambosols,and Udi-Orthic Primosols;and four Subgroup:Typic Ferri-Udic Cambosols,Typic Hapli-Udic Cambosols,Typic Udi-Orthic Primosols,and Lithic Udi-Orthic Primosols.Of these,6 profiles are Cambosols and 8 profiles are Primosols in yellow bedrock profiles;among the red bedrock profiles,only S19 is Cambosols,the rest of the profiles S2,S5,S10,S11,and S15 are Primosols.