The Clay Minerals Characteristics Of Late Devonian Palaeosol And Their Paleoclimate In Baiyin,Gansu Province

Paleosol is the soil formed in geological period,which records abundant paleoclimate and paleoenvironment information.Clay minerals are extremely sensitive to paleoclimate change,and becomes an important paleoclimatic alternative index.The Late Devonian was an important climate changes period in earth history,while the research on paleoclimate using paleosol and clay minerals is lack for now.Paleosols have been discovered in the late Devonian near Pingchuan City,Gansu province,China,and are recognized by evidence of root traces,soil horizons and soil structures,determine the pedotype,paleoclimate characteristics are discussed by clay mineral,element geochemistry and carbonate contents,the mainly conclusions are as follow:(1)The red deposits are identified as paleosol according to the diagnostic characteristics of root traces,soil horizons and soil structures in the red sedimentary strata.A large number of calcareous nodules,drab-haloed root traces,mud cracks,lungfish cave remained in paleosol and sandstone and other field characteristics reflect the semiarid to humid paleoclimate during the late Devonian.(2)Three different types of Paleosol sections were selected in the study area.According to the classification basis of paleosol such as soil color,root trace,Soil cutans,massive structure,calcium nodule and calcic horizon,the paleosol types of the three sections were Aridisol,Vertisol and Alfisol.(3)The clay minerals in three sections were extracted and analyzed.The types of clay mineral assemblage are illite,kaolinite,chlorite and illite/smectite formation(I/S)without montmorillonite,but their contents are quite different.The content of I/S is the highest,followed by illite,chlorite and kaolinite,with an average content of 43%,33.5%,3.9%and 5.5%respectively.(4)The results show that the IC value,Rb/Sr ratio and CaCO₃ content in Alfisol decrease gradually from horizon A to C,while increase gradually in Vertisol and Alfisol,which shows that the weathering and leaching degree of the Aridisol decreases from horizon A to C,while the Vertisol and Alfisol increase from horizon A to Bt.The high ratio of IC and Rb/Sr and the lowest content of CaCO₃ in the profile of Alfisol reflect the climate of strong eluviation and hot-humid.The highest content of CaCO₃,I/C,illite,kaolinite and chlorite is in the profile of Vertisol,but the low ratio of IC and Rb/Sr reflects the semi-humid environment with alternation of dry and wet,and the calcium is concentrated at the bottom of the profile;the lowest ratio of Rb/Sr in the profile of Aridisol and the higher content of CaCO₃ indicate the weathering eluviation CaCO₃ is mainly enriched in horizon A,reflecting the arid and semi-arid climate.In total,different paleosol types reflect from the arid to semi-arid,to humid climate during the late Devonian.