The Lower Member of the Longrnaxi Forma-tion is generally dominated by siliceous shale,but recently we found some siltstone-mudstone rhythm sections developed in the Lower Member of the Longmaxi Formation.The study of formation mechanism of siltstone-mudstone rhythmic sedimentary sections may provide new insights into the shale sedimentary environ-ment.Therefore,we studied the characteristics and formation mechanism of siltstone-mudstone rhythmic sedimentary sections in the Lower Member of the Long-maxi Formation in the Changning area based on core observation,thin section identification,major elements and trace elements analysis.The results show the following:1) Two siltstone-mudstone rhythmic sedimentary sections are characterized by frequent interbed between black or gray-black shale and light gray siltstone,abundant argillaceous laminas and silty laminas,with obvious lithological boundaries having developed.Horizontal laminas and rhythmic laminas are well-developed in the shale layer,while the wavy laminas are well-developed in the siltstone layer.2) The major compositional elements are SiO2,Al2O3 and CaO,followed by Fe2O3,MgO,K2O and Na2O.3) Compared with the world average shale,these siltstone-mudstone rhythmic sedimentary sections are rich in Mo,U and Ba,but less in V,Co,Ni,Cu.Compared with the shale layer,the siltstone layer has lower contents of V,Co and Ni.4).The geochemical redox indices,Mo-U and CIA values suggest the formation of the siltstone-mudstone rhythmic sedimentary sections are related to influences from bottom currents in an oxic condition with a warm and humid paleoclimate.