Characteristics and formation mechanism of siltstone-mudstone rhythmic sedimentary sections in the Lower Silurian Longmaxi Formation in the Changning area, South Sichuan Basin, southwest China

The Lower Member of the Longmaxi Formation 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 environment. Therefore, we studied the characteristics and formation mechanism of siltstone-mudstone rhythmic sedimentary sections in the Lower Member of the Longmaxi 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.