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.

龙马溪组下段普遍以硅质页岩为主，近期发现龙马溪组下段发育一些粉砂岩-泥岩韵律剖面。粉砂岩-泥岩韵律沉积剖面形成机制的研究可为页岩沉积环境提供新的认识。为此，我们在岩心观察、薄片鉴定、主量元素和微量元素分析的基础上，对长宁地区龙马溪组下段粉砂岩-泥岩韵律沉积剖面特征及形成机制进行了研究。结果表明：1）两个粉砂岩-泥岩韵律沉积剖面，黑色或灰黑色页岩与浅灰色粉砂岩互层频繁，泥质纹层和粉质纹层丰富，岩性界线发育明显。页岩层水平纹层和韵律纹层发育, 粉砂岩层波浪纹层发育。2) 主要组成元素为SiO₂、Al ₂ O ₃和CaO，其次是Fe ₂ O ₃、MgO、K ₂ O和Na ₂ O。 3）与世界平均页岩相比，这些粉砂岩-泥岩韵律沉积剖面富含Mo、U和Ba ，但在 V、Co、Ni、Cu 中较少。粉砂岩层与页岩层相比，V、Co、Ni 含量较低。4）。地球化学氧化还原指数、Mo-U 和 CIA 值表明粉砂岩-泥岩韵律沉积剖面的形成与暖湿古气候下含氧条件下底流的影响有关。