The Sedimentary Characteristics and Geological Significances of Carbonate Tempestites near the Boundary of Late Permian to Early Triassic at Nanjiang Section, North of Sichuan Basin

Key words: 
• Sichuan Basin /
• Nanjiang section /
• Early Triassic /
• tempestite

Abstract: To further explore the significance of the tempestites that exposed near the boundary of Permian and Triassic at Nanjiang section, and investigate the effect of Early Triassic monsoon on the sedimentary characteristic in northern Sichuan Basin, several set of tempestites have been researched in this article. The paper focuses on the characteristics of sedimentary and petrology in both outcrop section and thin sections of the storm deposits at Nanjiang section. According to the different sedimentary characteristic, each set of tempestite can be divided into three section: Section a is developed at storm active period, Section b is developed at storm energy declining period, Section c is developed at storm rest period. Depending on the combination of Sa, Sb, Sc, tempestites can be divided into four kinds of storm sequence structure type to represent different sedimentary environment. Then based on the grain-size analysis of storm particles and the comparison of sedimentary thickness, we found that the storm energy gradually increased upward. Furthermore, when the tempestites at Nanjiang section are contrasted with Shangsi section, showed that the thickness and the characteristic of them is similar and that they are both developed at the Early Triassic sedimentary strata near the boundary of P/T. Therefore, a conclusion can be made that they are the same deposition and both of them are influenced by frequent storm events under the giant monsoon system at Early Triassic Period. What`s more, they reveal the deposit process that the shelf margin gradually was in movement in the direction of the wide sea at Early Triassic, and the Trough gradually being filled simultaneously.