Features and origin of calcirudites in the first member of Feixianguan Formation in Guangyuan area, northwestern Sichuan Province

The Early Triassic calcirudites in the Shangsi section, Guanyuan city, have great significance for the study of paleoenvironmental changes after the latest Permian mass extinction (LPME) in the Upper Yangtze region. In this paper, we studied the possible cause of formation of the Griesbachian calcirudites from the first member of the Feixianguan Formation, using the methods of sedimentology, petrology, and C-O isotopic geochemistry. The results show that these calcirudites can be subdivided into four types: Type-Ⅰ calcirudites appear in the lower part of the section, with irregular shapes, mixed sizes, and a lack of lath-shaped gravels, indicating a gravity flow origin; Type-Ⅱ calcirudites contain an abundance of decimeter-sized flat pebbles with large gravel diameters, suggesting storm-induced deposition with increased intensity and frequency; Type-Ⅲ calcirudites show flat gravels distributed in parallel with the bedding plane and "bamboo leaf-shaped" gravels in the vertical section, with the characteristics of a chrysanthemum-shaped structure and imbricate arrangement, suggesting a mechanical and pressure solution origin; Type-Ⅳ calcirudites are vermicular limestones, representing a relatively weak water flow condition, suggesting a microbial and mechanical combined origin. Stromatolites altered the earlier limestones, which are the foundation for the formation of flat gravels. Later, argillaceous fillings and differential compaction of calcareous gravels during the diagenetic period constitute another reason for the formation of flat gravels. Vermicular limestones could be a result of the combination of biology and storms with diagenesis, which have a potential connection in cause with flat calcirudites. The carbon isotope curve of carbonate rocks from the first member of the Feixianguan Formation shows a slowly ascending trend, and the carbon isotope curve exhibits a positive shift in brecciated limestone (Type-Ⅱ calcirudite), indicating a transition of sedimentary environment, which could be the result of thriving planktonic microorganisms in oceans related to storms during this period.