Siliceous Nodules In The Upper Triassic Carnian Marine Carbonate Rocks In Northwestern Sichuan

The Carnian pluvial event(CPE)is the most significant global climate event in the Triassic period.The event was identified in Middle Europe,northwest Sichuan,Guizhou,Yunnan Provinces in China.Our recent research data show that the Carnian siliceous nodules observed in the carbonate rocks of Ma'antang section and Huanglianqiao section in Jiangyou,Northwest Sichuan,was around the CPE sediments in the upper Triassic,which may be related to this global climatic event.What's more,its origin is important for the study of the paleoclimate and paleoenvironment in the late Triassic in the area.In this paper,the genesis of siliceous nodules in the study area has been studied from the macroscopic,microscopic petrological characteristics and geochemical analysis.The siliceous nodules in the Ma'antang section are mainly developed in the lower part of the Ma'antang Formation,and gradually increase from down to up,but the distribution of siliceous nodules in the middle(Layer 29)of the Ma'antangFormation is relatively less and along the layer along the layer.In the Huanglianqiao section of Jiangyou,siliceous nodules are mainly developed in the middle and lower parts of the Ma'antang Formation,appearing suddenly in the fourth layer and appearing at the same time with a large number of bivalves and acanthosis fossils.The shape of the siliceous nodulesare ellipsoid and varies in size.It is about 15cm-18 cm long and about 10-15 cm wide,scattered distributed in the white thick massive limestone at the bottom of the Ma Antang formation.Because it is difficult to weathering,it protruded at the layer surface and mutated in contact with surrounding limestone.The main means of lithology study include ordinary polarizing microscope observation and scanning electron microscope observation.In the microscope,we see a large number of siliceous sponge spicules in the siliceous nodules layer of the Huanglianqiao section,indicating the effect of biogenesis;while in the siliceous nodules of Ma'antang section,they are filled with microcrystalline quartz,and there is no obvious bioclast.Sponge spicules develop in the layers above the siliceous nodules layer.These spicules are mostly calcified,and the lost siliceous rocks may be enriched into the lower layers and diagenesis.The related geochemical analysis include principal element analysis,trace element analysis and rare earth element analysis.The analysis results of these geochemical data indicate that siliceous modules in the study area is a marine biogenic cause.In addition,our latest C/O isotopes curve shows that the occurrence period of siliceous nodules is equal to that of carbon isotopes negative shift,indicating that the forming of siliceous nodules in the study area is closely related to the Carnian climate events.