The Response To The Key Sediments Of The Carnian Climate Event In Hanwang Town, Mianzhu City In Northwestern Sichuan

The mid-Carnian(Late Triassic) Extreme Carnian Climate Event, one of significant global climatic events after the geological event in the end of late Permian, was identified in Guizhou, Yunnan province and Hanwang town, Mianzhu city in northwestern Sichuan. Sedimentary sequences in Carnian of Q ingyangou and Guanyinya reveals the lithological change from carbonate rocks to clastic rocks, oolitic limestone, bioclastic limestone, sponge reef limestone, tuff, black shale and siltstone appeared from the bottom to the top. It is evident that the productivity in carbonate rocks are decreased, terrigenous detrital materials are increased and volcanic activities appeared in tuff layer in the Carnian Extreme Climate Event. Based on the field survey, sampling, thin sections identification and geochemical detection of Q ingyangou and Guanyinya in Hanwang town, Mianzhu city in northwestern Sichuan, it can be known that the key sediments in Guanyinya section are layers from sponge reef limestone to black shale and the key sediments in Q ingyangou section are layers from paleo-karst to black shale above tuff layer. After analyzing the lithological characteristics, paleontological characteristics and sedimentary characteristics, the Carnian Extreme Climate Event is divided into three stages—before, after and when it happened. It is found that the sedimentary environment changed from the oolitic beach in inner ramp, bioclastic beach in mid-ramp and shoal in outer ramp in the early period to the shallow shelf environment in middle period and the littoral environment to tidal environment in the late period, and the changes is closely associated with the fluctuation of sea level and plate tectonic movements. The U-Pb dating analysis of tan tuff in faces with lithological muta tion in Carnian gives a weighted mean age of 233.2±4.9Ma, which indicates that the tuff layer appeared in the middle Carnian, considering the age of Carnian is 228.4~237Ma in 2012 Global Stratigraphic Chart. The U-Pb dating analysis, combined with the data material about ammoniod and conodont discovered and measured in observed profile, implies that the stratum of the sample was in the middle to late Carnian. Thus it is concluded that the Carnian Extreme Climate Event occurred in the middle Carnian with weighted mean age of 233.2±4.9Ma. Based on the previous analysis and reaearch results about the cause of the Carnian Extreme Climate Event, the author studies the Carnian climate change in northwestern Sichuan, it is found that the extinction of sponge reefs and large area deposition of black shale is the response to the anoxic seawater and humid environment in Carnian Extreme Climate Event. Moreover, because of the plate tectonic movement, the eruption of a large Wrangellia igneous province, global climate change, the fluctuation of sea level, and so on, the Carnian Extreme Climate Event occurred in the Eastern Tethys with the extinction of sponge reefs, occurrence of tuff layer and the large area deposition of black shale.