The Sedimentary Microfacies And The Environmental Evolution Of The Carnian (late Triassic) Climate Event In The Northwestern Sichuan

The mid-Carnian Climate Event is one of the most pronounced climatic event in the Triassic. It can be isochronic identified in the section of Hanwang Guanyinya, Hanwang Qinyangou,Anxian Jushui,northwest Sichuan Basin,which is mainly reflected by the sudden death of carbonate rocks covered by black shales and terrigenous clastic rocks. Under the effect of Indosinianmovemen in the latest Permian, the fantasticality changing paleoclimate in the Late Triassic leads to the complex sedimentary environment of Maantang Formation in western Sichuan basin. The sedimentary facies classification of Maantang Formation is also controversial. In order to ascertain the sedimentary microfacies characteristics in CPE and explore the environment evolution process and its response in paleontology, geochemistry ahead-after the CPE, the research observed and sampled outcrops and statistically analyzed thin sections. With identification of fossils, geochemical test methods, foreign research data and methods to recover paleclimate by sedimentary micofacies, it mainly analyzes the microphysiography, paleontology and geochemistry characteristics of rocks changed from carbonate to terrigenous clastic rocks. It is concluded that the lithology characteristics changed from the early-middle Carnianoolitic-bioclastic bank and siliceous sponge reef to the late Carnian calcareous shale-calcareous argillaceous siltstone. The carbonate formations can be divided six microfacies and seventeen secondary microfacies. The terrigenous clastic rock can be divided five microfacies. Based on the above characteristics of microfacies, we foud that the environmental evolution of the Carnian Climate Event in Upper Triassic changed from bioclastic-ooliticshoal shoal on the back-slope to bioclastic-ooliticshoal shoal on the middle-slope and siliceous sponge reef to the neritic argillaceous to the coastal tidal flat, and to the delta-front subfacies of Xiaotangzi Formation. The black calcareous shale contains marine fossils, plant leaves, stems fossils and pyrite. It shows that fresh water was input in the environment with deficient energy and oxygen. Such apparent facies change is closely related to the large input of fresh water and terrigenous siliceous clastic in the Carnian. The carbonate compensation depth rised and the carbon and oxygen isotopehas shifted. The ammonoids in Maantang Formation were also found in region of West Tethys Ocean. CPE in Triassic have exerted an extensive influence on global biosphere, which means that the global sea-level fluctuation rate in Upper Triassic Carnian which is higher than the growth rate of sponge reefs, coupled with the frequent plate tectonic during this period, the concentrated outbreak in fire rock provinces and prevalence of monsoon climate, leads to the extinction of siliceous sponge reefs.