Changes In Sedimentary Environment And Terrestrial Paleoecology Across The Triassic-Jurassic Transition In Eastern Sichuan Basin

The Late Triassic(237-201 Ma)is a critical period with active geological processes in the history of "Deeptime Earth".The end Triassic mass extinction provides us an ideal window to the response mechanisms of different species and ecosystems to the environmental and climate fluctuations.Based on the Triassic-Jurassic strata exposed in Sichuan Basin and environs,we have reconstructed the evolutionary history of the sedimentary environments.More than ten subfacies of five sedimentary system are recognized.The meander river,delta and lake sedimentary systems are widely distributed in Sichuan Basin.The alluvial fan and braided river sedimentary systems are restricted near the source area.The Indosinian Tectonic Movement is the primary factor for the changes in the sedimentary environments.Changes in climatic conditions and lake level also have a certain influence to the changes.There has been a significant change in the composition of the paleophyte community and the vegetation landscape across the Triassic-Jurassic boundary transition.However,the decline of the biodiversity of the Xujiahe Flora has occurred in the middle to late Rhaetian rather than the transition of the Triassic-Jurassic boundary.Moreover,the evidence of transitional flora presents at the latest Rhaetian.Premised on the reconstruction of sedimentary environment,the Xujiahe Flora and Zhenzhuchong Flora buried in the Qilixia Section are divided into 6 eco-communities,including marine aquatic plant eco-community,fluvial aquatic eco-community,delta wetlands plants eco-community,swamp wetlands plant eco-community,transitional plant eco-community and highland plant eco-community.The succession of the eco-communities provide a contrasting scale and regional framework for the evolutionary history of floras across the Triassic-Jurassic transition in Sichuan basin.The rise,prosperity and demise of the Xujiahe Flora are closely linked to the evolution of the sedimentary environment.The tempo-spatial distributions of the palaeophyte community and representative groups show us that he changes in the composition of the palaeophyte community and the turnover of the plant species have been influenced by the global warming and wildfire during the Triassic-Jurassic transition.Furthermore,the environmental and climatic effects of the Indosinian Movement and changes in land-ocean distribution and paleogeographical patterns also have played an important role in the crises of the biodiversity and the ecosystem.The contrasting scale and regional framework have been reconstructed for the evolutionary history of floras across the Triassic-Jurassic transition in Sichuan basin.The scope and extent of the end-Triassic Mass Extinction(ETE)in terrestrial ecosystem of the Sichuan Basin are revealed.