A Study On Paleogeography And Paleoecology Of The Xujiahe Period In The Late Triassic Of Guangyuan, Sichuan

The famous mass extinction event occurred during the Mesozoic inbetween Triassic-Jurassic period.With the frequent occurrence of global volcanoes eruption,the increase of CO₂ concentration and the global warming,the paleoecology and paleoenvironment of the earth underwent drastic changes.And the late Triassic was the key epoch for the formation of the Western Sichuan depression and the uplift of Longmenshan Mountains.At the same time,the terrestrial ecosystem of Sichuan Basin had completed the transformation from sea to land.The Triassic-Jurassic strata in the northern part of the Sichuan Basin well developed and exposed,it was the good material records for paleoclimate and paleoenvironmental studies.In this paper,the Xujiahe Formation in Guangyuan,North Sichuan Basin is investigated,mainly focused on the analysis of the sedimentary structures and facies analysis,so we can summarize the sedimentary system and environment evolution of this place during the stage of Norian-Rhaetian in late Triassic and restore the paleogeographic environment.The paleoclimate and the community succession were studied with the fossil data of paleobotany fossils and sporopollen through statistical classification.And the palaeoecological environment was studied based on Sporomorph EcoGroup?SEG?,a semi-quantitative mathematical model.Meaningwhile,this paper discusses the causes of palaeoecological changes in guangyuan area based on comparing the characteristics of palaeogeography and palaeoclimate with other regions.The results show that:?1?The lagoon marshes,delta plains and braided rivers facies are developed in the section in Guangyuan region,which is a remarkable terrestrial transitional—terrestrial facies system.Through the influences of the formation and development of Ganzi-Aba back-arc basin,the transgression began in Carnian stage affected the Guangyuan region during the late Norian.Due to the uplift of the Qinling orogenic belt and Longmen mountain,marine facies ended in Guangyuan area,and changed to terrestrial facies in the 1^st member of Xujiahe Formation.?2?We have reconstructed the paleoclimate in the later Norian stage of late Triassic in Guangyuan area by analyzing the plant fossils assemblage combined with sedimentary cycle and sedimentary facies characteristics.We concluded that Guangyuan area is the subtropical coastal humid climate in the 1^st member of Xujiahe Formation,and this coastal humid climate has continued from the 1^st member of Xujiahe Formation to the late Norian stage.The appearance of Xenoxylon guangyuanense indicated that cooling event happened in the 2^nd member of the Xujiahe Formation,and the temperature rose somewhat in the period of 3^rd member of the Xujiahe Formation than previous stage.However,it is still not so warm enough,and there is a trend change to the temperate climate.?3?Through the statistical analysis of paleobotanies and sporopollen fossils,the community succession of the period of Xujiahe Formation in Guangyuan area was summarized as follows:For the period of the 1^st member of the Xujiahe Formation,there were mainly humid-plain subtropical communities dominated by ferns and cycads.Due to the cooling of the climate,the plant community changed to a wet community including arbor in temperate climate during the stage of 2^nd member of the Xujiahe Formation.In the stage of the 3^rd member of the Xujiahe Formation,the coastal aquatic community dominated by Equisetophyta gradually changed to the one dominated by subtropical wet plants including ferns and cycads,which also containing temperate trees such as ginkgo and cypress.?4?The ecological environment of the study area was the lowland riparian areas during the stage of 1^st member of the Xujiahe Formation,and the environment of the stage of 3^rd member of the Xujiahe Formation is warm and wet riverine plain spore assemblages show cooler climate features.?5?The paleogeographic environment of Xujiahe period in Guangyuan area during the stage of 1^st member of the Xujiahe Formation,the area in Guangyuan changed from lagoon-swamp to braided river plain.and oxbow lake developed.The paleogeography becomed to braided river during the stage of 2^nd Xujiahe Formation,and then turned to river plain with lots of lake and marsh at stage of 3^rd Xujiahe Formationthe.Until the period of 4^th member of Xujiahe formation,it was retured to braided rivers.?6?The changes of palaeogeography and sedimentary environment during Xujiahe stage controlled the community evolution.And the trend of climate change is consistent with other place's within the basin.