The Permian-Triassic Boundary And Geological Events In The Upper Yangtze Area

The earth on which human beings depend is experiencing dramatic changes in the environment,such as climate warming and the disappearance of a large number of species.Is this the result of environmental deterioration such as the rise of carbon dioxide and environmental pollution caused by human activities? Or is it a "normal" replacement event of biological evolution in the history of the Earth's evolution? At present,this scientific problem has become one of the major scientific problems in the study of earth system.Studying similar events in deep geological records should be an effective way to find the answer to this scientific problem.Since the Phanerozoic,there have been five mass extinctions on the earth,of which the Permian-Triassic period,the P-T period,is the most prominent one,and has become the best object to study this issue.The Permian and Triassic in China have developed continuous geological record profiles including marine facies zones,which have unique geological conditions for studying the mass extinction event at the P-T period.In this paper,the Permian-Triassic boundary strata of Yangjiayan section in Guangyuan,Jianshuigou section in Huayingshan area and Jiandaoshan section in Zhongliangshan area in Chongqing are selected as the research objects.Through field boundary strata profiles,conodont fossil samples and various sedimentary rock,such as tuff,carbonate,siliceous and mudstone samples are systematically collected and studied.The lithostratigraphic and stratigraphic characteristics,sedimentary characteristics,volcanic eruption periods and intensity,and sedimentary geochemical response characteristics at the P-T period are investigated.The paleostructure,paleoenvironment and paleoecological characteristics at the P-T period in the Upper Yangtze region are reproduced.The major geological events and their geological evolution processes at the P-T period are identified,and the genesis,deepening and objectivity of the mass extinction events at the P-T period are explored.In order to understand the cause and mechanism of the mass extinction,we have obtained the following important understandings:(1)There are two types of PTB boundaries in different locations and properties in the Upper Yangtze region: one is the lithostratigraphic PTB(R)boundaries,under which there are many paleo-exposed surfaces,paleo-soil layers and percolation beans layers related to the dramatic decrease of sea level,which are overlapped by the purple-gray thin plate mudstone deposits at the bottom of the first section of the Feixianguan Formation,and the other is the biota PTB(B)boundaries with the first appearance of Hindeodus parvus conodonts are now used as markers to divide the P-T boundary.Both types of boundaries have specific identification marks and good regional correlation.(2)Yangjiayan,Jianshuigou and Shanjianshan profiles are located in the transitional zone of deep-water basin-front slope,shallow platform shoal and platform margin shoal-platform slope,respectively.They have different lithologic assemblages,profile structure and sedimentary evolution sequence,and can be used as the representative of PTB boundary strata of different lithofacies types and palaeogeographic locations.(3)The conodont spine fossil belts at the P-T period in the Upper Yangtze region have been established,which are Neogodolella changxingensis belt,Neogodolella yini belt,Neogodolella meishanensis belt,Hindeodus parvus belt and Isarcicella lobate.The conodont spine fossil belts in all sections can be compared with the conodont spine belts in the Meishan section of Changxing,Zhejiang Province,and have good correlation with each other.(4)The major geological events identified in the profile are mainly sea-level decline events,mass extinction events,volcanic eruption events and geochemical events.All the events are carried out in stages,including: the sea level decline events are superimposed by four stages of gradual increase in sea level decline;the biological mass extinction events are divided into four stages of gradual evolution: the bioconcentration stage,the biological attenuation stage,the biological extinction stage and the biological depression stage;and the volcanic eruption events are divided into four cycles of gradual intensification first,gradual frequency later,exhaustion and extinction.The geochemical events represented by C,O and Sr isotope negative migration also have multi-episodic evolution characteristics.Significantly,these major geological events have corresponding synchronous evolution properties.(5)The analysis of the geochemical characteristics of the constant,trace and rare earth elements of tuff at the P-T period of Yangjiayan section shows that the sedimentary environment of tuff in the Upper Yangtze area has evolved from hypoxic to hypoxic environment through hypoxic to oxygen-enriched environment to weak reduction-weak oxidation environment from bottom to top.It is proposed that the intrinsic dynamic process during the gradual and gradual volcanic eruption occurs.The intermittent convective circulation of oxygen-enriched water from the surface of the ocean to the deep seabed results in the transformation of strong reduction environment into weak reduction-weak oxidation environment,which is contrary to the fact that the Yangjiayan section at the P-T period is in the deep seabed environment with strong reduction,but it is proved that the internal dynamic action during the volcanic eruption can cause the convective circulation of oxygen-enriched water from the surface of the ocean to the deep seabed,resulting in deep seabed outflow.The frequent alternation of strong reduction and weak reduction-weak oxidation environment also resulted in the appearance of gray-white interbedded tuff in a large set of dark fine-grained rock series.(6)According to the chemical analysis results of the main elements of the tuff in Yangjiayan section,combined with the unusually high zircon crystal debris abundance,grain size and wide oscillating ring structure of the tuff,and its angular or tearing shape lacking transportation and abrasion modification,the primary magmatic properties of the basalt with high alumina and alkaline basalt are generally shown.It is believed that the tuff is supplied.The volcanic detritus of the rocks is the recent source.It is speculated that there exists a volcanic island arc in the northwestern margin of the Yangtze Plate at the same time as that in Siberia.The frequent eruption of the volcanic island arc at the P-T period provides the main source of volcanic ash near the western margin of the Yangtze Plate.(7)Volcanic eruptions caused by large-scale plate tectonic movement at the P-T period are global,and a series of major geological events caused by global volcanic eruptions are not only global.(8)Considering the synchronous evolution process of four volcanic eruption cycles,four sea-level declines and four biological extinction stages,and the nature of marine living space and ecological environment,it is proposed that a series of major geological events with high intensity and high frequency volcanic eruptions as the dominant factor are the main causes of the biological extinction event at the P-T period.