| During the whole Phanerozoic,the most severe extinction event occurred at the end of the Permian,what followed was the long biotic recovery period in the Early Triassic,which led to the Paleozoic ecosystem giving way to the Mesozoic ecosystem.Many geologists have been attracted by the mechanism of the mass extinction,paleo-oceanic conditions and paleoclimatic background during the period of biotic recovery.According to these,this paper uses six Permian-Triassic Boundary(PTB)sections(e.g.,Guangyuan Shangsi,Chongqing Beifengjing,Beibei Baimiaozi,Nanchuan Dongsheng,Jiangyou Yudongzi and Qingchuan Dagouli)which deposited in different environments as the research carriers,with sedimentology as the main research method and paleontology and geochemistry as the auxiliary means,to analyze and study systematically the geological events and sedimentary features near the PTB in the Upper Yangtze region.Meanwhile,the impact of regression and volcanic activity on the end-permian mass extinction,and paleo-oceanic physical and chemical conditions reflected by some Early Triassic unusual sedimentary fabrics and features on biotic recovery are also discussed in this paper.The relative sea level drop occurred in the upper Yangtze region at the end of the Permian,which had little effect on the mass extinction event during this period.Two paleo-exposed surfaces,karstified and filled with paleo-soil,can be seen in the Changxing Formation of the Beifengjing section which deposited in carbonate platform,representing two relative sea level drops.Because it is difficult to identify the sedimentological markers indicating sea level changes in the Guangyuan Shangsi section which deposited in relative deep water,the high-frequency sequence division is carried out by using the wavelet analysis method based on natural gamma data.And on this basis,the Fischer chart is helpful to determine sea level changes.The results also show that regional sea level drop occurred at the end of the Permian and continued until the Early Triassic.In addition,the response of organisms in the Beifengjing section to these two relative sea level drops shows that the first sea level drop lead to species replacement or death of some organisms.Although the second sea level drop coincides with the mass extinction in terms of time,this paper holds that the effect of sea level drop on mass extinction is limited,and the main reason for the mass extinction at the end of Permian is volcanic activity during the Late Permian regression.The upper Yangtze region experienced several volcanic activities of different scales in the Permian-Triassic boundary interval,which may be the main factor of the mass extinction at the end of Permian.The PTB strata in the study area contain many beds of clay.According to the analysis results of REE in boundary strata,the distribution characteristics of REE in boundary strata are similar to those of volcanic ash near the PTB of South China,indicating that the upper Yangtze region was continuously affected by volcanism during this period.Most trace elements in boundary clay and non-boundary clay layers show abnormal and consistent vertical variation characteristics,suggesting that clay layers are indicators of environmental change.Because there are other layers of clay near the PTB,but the extinction event is only coupled with the boundary clay,it is speculated that the cumulative effect of multi-pulse volcanic eruption may not be directly related to the mass extinction.Moreover,volcanic activities in South China have limited influence,the triggering mechanism of the end-Permian mass extinction may be related to the simultaneous eruption of large-scale and multi-source volcanoes around the world in a short time.Druing the Early Triassic,the unusual carbonate deposits,represented by microbialites,calcirudites and giant oolitic limestones,were widely developed in the Upper Yangtze area,indicating that the paleomarine environment in this period was in an extremely disturbed state.There are many kinds of microbialites,such as lamellar microbialite,micro-dome microbialite,thrombolite-like microbialite.Mcrobial mounds,giant ooids and microbial spheroids are all microbial origins,indicating that the Early Triassic was a period of microbial bloom.According to the shape and structure,the calcirudite can be divided into breccia limestone and Flat-Pebble Conglomerates.Large seismogenic structures such as synsedimentary deformation structure,liquefied vein,gravity fault,"piercing bedding" and convolution structure can be seen on the outcrop,as well as adjacent storm crust layer and gutter cast structures with different shapes.It shows that the formation mechanism of the calcirudite may be related to storms and earthquakes.Based on the comprehensive analysis of the formation mechanism of these unusual deposits,the oxygenous environment indicated by the gastropods and ostracods in the microbial mounds,and the sedimentary markers of the storm and seismic events,it is shown that the marine physical and chemical conditions of the Early Triassic in the study area are global anoxia,local and intermittent increase of oxygen content in seawater,ocean water is in the state of calcium carbonate supersaturation,and frequent storms caused by monsoon climate and multi-period earthquakes caused by regional tectonic activities further lead to more turbulence of the ocean.In summary,it is speculated that the extremely harsh paleoclimate,paleomarine environment and strong regional tectonic activities during the Early Triassic are the main reasons for the delayed biotic recovery in the Upper Yangtze region.This paper systematically study s a series of major geological events from end-Permian to Early Triassic in the Upper Yangtze region,such as sea level drops events,volcanic events,microbial bloom events,storm events and earthquake events,and establishes the time series of geological events under the disordered paleo-ocean during this period.At the end of the Permian,the volcanic eruption was accompanied by relative sea level drops,however,the sea level drop stopped at the the Earliest Triassic,and volcanic activity did not stop during the Early Triassic.Microbial bloom started in the Earliest Triassic,and the initiation of storm and earthquake events was slightly later than microbial bloom evert,both of which lasted throughout the whole Early Triassic. |
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