Late Permian To Early Triassic Redox Variations Of South China And Unusual Biosedimentary Responses

During the Permian-Triassic transaction 252.0 Ma ago,the largest extinction events occurred in the Phanerozoic.Closely related to the mass extinction and the subsequent long-term environmental stress in the Early Triassic is the appearance of a large number of abnormal sedimentary structures.These special types of carbonate deposits are commonly found in the pre-Cambrian Proterozoic marine environment,and they appeared in the shallow sea environment after the Cambrian.Such sedimentary structures are considered to be "dislocations in time and space" and are called “Anachronistic facies”.The most representative of which is the Permian-Triassic boundary Microbialites(PTBMs).They widely developed in the shallow marine facies at the margin of the Tethys Ocean.Although predecessors have made more detailed studies on the macroscopic morphology of PTBMs,their redox state,or paleo-oxygen phase,has been controversial.The study of other types of Anachronistic facies in the Early Triassic above the PTB boundary in South China is more superficial.To this end,this paper has carried out the following research work,as well as the results and understandings obtained:Summarized the palaeogeographic distribution and microbialites types of PTBMs reported globally,including South China,and found that the PTBMs in South China are quite different in terms of their macrostructure and microbial fossil composition.South China PTBMs were mainly located in the Hindeodus parvus conodont zone in terms of their formation time.They are dominated by thrombolites.A few sections develop stromatolites at the bottom,and some microbial rock sections develop digitated thrombolites in the middle and upper sections.Moreover,all the microbialites sections in South China and the Upper Permian bioclastic limestone have sedimentary discontinuities and may even be exposed.Calcareous microorganisms generally exist in microbialites,which are divided into Gakumella and calcareous microspheres in terms of morphology.Calcareous microspheres can be divided into three types from the microscopic morphology:microcrystalline spheroids,splendid spheroids enclosing the outer shell of microcrystals,and spheroids with radial calcite crystals with microcrystals as the core.The statistics of the size and diameter of the calcareous microorganisms in the Yudongzi and Dajiang sections indicate that they change significantly in the microbialites.As the degree of anoxia increases,Gakumella does not change significantly in number,but its diameter increases upwards.The number of calcium microspheres in Yudongzi profile decreased significantly with increasing anoxia.It shows that these two types of calcareous microorganisms may represent different groups of microorganisms.In order to study the paleo-redox facies of PTBMs,the author selected four microbialite sections in Yudongzi in Sichuan,Xiejiacao in Sichuan,Dajiang in Guizhou,and Tieshikou in Jiangxi for the study of pyrite framboids.The results showed that pyrite framboids always appeared simultaneously with microbialites.The statistical results of 2,972 pyrite framboids pyrites in the microbialites from Yudongzi section show that the average particle size is between 5.49 and 9.52?m;the average particle size of 2,208 pyrite framboids in the microbial rock from the Dajiang section is 5.5?11.4?m.The average particle size of 1829 pyrite framboids in Xiejiacao microbial rock is between 5.3 and 8.6?m;the Tieshikou section only finds pyrite framboids in bowl-shaped thrombolites with an average particle size of 6.28?8.76?m,and the bowl-shaped thrombolites muddy stone does not contain pyrite framboids.The average particle size to standard deviation of the pyrite framboids results of all samples were plotted to determine the redox state represented by each sample.The results showed that the pyrite framboids in PTBMs all indicated an dysoxic to anoxic state.In addition,the degree of redox state increased from the lower dysoxic to the upper dysoxic of the microbialites in the Yudongzi,Dajiang and Xiejiacao sections.The author compared the paleo-redox facies with other undeveloped microbialiltes in the H.parvus conodont zone,and selected the shallow sea non-microbial rock Zunyi section,Yangou section and Liangfengya section,slope facies from Chaohu section and the deep-water Shangsi section.The results show that in addition to the microbialitess section,the shallow-water platform facies and slope facies areas in the H.parvus zone,and the deep-water basin facies areas are all oxygen-rich environments.In the slope facies and basin facies,obvious two-facies anoxia phenomena were identified.The first face was from the conodont C.meishanensis to the C.taylorae zone,but immediately in the H.parvus to the lower part of the I.staeschei zone.It returned to the upper dysoxic to oxygen-rich state,and then it was brought to the I.isarcica zone in I.staeschei to become anoxic.In addition,the author also reported for the first time unknown structures that can be compared with Ediacaran macrofossils in the lower part of the Daye Formation of the two Lower Triassic in South China.And it is called Ediacaran-like macrofossil in this article.Ediacaran-liked macrostructures were found in the Shatian section of Daye,and Sanxi section of Yangxin,Hubei,which can be divided into three categories according to their morphology.Through the study of its geometric form,it is found that this kind of special structure is very similar to Ediacaran macrofossils and has similar biological characteristics.The author believes that this is a kind of soft biological fossil impressions deposited and recorded in the Anachronistic facies.According to the analysis of pyrite framboids,its depositional environment is dysoxic to anoxic.