| Well preserved marine carbonate strata of the Permian–Triassic transition occur in the Niejiagou section,Zhen’an,Shaanxi.It records the succession of biota and environmental changes of shallow-marine carbonate platform in the Permian-Triassic transition,and is a perfect area for studying the ecological response and palaeoenvironmental changes after the end-Permian mass extinction.On the basis of field measurement,this study systematically collects specimens,makes thin sections,and observes the microscopic characteristics of thin sections with polarized light microscope.Further analysis of the sedimentary environment and its paleoecological and paleoenvironmental significance.12 microfacies types are identified based on the detailed carbonate microfacies analysis methods in the Permian-Triassic transition of the Neijiagou section,including bioclastic argillaceous limestone,patched agglomerates,stromatolites,foraminiferal grainstone,algal-crinoid grainstone,wackestone containing coated and eroded bone debris particles,oolitic grainstone,aggregate grainstone,oolitic wackestone,mudstone and micrite.The microfacies reflect the characteristics of frequent changes of sedimentary facies in the Permian-Triassic transition.The rock types of the Lopingian Longdongchuan Formation mainly consist of sparry grainstone and bioclastic packstone,followed by muddy limestone and oolitic limestone.The particle types are mainly bioclasts,oolitic and spherulite particles.Bioclastic particles mainly consist of foraminifera,crinoids and calcareous algae,with a small number of brachiopods,bryozoans,bivalves and gastropods.Nine microfacies types are identified based on the detailed carbonate microfacies analysis methods in the Lower Triassic strata of the Neijiagou section,including patched agglomerates,stromatolites,oolitic-layered,algal-crinoid grainstone,wackestone containing coated and eroded bone debris particles,oolitic grainstone,aggregate grainstone,oolitic wackestone,mudstone,micrite.Compared with the Longdongchuan Formation,the abundance and types of bioclasts in the Jinjiling Formation are less and are not evenly distributed in the strata,showing an upward trend.The main types of biological particles are bivalves,brachiopods,foraminifera and ostracodes.Based on the analysis of the microfacies characteristics of carbonate rocks in the Permian-Triassic transition in the Nijiagou section from the perspective of ecology,it is found that the paleoecology and biocommunity succession of shallow marine carbonate platform changed significantly before and after the end-Permian mass extinction,namely the normal shallow sea ecological environment with abundant biotypes at the end of Permian.It was replaced by a simple ecosystem dominated by microorganisms and containing a few endemic organisms in the Early Triassic.The characteristics of these changes are consistent with other shallow sea carbonate platform profiles of the same period in South China.The evolution of Marine ecosystem in the Niegiagou section during the Permian–Triassic transition reveals that there was a complex coupling relationship between microorganisms and metaorganisms in shallow-marine carbonate platform under the influence of environmental changes after the end-Permian mass extinction.The development of this relationship can be divided into three stages dominated by different organisms.In the first stage,before the mass extinction event,marine ecosystems were dominated by diverse epigenetic organisms.In the second stage,after the mass extinction event,the marine ecosystem was dominated by single species of microorganisms.In the third stage,Marine ecosystems are dominated by simple,widespread organisms.Combining the characteristics of each stage,we can find that the relationship between metazoan and microbe is not simply one and the other,but a dynamic equilibrium relationship of mutual influence.This paper includes 36 figures,3 tables and 194 references. |
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