Trace Fossils And Pyrite Formboids Of The Dalong Formation In Guangyuan Shangsi,Sichuan And Their Implications For The Palaeooceanic Redox Condition

The Shangsi section is considered as one of the famous Permian-Triassic Boundary(PTB)section in the western of Yangtze platform.Trace fossils enriched in the top siliceous rocks and siliceous limestone of the Dalong Formation are taken as the entry point.By using the method of geobiology,the most topmost trace genera and species,ichnofabric,and pyrite framboids rich in trace burrows and surrounding rocks of the Dalong Formation are described and analyzed in detail,achieved the mainly points as follows:(1)Abundant trace fossils in marine siliceous rocks at the top of Lopingian Dalong Formation in Guangyuan,Sichuan Province.The ichnoassemblages represent types of domichnia and fodinichnia,which consist of 5 ichnogenera and 7 ichnospecies of Chondrites targionii,Chondrites isp.,Paleophylcus isp.,Planolites montanus,Planolites isp.,Thalassinoides isp.and Zoophycos isp..From the upper part of Dalong Formation of the Upper Permian to the bottom of the Feixianguan Formation of Lower Triassic,the diversity of trace fossils in different beds shows a decreasing trend until its disappear.This may be due to the deterioration of marine ecological environment at the end of the Late Permian,which is not conducive to the survival of benthic tracemakers.(2)According to the trace fossils distribution and assembly features,three ichnofabrics were established: Chondrites-Planolites ichnofabric,representing relatively stable deep-water hypoxic and hypoxic environment;Planolites-Thalassinoides ichnofabric,which are formed in relatively oxygen-rich environment of sedimentary bottom water;and Planolites-Zoophycos ichnofabric,which are developed in oxygen-poor or relatively anoxic environment.(3)The pyrite framboids in the top layers of the Lopingian Dalong Formation is mainly biogenic,while in a few layers there are non-biogenic pyrites.The majority of the framboids pyrites in the burrows of trace fossils are biogenic,indicating that microorganisms are involved in the process of marine benthos burrowing.The pyrite framboids in the whole section is small in size,abundant in distribution,but the average size of the pyrite framboids in some bedsets is larger,suggesting that the marine environment of the study section are not continuous anoxic,but intermittent strong reductive sedimentary environment and sulphate-rich hydrostatic water and oxygen-rich or oxygen-poor sedimentary environment.(4)The features of trace fossils,geochemistry and pyrite framboids in different beds at the top of the Dalong Formation in the Shangsi section,Sichuan Province,suggest that the ocean in the late Late permian was not characterized by persistent anoxia,which generally reflects the periodic fluctuation of ocean oxygen content,and may be caused by the same-cycle climate change due to Milankovitch Cycle.The temperature of anoxic bottom water and aerobic pore water as well as the supply of organic matter are affected,and the periodic trace fossil assemblages and distribution featuress of pyrite framboids are formed.