Evolutionary Characteristics And Ecological Significance Of Trace Fossils Of The Lower-Middle Members Of The Middle Triassic Ermaying Formation In Jiyuan,Western Henan Province

The Triassic strata are fully developed and well-preserved in Jiyuan basin,situated in the southern margin of North China plate.Abundant trace fossils are found in the Middle Triassic Ermaying Formation and has become good materials to study the evolution of paleoenvironments,biological recovery and ecosystem reorganization after the Permian-Triassic Mass Extinction,PTME.Taking lower-middle members of the Middle Triassic Ermaying Formation in Jiyuan area of North China as the research object,in the study area,we reconstruct the paleoenvironment and paleoclimate in the early Middle Triassic,using petrographic,sedimentological,geochemical methods.The ecological space occupation model and ecosystem engineering model are used to explore the process and stage of early Middle Triassic biological recovery.It is the further supplement the data of early Middle Triassic biological recovery in western Henan Province,trying to restore the structure of palaeoecosystem food web in this sedimentary period.The current conclusions are as follows:(1)The lower-middle members of the Middle Triassic Ermaying Formation in Jiyuan area,western Henan Province is dominated by fluvial-lacustrine facies deposition.From bottom to top,braided stream-meandering river-lacustrine deposits were formed successively.It can be divided into six sedimentary subfacies: meandering channel subfacies,embankment subfacies,overbank subfacies,braided channel subfacies,floodplain subfacies and shore lacuetrine subfacies.These sediment deposited in the oxygen-rich,fresh water environments,but several strata show brackish water state for the evaporation is greater than the precipitation in a short time.The palaeoclimate shows the trend of warm and humid-dry-warm and humid,characterized by semi-humid semiarid with low-medium chemical weathering intensity.(2)There are 9 ichnospecies,belonging to 7 ichnogenera,of trace fossils in the study area,including Skolithos linearis,Palaeophycus tubularis,Palaeophycus alternatus,Scoyenia gracilis,Planolites beverleyensis,Planolites montanus,Taenidium barretti,Didymaulichnus roualti and Rhizoliths.Basing on analyze the distribution characteristics of trace fossils,five ichnofabrics are identified,including:(1)Skolithos ichnofabric represents the fluvial or shore lacuetrine sedimentary environment with mud,fine sand deposits;(2)Palaeophycus-Planolites ichnofabric appears in the low-medium hydropower fluvial embankment or lacustrine mudflat deposits;(3)Scoyenia ichnofabric forms in the periodically submerged flood plain;(4)Taenidium ichnofabric emerges from the flood plain environment under semi-arid climate;(5)Rhizoliths ichnofabric preserves in the embank or overbank environments under semi-arid climate conditions.(3)Trace fossils are relatively developed in the lower-middle members of the Ermaying Formation.Skolithos and Palaeophycus are dominent in the lower Members.It is relatively small but abundant,and the biological disturbance index is mostly 2 ～ 4.Upward,trace fossils is significantly affected by the sedimentary environment,the intensity of biological disturbance is reduced,and Planolites occupy a dominant position.Trace makers show strong ability to develop and utilize ecological space in the study area.Diffusion disturbance-backfill,diffusion disturbance-compression and regeneration-compression are common ecosystem engineering behaviors and high-impact ecosystem engineering behaviors are absent in the Ermaying Formation.The biological recovery after PTME in Jiyuan area follows the model of extinction-survival-recovery.The terrestrial benthic ecosystem was basically restored and the biological recovery has been basically realized in the early Annian age.(4)The ecosystem components mainly include primary producers,primary consumers and secondary consumers in the early and middle Annian age in Jiyuan area,western Henan Province.Plants,phylum platyhelminthes,molluscs,annelids and small arthropods constitute the basic structure of terrestrial benthic ecosystem in this sedimentary period.The absence of benthic fishs and macrovertebrates may reflect that the terrestrial ecosystems is still fragile in this sedimentary period.Body fossils are still needed to support and further supplement in the study about terrestrial ecosystem.