Evolution Of The Early Middle Triassic Terrestrial Benthic Ecosystem In Dengfeng, Henan Province

The Triassic strata in Dengfeng area of western Henan province well exposed and abundant in trace fossils.From Early Triassic to early Middle Triassic,they include Liujiagou Formation,Heshanggou Formation and Ermaying Formation.Using the methods of ichnography,sedimentology and geochemistry,this paper studies the sedimentary environment,ichnoassemblage and ecological spatial evolution of trace fossils in detail in the middle and lower members of Ermaying Formation in this area.Combined with the ecological indicators of the Early Triassic Liujiagou Formation and Heshanggou Formation,the spatial evolution characteristics of the ecosystem in the Early-Middle Triassic are established.Comparing the results of north China strata with marine strata in the same period,we can further explore the evolution and recovery of terrestrial ecosystem after the Permian mass extinction.The current conclusions are as follows:1)Sedimentary environment and geochemical characteristics: Based on the survey of field outcrop,combined with petrography,particle analysis and geochemical analysis,it is concluded that the middle and lower members of Ermaying Formation is a set of fluvial-lacustrine deposits with freshwater and most of oxidizing environment,and the intensity of chemical weathering is low to medium.Furthermore,the paleoclimate in the area was mainly warm and humid.These results would provide some evidences for restoring the evolution process of paleoenvironment in this period.2)Ichnology: 6 ichnogenera and 12 ichnospecies have been identified in the lowermiddle Members of Ermaying Formation in the Baiping section.According to the distribution of ichnofossils,4 ichnoassemblages have been identified,including:(1)Planolites ichnoassemblage represents the floodplain sedimentary environment.(2)Skolithos ichnoassemblage represents the river with soft bottom and fine sand and medium hydraulic energy or shore-shallow lake sedimentary environment;(3)Palaeophycus ichnoassemblage represents embankment and flood plain sedimentary environment;(4)Beaconites-Scoyenia ichnoassemblage represents the embankment sedimentary environment.Through the analysis of ichnoassemblages and ichnology in sedimentary period,they provide a reference for the study of biological recovery of continental strata after the mass extinction.3)Trace fossils and ecosystem evolution: According to the ecological indicators of trace fossils in the Early-Middle Triassic Liujiagou Formation,Heshanggou Formation and Ermaying Formation.it generally presents: The increasing trends of burrow depth;The bioturbation index shows an increasing trend and then tends to be stable;The diversity of trace fossils increases significantly.Trace fossils are the direct evidence of benthic organisms activities,which can provide a basis for studying the evolution of benthic ecosystem engineers during the process of extinction,survival and recovery in the Early-Middle Triassic and evaluating their ecosystem engineering impacts.In the ecological ecospace occupation and ecosystem engineering model of Dengfeng area in the Early-Middle Triassic,trace fossils mostly occupied the surficial tier,semi-infaunal tier,shallow infaunal tier,intermediate and deep infaunal tier;The behaviors of ecosystem engineering were diffusion disturbance backfill,transporter compaction and reconstruction compaction.From bottom to top,the number of trace makers in the ecospace occupation and ecosystem engineering model showed a gradual growth trend,indicating that the communites of trace makers were in the stage of gradual recovery.4)Early-Middle Triassic continental ecosystem evolution: According to the ecological indicators of Early-Middle Triassic trace fossils in Dengfeng,western Henan,this paper studies the spatial evolution characteristics of ecosystem,and further explores the evolution and recovery of terrestrial ecosystem after Permian mass extinction by comparing the results of continental strata with marine strata in the same period.We conclude that the terrestrial benthic ecosystem could be basically restored in the Middle Triassic Ermaying Formation in the study area.The basic structure of terrestrial benthic ecosystem mainly include primary producers,primary consumers and secondary consumers.Worms and small arthropods constitute the basic structure of terrestrial benthic ecosystem.It is speculated that after the extinction event at the turn of Permian Triassic,the recovery of continental ecosystem in western Henan has been basically carried out in the Middle Triassic.