The Study Of Devonian Trace Fossils And Its Environmental Significance From Longmenshan Area,Northern Sichuan

Abundant marine trace fossils were preserved in the Devonian strata of the Guixi-Shawozi section from Longmenshan area,Sichuan Province.According to the shapes,five types of trace fossils which are identified include Domichnia as Arenicolites,Diplocraterion and Skolithos,Fodinichnia as Chondrites,Phycodes,Planolites,Rhizocorallium,Thalassinoides and Zoophycos;Food-seeking traces including Cylindrichnus and Palaeophycus;Repichnia as Cruziana,Diplichnites and Monomorphichnus;Cubichnia as Asterichnus,Lockeia and Rusophycus.17 kinds in total trace fossils can be identified clearly.The ichnocoenoses represent types of Domichnia,Fodinichnia,Food-seeking traces,Repichnia and Cubichnia.Five ichnocoenoses represent different sedimentary environments which have been recognized as follows: 1)Arenicolites—Diplocraterion ichnocoenosis was formed in ample food intertidal zone,which was influenced by waves;2)Chondrites— Cruziana ichnocoenosis indicated shallow shelf which represented lower water oxygen level and lower water energy deposit environment;3)Zoophycos—Chondrites ichnocoenosis was considered as Fodichnia from higher-energy shallow shelf between normal wave base and storm wave base;4)Rusophycus—Phycodes ichnocoenosis mainly consists of Cubichnia and Fodichnia generated in higher-energy nearshore;5)Skolithos-Thalassinoides ichnocoenosis can be explained as Domichnia from higher-energy foreshore.In terms of these ichnocoenoses,their living environments and distribution pattern of trace fossils of hybrid facies were set up from Devonianin Longmenshan area.Trace maker behaviors were analyzed from each ichnofacies,combined with thei deposit surrounding,further explained,the link between trace maker behavior pattern and sedimentary environment.Furthermore,the Chondrites surrounding rock samples were collected from condensed section and tempestite,then obtained microelement data,using methods by U/Th?Ni/Co?V/Cr and V/(V+Ni),reduzated Chondrites trace maker living environment and palaeo oxygenation facies,reduzated paleo water depth by V/Ni,then discussed its palaeoenvironment and living habits.Three kinds of Zoophycos fossils were observed,coexisted with concentration of brachiopoda and shell,ripple marks,hummocky and swaley cross bedding,which reflects shallow environment sedimentary characteristics from Devonian in Longmenshan area,indicating that Zoophycos fossils were formed from shallow environment.A further understanding for Zoophycos ichnofacies can be made by this phenomenon.