四川龙门山地区早—中泥盆世混积相遗迹化石及其环境分析

摘要

龙门山地区早—中泥盆世碳酸盐岩和碎屑岩组成的混合沉积相富含大量海相遗迹化石,可识别居住迹、进食迹、觅食迹、爬行迹和停息迹5大类14属,包括居住迹:Skolithos;进食迹:Chondrites、Phycodes、Planolites、Rhizocorallium、Thalassinoides和Zoophycos;觅食迹:Cylindrichnus以及Palaeophycus;爬行迹:Cruziana和Monomorphichnus;停息迹:Asterichnus、Lockeia和Rusophycus等,这些遗迹化石分属于4个遗迹化石组合,反映了不同沉积环境,分别为:(1) Skolithos—Thalassinoides遗迹组合,沉积于高能砂质环境中,为水动力较强的无障壁滨岸前滨;(2) Rusophycus—Phycodes遗迹化石组合反映了水动力较弱的近滨环境;(3) Chondrites—Palaeophycus遗迹化石组合反映了贫氧、水动力条件较弱的近滨下部环境;(4) Zoophycos—Palaeophycus遗迹化石组合反映的环境为正常浪基面以下,风暴浪基面以上的风暴沉积环境,据此建立了龙门山地区早—中泥盆世混积相遗迹化石生活环境分布模式.龙门山地区早—中泥盆世发现的3种类型Zoophycos遗迹化石,与腕足介壳富集层、波痕、丘状和洼状交错层理等反映浅水环境的沉积特征共存,表明泥盆纪Zoophycos形成于浅水环境.%Abundant marine trace fossils are preserved in the Early-Middle Devonian strata of the Guixi-Shawozi section in Longmenshan area,Sichuan Province.According to the shapes,three types of trace fossils were identified that include Domichnia as Skolithos,Fodinichnia as Chondrites,Phycodes,Planolites,Rhizocorallium,Thalassinoides and Zoophycos;Food-seeking traces including Cylindrichnus and Palaeophycus;Repichnia as Cruziana and Monomorphichnus;Cubichnia as Asterichnus,Lockeia and Rusophycus.14 kinds in total trace fossils can be identified clearly.The ichnocoenoses represent types of Domichnia,Fodinichnia,Food-seeking traces,Repichnia and Cubichnia.Four ichnocoenoses representing different sedimentary environments were recognized as follows:(1) Skolithos-Thalassinoides ichnocoenosis can be explained as Domichnia from higher-energy foreshore;(2) Rusophycus-Phycodes ichnocoenosis mainly consists of Cubichnia and Fodichnia generated in higher-energy nearshore;(3)Chondrites-Palaeophycus ichnocoenosis is from shallow shelf,which suggests sub-oxic and lower energy;(4) Zoophycos-Palaeophycus ichnocoenosis is considered as Fodichnia from higher-energy shallow shelf between normal wave base to storm wave base.In terms of the ichnocoenoses,living environment and distribution pattern of trace fossils of hybrid facies were set up from Early-Middle Devonian strata in Longmen-shan area.Three kinds of Zoophycos fossils were observed,which were coexistent with concentrations of brachiopoda and shell,ripple marks,hummocky and swaley cross bedding,which reflects shallow environment sedimentary characteristics from Early to Middle Devonian in Longmenshan area and indicates that Zoophycos fossils were formed in a shallow environment.