Late Triassic Deep Water Trace Fossils and Their Sedimentary Environment in the Jinmuda,Rangtang,Sichuan

Key words: 
• trace fossil /
• ichnoassemblage /
• ichnofacies /
• depositiond environment /
• Late Triassic /
• Rangtang Sichuan

Abstract: The study area,Jinmuda,Rangtang county, Sichuan province is located in the southeast of Aba Mass. During the early period of Late Triassic, deep water detrital sediments, which can be divided into Zhuwo Formation and Xinduqiao Formation, deposited in this area.The area is one part of Songpan-Ganze trough. For the first time,plenty of trace fossils have been discovered in Late Triassic Zhuwo Formation and Xind uqiao Formation in Riji Gully of Jinmuda area. After determination, these trace fossils have been classified into 38 ichnogenera and 63 ichnospecies.Most of them belong to deep water trace fossil. According to their composition, the relative abundance in different stratigraphic horizons and distribution features, five ich- noassemblages have been established, namely： (1) Helminthopsis-Paleodictyon ichnoassemblage, (2) Paleodictyon-Neonereites ichnoassemblage, (3) Megagrapton-Imponoglyphus ichnoassemblage, (4) Paleodictyon -Phycosiphon ichnoassemblage, (5) Megagrapton-Helminthoida ichnoassemblage. Based on detailed study it,turns out that all these five ichnoassemblages belong to Nereites ichnofacies. In order to analyze depositional environment, habit classification, characteristics and ichnofacies of trace fossils have been adopted. This paper concludes that each ichnoassemblage represents different depositional environment,ie. the first ichnoassemblage represents bathyal middle-lower slope environment, the second repre- sents abyssal basin margin environment,the third represents bathyal lower slope environment, the fourth repre sents abyssal basin and the fifth represents bathyal lower slope environment. Therefore it could be concluded that the sea level of Late Triassic Zuwo Formation in Riji Gully of Jinmuda area occurred secondary fluctuation with the general tendency of ascent, while from the top part of Zuwo Formation to Xinduqiao Formation, sea level began to descend. Thus, early Late Triassic depositional environment in Jinmuda area experienced the evolution of lower slope-abyssal basin margin-middle-lower slope-abyssal basin-lower slope. The study of deep water trace fossil brings another evidence for clarifying sedimentary water depth and eustatic fluctuation of the study area. Besides, it also provides new evidence for restoring paleogeography of the study area. In the study area,there are many gold deposits occurred in the turbidite of Late Triassic Xinduqiao Formation. Based on the study on trace fossils, the host strata of gold deposit,Xinduqiao Formation,was fomed in lower slope. So the study of trace fossil has a certain significance for searching for gold deposit and studying the formation environment of gold deposit.