四川省壤塘县金木达晚三叠世深水沉积中的遗迹化石及沉积环境分析

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

四川壤塘金木达日基沟晚三叠世地层中首次发现了大量的遗迹化石 ,经鉴定有 38个遗迹属、6 3个遗迹种。根据不同遗迹属在不同层位的相对丰度 ,建立了 5个遗迹组合 ;全部遗迹组合均属于Nereites遗迹相。同时应用遗迹化石的生态习性分类、特征遗迹化石和遗迹相来进行沉积环境的分析 。

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) Paleo dictyon-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.