西秦岭二叠纪－三叠纪遗迹化石及其环境意义

TRACE FOSSILS AND THEIR ENVIRONMENTAL SIGNIFICANCE OF PERMIAN-TRIASSIC,WESTERN QINLING MOUNTAINS

本文系统描述和研究了西秦岭合作至夏河间晚二叠世和临潭至岷县以南早、中三叠世复理石相中发现的遗迹化石，并得出以下认识：１）各遗迹组合都属与浊流有关的深海“Ｎｅｒｅｉｔｅｓ”遗迹相，都不具远洋沉积中遗迹组合特征；２）各遗迹组合所指示的海水深度都介于深水斜坡环境的中等水深范围内，最深不超过２０００ｍ；３）早三叠世遗迹化石潜穴管普遍细于中三叠世，表明前者属水体循环差的缺氧环境，后者为开放型海盆；４）根据遗迹组合空间分布的差异，认为在三叠纪时北特提斯北缘的深水斜坡带的海水深度呈现出西深东浅的变化趋势。

A systematic description and study of trace fossils that were found in Late Permian flysch factes in the region of Hezuo-Xiahe and Early to Middle Triassic flysch deposits, to the south of Lintan-Minxian are presented in this paper. Six ichnogenera, including two 'facies crossing' ichnogenera (Neonereites and Scolicia) and four 'deep water' ichnogenera (Cosmorhaphe, Helminthoida, Helminthopsis and Megagrapton) were found in the late Permian. Six ichnogenera were found in the Early Triassic deposits that include four 'facies crossing' ichnogenera (Chondrites, Glocheria, Lennea and Palaeophycus) and two 'deep water' ichnogenera (Megagrapton and Protopaleodicton), and ten were found in the Middle Triassic formations that composed of five 'factes crossing' ichnogenera (Chondrites, Dictyodora, Gordia, Neonerites and Fucusopsis) and five 'deep water: ichnogenera (Helminthoida, Lophoctenum.Megagrapton, Nereites and Paleodicton).The following conclusions are obtained through the comprehensive studies of the assemblages of the trace fossils:1. All of the trace fossil assemblages are of the abyssal 'Nereites' ichnofacies that related to the turbidite, no pelagic sediments-related assemblages were found.2. The assemblages of trace fossils indicated that the water depths are of profundal clinoform to medium deep sea with in the limit of 2000 m.3. Tee diameters of the domichnia that developed in the Early Triassic are generally thinner than those of the Middle Triassic for the reason that the former were formed under anoxic water body with a bad circulation, while the later were formed in an open marine basin.4. The spatial distribution of the trace fossil assemblages suggested that the northern margin of North Tethys belonged to profundal clinoform during Triassic,and there was a tendency of shallowing up from the west to the east.