新疆伊犁晚二叠世Ophiomorpha的环境意义

The Environmental Implication of the Trace Fossil Ophiomorpha from the Upper Permian of the Yily Basin, Xinjiang

新疆伊犁盆地上二叠统主要为一套湖相碎屑岩夹碳酸盐沉积，厚约２７００ｍ，在晓山萨依组下部灰岩夹层中产丰富的Ｏｐｈｉｏｍｏｒｐｈａｃｆ．ｎｏｄｏｓａ遗迹化石。标本由一系列代表不同阶段个体所留下的粗细不同的潜穴组成。潜穴具一个明显的加厚的、由内外两层粪瘤构成的衬壁构造。潜穴呈简单直管状或盲肠状，不分叉，以水平和斜伸类型为主；内部充填具粒序性，说明沉积速度较快，且可作为良好的地层示顶构造。这些特征说明沉积基地的沉积速度较快，松软。Ｏｐｈｉｏｍｏｒｐｈａ分布特征表明这种造迹生物的掘穴主要与水体的含氧性和沉积基地的性质有关，而与基地成分、水体含盐度、水深等无明显的关系。

Ophiomorpha is amply produced in the Lower part of Xiaoshansayi Formation which consists of thin-bedded limestones interbedded with massive sandstones.Graded pebble-bearing sandstonaes,cross-bedded sandstones and rippled fine sandstones often constitute cyclic fining-upward deposits in section,showing mountainy river fan deposits.The limestone usually contains sands,bioclastics and mud with few plant and freshwater bivalve fossils in them revealing shallow lacustrine deposits.It is absent in black laminar limestone and shales of Tiemulike Formation and Bakaluesayi Formation because this lithology represents an exaerobic condition. Ophiomorpha exists in limestone beds without laminae in them,preserved as a set of different burrows in diameter from 0.6 to 1.6 cm,probably representing developing stages of the burrow-maker.The burrow wall,0.5～2.5 cm in thickness,is made up of dark or black pellets ovoid,or irregular in shape,but it is conspicuously double-layered.Their maximum length observed in the section can reach 35 cm .They are tube-like or caecum-like,predominantly horizontal and oblique in level,neither showing divergent branching pattern of typical Ophiomorpha So,they are similar to Ophiomorpha nodosa Lundgren in size and pellet wall,but different in wall texture and branching pattern.Infill is passive and its composition is like country rocks. Ophiomorpha was once accepted as a marine indicator(Kennedy and Sellwood,1970),and afterwards documented in continental strata(Stewart,1978;Bown,1982; Merill,1984).But Goldering and Pollard (1995) thought that the continental appearance of Ophiomorpha is questionable.Samples in this study are collected in absolutely lacustrine sedimentary rocks,This supports the point of view that Ophiomorpha can occur in nonmarine environments.Some samples possess upward grading fill (Plate I-E),reflecting a rapid depositional condition. It could be also used as top-indicating structure.Above characteristics demonstrate that the burrow-maker is inclined to penetrate soft sedimentary floor where there are relatively a high rate of deposition and a high level of oxygen content,and they are scarcely controlled by composition of the substrate,water-depth and salinity.