浙西上奥陶统于潜组Spirodesmos遗迹化石组合的环境意义

ENVIROMENTAL SIGNIFICANCE OF A Spirodesmos ICHNOFOSSIL ASSEMBLAGE FROM UPPER ORDOVICIAN YUQIAN FORMATION IN THE WESTERN ZHEJIANG PROVINCE,CHINA

在概述了浙江西部上奥陶统于潜组的沉积学特征，并进而描述了发现的１４个遗迹化石属基础上，从４个方面对Ｓｐｉｒｏｄｅｓｍｏｓ为代表的遗迹组合进行了环境分析：（１）１４个遗迹属中仅３个属为穿相型分子，其余占遗迹总属２／３以上的遗迹化石均为典型的深水型分子，它们都是Ｎｅｒｅｉｔｅｓ遗迹相的主要成员，因此该遗迹组合可归属于深海Ｎｅｒｅｉｔｅｓ遗迹相中；（２）大部分遗迹化石呈复杂规则弯曲的形态，这种形态的遗迹化石是深海环境的遗迹化石标志；（３）１４个遗迹属均为觅食迹和牧食迹，它们的行为习性，尤其是牧食迹主要分布于深海环境；（４）多数遗迹属的水深分布均在２００ｍ以下。由此可认为Ｓｐｉｒｏｄｅｓｍｏｓ遗迹组合是半深海－深海环境的产物。

The Upper Ordovician Yuqian Formation, a rhythmic unit composed of silty to fine sandstones and mudstones with different thickness, is widely developed in the aera from Lingan to Jiande in western Zhejiang Province. The thick terrigenous clastic rock series clearly characterizes turbidity deposits in which lenticular or wavy beddings occasionly occour. On the Te surface or under the Ta bottom surface of the Bouma sequence,abyssal ichnofossils of Nereites ichnofacies appear. Graptolites shales are commonly intercalated in the Te member. Based on the facts above, the formation is considered as a flysch facies. Fourteen ichnospecies were found in the facies. They are Chondrites, Granularia, Helminthoida, Helminthopsis, Megagrapton, Paleodictyon, Paleomeandron, Planolites, Protopaleodictyon, Spirodesmos, Subphyllochorda, Subularia, Tuberculichnus and Urohelminthoida. The Spirodesmos ichnogenuses are abundant in the ichnocoehose, so the ichnofossil assemblage can be named a Spirodesmos one. Besides, because the ichnofossils above are usual in the Nereites ichnofacies, the Spirodesmos assemblage can be included in it.The enviroment of the assemblage is investigated in terms of the following four aspects:1. A Nereites ichnofacies is generally thought as a common ichnofacies in a bathyal to abyssal area where turbidity currents can flow.2. Fourteen ichnofossils are predominant in winding, meandering and network shapes, which characterizes shapes of abyssal ichnofossils.3. Fourteen ichnofossils are superior in ethology of Fodichnia, especilly Pasichnia which are common in a abyssal environment.4. According to water depths at which the ichnofossils were formed, the fourteen ichnogenuses were mainly generated at the depths less than 200m below sea level.In light of the analysis above, it can be concluded that the Spirodesmos assemblage is an abyssal indicator.