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.
Jin Huijuan,Li Yuci. ENVIROMENTAL SIGNIFICANCE OF A Spirodesmos ICHNOFOSSIL ASSEMBLAGE FROM UPPER ORDOVICIAN YUQIAN FORMATION IN THE WESTERN ZHEJIANG PROVINCE, CHINA[J]. Chinese Journal of Geology, 1998, (3): 282-289.