Abstract:

The depositional environment of Middle Ordovician sediments in Fuping area of the north of Weihe River has long been disputed.The evidences provided here,such as algal mat,bird,s eye structure,biodisturbation,vertical and oblique intersected burrow,dry crack,gypsum pseudocrystal etc,are the features of intertidal zone.In accordence to chemical element analysis,Fe content is much greater than that of Mn,the content of minor elements is much less than that in deeper marine facies but close to that of fresh water.This also shows that the sediments were deposited in shallow water instead of deep water.Radiolarian siliceous rocks,according to Folk,s study,can be formed in various depositional enviroments.A great deal of acidic volcanic ashes deposited in the sediments of the Middle Ordovician within Fuping area would be bound to change water medium temporarily,providing certain favourable nsconditio for radiolarian to live.In particular,the occurrence of both small amount of radiolarians associated with tubular blue algae and positive elongated chalcedonite has further proved that the radiolarian siliceous rocks were formed in shallow water instead of deep water.Based on the above mentioned,the auther suggests that Middle Ordovician sediments were deposited in the enviroment of the tidal flat on the background of the lagoon,which has the properties of both subtidal and intertidal deposits.