Bedded Radiolarian Cherts in the Ordovician of Weibei, Shaanxi

The radiolarian cherts in the Wenbei region of Shaanxi Province are a formation of pelagic sediments formed within the North China Platform. These radiolarian cherts are black in color, composed mainly of chalcedony, abound in "ghosts" or netted skeletons of radiolarians and show well-devoloped liminations. They contain some interbedded thin siliceous shales, and in stratigraphic section are associated with turbidite limestones and tuffs. The well visible relict organic texture and diagenetic replacement fabrics in the cherts suggest that the rocks are chiefly derived from biogenic opals by dissolution-reprecipitation processes during the early diagenetic stage, while free SiO2 released by volcanic ash constitutes a very limited content. In terms of age, the radiolarian cherts in the Weibei region can be correlated to the Caradocian stage of the Middle Ordvician. This corresponds to a time when there was a rise of the global sea level, and when the ancient Qinling ocean basin was undergonig vigorous spreading and subduction. The subsidence occurred in the southern margin of the North China Platform made a deep-water back-arc basin on its northern margin which lay in the Weibei region. During this period, the study area was located right within low latitudes of the eguator. A broad shallow water carbonate platform which existed to the north of North China impeded effectively the introduction of terrigenous clastic sediments into the basin from the north. The widespreded bedded cherts in the Qinling region indicates that the open sea was a zone of up welling ocean currents characterized by high planktonic organism productivity. All these factors were favourable conditons for the deposition of radiolarian cherts. Becaues calcareous planktonic organisms had not yet flourished at that time, radiolatian chert deposition might not have been controled by CCD, and the water depth probably was much shallower than that of their modern analogue.