渭北奥陶系的放射虫燧石岩

Bedded Radiolarian Cherts in the Ordovician of Weibei, Shaanxi

渭北奥陶系的放射虫燧石岩是在稳定的华北地台上形成的远洋沉积.它们产在赵老峪组深水碳酸盐地层的下部,并以层位稳定、纹层发育、富含放射虫骨骼、成岩交代组构清楚,以及在剖面上与薄层的硅质页岩呈韵律互层等为等征,表明是深水的放射虫软泥在成岩作用早期由生物蛋白石经溶解-再沉淀反应快速转变而成.这种产在地台上的远洋沉积虽不多见,却是海平面大规模上升及其伴生的重大地质事件的反映.渭北放射虫燧石岩的时代相当于中奥陶世卡拉道克期.当时正值全球性海平面上升,秦岭古海盆也在发生强烈扩张与俯冲,因此,华北地台南缘下沉,使渭北地区变成了弧后深水盆地的北部边缘.当时研究区的古地理位置也恰好处于低纬度的赤道附近.这些都为放射虫燧石岩的堆积提供了有利的条件.由于当时钙质浮游生物尚未大量兴起,所以其沉积作用不受CCD的控制,水深较之现代的类似物可能要浅得多.

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.