EPIPHYTON BOUNDSTONE AND PALAEOGEOGRAPHY OF THE MIDDLE CAMBRIAN ZHANGXIA FORMATION IN THE EAST OF NORTH CHINA PLATFORM

Abstract: Epiphyton boundstones are widely distributed in Zhangxia Formation of the Middle Cambrian in the middle of Shandong Province, Fuxian of Liaoning Province, Hunjiang City of Jilin Province and Suxian of Anhui Province. The Epiphyton boundstones exposed in the field are light grey massive limestone. The Epiphyton boundstones are mainly composed of Epiphyton, and the boundstone has not laminated structure. Therefore, the Epiphyton boundstones not only differ from stromatolithic boundstone, but also differ from thrombolitic algal boundstone. The Epiphyton boundstones are pure limestones, containing less than 2% of insoluble residue and less than 0.5% of MgO. The characters of chemical composition in the Epiphyton boundstones have demonstrated that Epiphyton had grown and fliourished in clear water at that geological time. In this area, Epiphyton boundstones occur as three forms: domed bioherm, tabular biostrome and medium to thick-bedded boundstone. The different forms of Epiphyton boundstone and their lithologic type assemblages suggest that Epiphyton boundstone develope in different depositional environments. Domed Epiphyton bioherms are mainly distributed in the upper part of Zhangxia Formation at Xiwangzhuang of Yiyuan County, Shandong Province, Mopan Hill of Fuxian, Liaoning Province and in the middle part of Zhangxia Formation of Changxing Island. The domed Epiphytos bioherm complex interbedded with oosparites in the upper part of Zhangxia Formation indicates that domed bioherm complex was formed in shelf edge shoal. Therefore, the sedimentary assemblages of domed Epiphyton bioherm complex and oosparites probably represented as shelf margin shoal-reef facies passing eastwards into outer shelf. The tabular Epiphyton biostroms are distributed in Jiulong Mountain of Laiwu County, where it spreads laterally over long distances. The tabular Epiphyton biostroms constitute cyclic repetition of beds within the section, each cyclic bed comprising oomicrites at the base, followed by massive Epiphyton boundstone (1-2.5m in thickness) grading upwards into calcareous shales, worm trace micrites, irregular nodular micrites. This sedimentary sequence seems to represent a back shoal environment. The medium to thick-bedded Epiphyton boundstones are distributed in Gushang of Changqing County, where they are interbedded with skeletal micrites, dolomttled micrites containing Epiphyton and Girvanella fragments. This sequence shows that the medium to thick-bedded Epiphyton boundstones was formed in inner shelf lagoon environment. As mentioned-above, the geographical distribution of the Epiphyton boundstones from Changqing County eastward to Juxian County of Shandong Province suggests that a model of depositional facies be divided into four facies belts from west to east: inner shelf lagoon, back shoal, shelf margin shoal-reef and outer shelf during Late Zhangxia time. Whilst depositional environments of the Early to Middle Zhangxia Age are shelf margin shoal and outer shelf at Juxian, Yishui County and Fuxian. From it one might infer that in the eastern reigon to Juxian, Shandong Province and Fuxian, Liaoning Province was not an erosional old land (Jiao-Liao old land) in reality as suggested before, but an open outer continental shelf during the Middle Cambrian Zhangxia Age.