华北地台东部中寒武世张夏期附枝藻(Epiphyton)灰岩及古地理

近年来,笔者在研究华北地台东部地区寒武纪沉积环境的野外考察中,发现本区中寒武统张夏组碳酸盐岩地层中发育有厚层块状附枝藻灰岩。它主要分布于山东省中部地区,辽宁省复县,吉林省浑江市和安徽省宿县等地的张夏组下部和上部。在长清县崮山镇,泗水县和辽宁省复县长兴岛、磨盘山一带张夏组中部也有分布。 附枝藻灰岩在苏联、美国、加拿大、南极洲的寒武纪地层中均有分布。在我国,除华北地台东部张夏期碳酸盐岩地层中有分布外,杨子地台边缘的黔东、湘西一带下寒武统清虚洞组和中寒武统也有发育。由于该类藻灰岩分布广泛,引人注目,具一定的古地理环境意义。本文主要叙述山东省长清县以东地区以及辽宁省复县一带的附枝藻灰岩特征和古地理环境意义。

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

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.