贺兰山中元古代三个叠层石组合及其地层意义

贺兰山原划归蓟县系王全口群中、下部的碳酸盐岩中新建立了两个叠层石组合 ,即下部以 Colonnella sp.、Gaoyuzhuangia sp.、Conophyton garganicum、C.cf.cylindricum等锥叠层石和块茎状柱叠层石为代表的闵家沟组合 ,其属种和总体面貌与长城系高于庄叠层石组合较为相似 ;中部以 L ochmecolumella和 Pseudogymnosolen等微小类型叠层石为代表的冰沟叠层石组合 ,其特征与蓟县系雾迷山叠层石组合完全一致。同时通过对王全口组叠层石分子的全面分析 ,判定王全口叠层石组合的时代应是蓟县纪中、晚期 ,其底界不低于雾迷山组底部。据此 ,重新厘定了贺兰山地区中元古代地层划分。

THREE MESOPROTEROZOIC STROMATOLITE ASSEMBLAGES IN THE HELAN MT. , NINGXIA AND THEIR BIOSTRATIGRAPHIC SIGINIFICANCE

The widely distributed Mesoproterozoic strata in Helan Mt. can be subdivided into two distinct sedimentary sequences, namely the lower clastic rocks intercalated with rare carbonate rock (the so-called Huangqikou Formation of Changcheng System), and the upper carbonate sedimentary sequence, mainly composed of siliceous ribbon dolomite and limestone (i.e the Wangquankou Formation of Jixian System). For the first time, two stromatolite assemblages were found in the lower part of the former Wangquankou Formation. The lower stromatolite assemblage contains diverse conical and bulbous column stromatolites, represented by Colonnella sp., Gaoyuzhuangia sp., Conophyton garganicum, C. cf. cylindricum etc., and is comparable with Gaoyuzhuang Stromatolite Assebmlage in the Jixian Section, and is also well conformable to the Minjiagou Assemblage in South Ningxia. Therefore, the lower part of the primary Wangquankou Formation is ascribed to the Minjiagou Formation of Changcheng System. The upper assemblage, however, is mainly made up of ministromatolites, such as Pseudogymnosolen shisanlingense, P. gratiose, Lochmecolumella gracilis, Straticonophyton cuihuashanense etc., and is principally constricted to the Lower Jixian System(1 400-1 200Ma) and widespread in many regions of North China, including the Yanshan Range, Yinshan Mountain and Xiaoqinling district. Therefore, the ministromatolite-bearing stratum in this area can be equated fairly well with the Wumishan Formation of Jixian System. Occurring in the upper part of the Wangquankou Formation, the third stromatolite assemblage in this area contains chiefly Baicalia minuta, B. capricornia, B. cf. baicalica, Conophyton sp., C.lituus, C. pinluoensis, Tungussia sp., as well as Jacutophyton cf. luonanensis, and showing some similarities to the Tieling and Fengjiawan Stromatolite Assemblages from Yanshan area and southern margin of the North China Platform respectively and is therefore of middle and late Jinxian age.