再论石炭-二叠系界线及古地磁学的证据

Further study of the Carboniferous-Permian Boundary in Yunnan and its correlation

滇东南八宝附近的安乐剖面,是石炭-二叠系界线地层的典型剖面,沉积连续、单相、富含牙形刺、类、四射珊瑚和腕足类化石,以牙形刺Streptognathodus elongatus带之底作为二叠系的开始,Pseudoschwagerina uddeni带稍高于这一界线。我们建议石炭-二叠系界线必须建立在沉积连续、单相,化石类型多样、丰富的剖面上,为了尽可能接近假希瓦格亚科分子的始现界线,以牙形刺Streptognathldus elegantulus到St.elongatus演化系列中St.elongarus始现作为界线,这一界线大致相当于前苏联阿舍林阶的底界。古地磁学研究表明,该剖面的第3层底部,在长期的基亚曼极性段中存在着一个短暂的正极性亚带,它具有广泛的代表性,而且它与Streptognathodus elongatus的始现相一致,强烈支持上述生物界线作为石炭-二叠系界线。

An excellent limestone sequence, continuous from the Late Carboniferous to Early Permian, occurs in the regions of Southeastern Yunnan, Western Guangxi and Southwestern Guizhou. The Anle section, 2km south of Babao Town, is typical of the Carboniferous-Permian Boundary Stratigraphy, rich in conodonts, fusulinids, corals and brachiopods. This section is a monofacial carbonate sediment. The boundary position is based on the distribution of conodonts, but not fusulinids. It lies at the base of the Streptognathodus elongatus zone, between beds 2 and 3 in the Anle section. Pseudoschwagerina uddeni makes their first appearance somewhat above the boundary. We try to give a definition of the Carboniferous-Permian Boundary. The Boundary Position must be based on the section of continuous sediment, monofacies, rich in conodonts and fusulinds etc.. In order to approch possibly the base of the 'Schwagerina' Bed, the boundary is drawn at the first appearance of Streptognathodus elongatus. It corresponds to the base of the Babao Stage in Yunnan and the Asselian stage in former USSR. St. elongatus and Pseudoschwagerina uddeni almost coincide with this level in Yunnan and Guizhou. The former appears a little earlier than the latter in the Anle section. This boundary can be apparently accepted in China, former USSR and North America. The famours Aidaralash and Usolka sections discussed by ICPSW (1991) are not monofacial. Their sedimentary environments were not stable. It is hard to say whether the base of Sphaeroschwagerina vulgaris-S, fusiformis zone and Daixina bosbytauensis-D, robusta zone are clear. There is not an exact Boundary Point in the Aidaralash section. Perhaps it is not proper to establish the Boundary Stratotype in the Aidaralash or Usolka sections. The paleomagnetic characteristics of Anle section have been examined. There is an Orthomagnetic Subzone in the Kiaman Magnetic Interval at the base of bed 3 of the Anle section. It possesses a worldwide representative. The paleomagnetic data strongly support the base of the Streptognathodus elongatus zone as the boundary.