“赫南特阶”和奥陶系—志留系界线的厘定

THE "HIRNANTIAN STAGE" AND THE IDENTIFICATION OF THE ORDOVICIAN/SILURIAN BOUNDARY

根据对湖北宜昌王家湾、黄花场、分乡以及新近发现的远安苟家垭奥陶系与志留系界线剖面生物地层、层序地层和化学地层的综合研究将该区最晚奥陶纪到最早志留纪的化石带自下而上重新厘定为Nankinolithus带,Dicellograptus complexus带,Paraorthograptus pacificus带,Nor-malograptus bohemicus-N.extraordinarius带,Hirnantia-Kinnella带,Glyptograptus? persculp—tus带,Akidograptus ascensus带,Parakidograptus acuminatus带和Orthograptus vesiculosus带,其中Paraorthograptus pacificus带可分为Pararetiograptus sinensis亚带,Tangyagraptus typicuas和Diceratograptus mirus亚带。指出目前尚无可靠证据证明国际奥陶系与志留系界线层型剖面,即英格兰Dob’s Linn剖面P. acuminatus带底部与该种首次出现（FAD）的层位一致。建议将英国Ashipill统改为阶,内部分两个亚阶,其上部“Hirnantian”亚阶和O/S界线应分别下移到P.pacificus带底部和G.?persculptus带底界。前者的底界与晚奥陶世Nankinolithus带顶部海退事件（NRE）后海侵所形成的最大海泛面的位置基本一致;后者与古冈瓦纳超大陆中部冰盖快速消融所引起的全球海退/海侵时间（HRE）和铱、碳同位素异常以及海洋及大气地球化学状况的突然改变相吻合,?

Through the study on the bio-stratigraphy, sequence and chemo-stratigraphy from the Or-dovician/Silurian boundary section in Wangjiawan, Huanghuachang, Fengxiang of Yichang County and the newly founded one in Goujiaya of Yuan 'an County, the fossil zones of the upper-most Ordovician and the lowermost Silurian are securely re-established in ascending order; Nankinolithus zone, Dicellograptus complexus zone, Paraorthograptus pacificus zone, Nor-malograptus bohemicus-N. extraordinarius zone, Hirnantia-Kinnella zone, Glyptograptus? persculptus zone, Akidograptus ascensus zone, Parakidograptus acuminatus zone and Or-thograptus vesiculosus zone. Moreover, the Paraorthograptus pacificus zone could be further divided into Pararetiograptus sinensis sub-zone, Tangyagraptus typicuas and Diceratograptus mirus sub-zone. Further more, the paper points out, that, no hard evidences can prove consistent between the basement strata of the P. acuminatus zone from the Dob's Linn Section of the International Typical Boundary Section of Ordovician/Silurian in England and the strata in which the FAD firstly appeared in this area. Therefore, it could be suggested that modifying Ashigill Series to Ashigill Stage contained two sub-stages, and the upper 'Hirnantian Sub-stage' and 'Ordovician/Silurian Boundary' are better shifted, respectively, downward to the bottom of P. pacificus zone and G.? persculptus zone. The basic boundary of the Hirnantian sub-stage is approximately identical to the locality of the maximum flooding surface which was formed during transgression period after regression (NRE) of uppermost part in Late Ordovician Nankinolithus zone. However, the O/S boundary consists with the global transgression/regression time (HRE) which was directly caused by the rapid icecap melting in the central super-continent Gondwana. It was also i-dentical to the iridium and carbon isotope anomaly, to the sharply variety of geo-chemical phase of atmosphere and ocean, and, which caused the mass extinction could be considered to be related to the sudden rising of CO2 components. Therefore, the Section in Wangjiawan of Yichang should be capable of the best selection one for the identification global Ordovician/Silurian boundary sequence and the newly redefined 'Hirnantian Sub-stage'.