关于奥陶系最上部赫南特(Hirnantian)亚阶全球层型的建议

Proposal of a GSSP Candidate Section in South China for a New Hirnantian Boundary Stratotype

在对鄂西宜昌王家湾和分乡、黔北桐梓红花园、黔东北松桃陆地坪 4条剖面的相关地层和主要门类化石做了新的无间断采集和重新鉴定笔石和腕足动物这两个重要化石门类的基础上 ,发现奥陶纪最末期—志留纪最早期的完整笔石序列与腕足动物等新的化石材料 ,初步了解奥陶纪末集群灭绝后生物幸存的基本特征。经过选择 ,特推荐宜昌王家湾公路旁穿越奥陶 -志留纪界线地层剖面作为建立国际赫南特亚阶的首位候选 (leading candidate)层型 ,并推荐桐梓红花园相关剖面作为辅助候选 (auxiliary candidate)层型。对建立全球赫南特亚阶界线层型的基本条件和国外相关剖面的优缺点作了分析。本文所识别的赫南特亚阶相当于两个笔石带 :下部 N ormalograptus ex-traordinarius- N.ojsuensis带和上部 N.persculptus带。典型的 H irnantia动物群的整体地质历程为奥陶纪最末期 ,相当于 N ormalograptus extraordinarius- N.ojsuensis带和 N.persculptus带的一部分。

Graptolites, brachiopods, and other fossils were collected and identified from four continuous stratigraphical sections spanning the Ordovician Silurian boundary at Wangjiawan and Fenxiang, north of Yichang, western Hubei, Honghuayuan, south of Tongzi, northern Guizhou, and Ludiping, north of Songtao, northeastern Guizhou. The Wangjiawan section, representing a deeper water sequence with its complete graptolite zonation intercalated with the widely distributed Hirnantia fauna, is considered as leading candidate for a new Hirnantian boundary stratotype. The Honghuayuan section is proposed as an auxiliary stratotype of the Hirnantian since it represents a shallower water sequence with a well recognized graptolite zonation and the Hirnantia fauna. The Hirnantian Substage defined herein corresponds to the Normalograptus extraordinarius N. ojsuensis Zone in the lower and N. persculptus Zone in the upper. The start of the former zone is consistent with the final disappearance of mid Ashgillian shelly fauna.