河北唐山达瑞威尔阶(Darriwilian)牙形刺生物地层的新认识

A REVISION OF THE DARRIWILIAN BIOSTRATIGRAPHIC CONODONT ZONATION IN TANGSHAN,HEBEI PROVINCE BASED ON NEW CONODONT COLLECTIONS

本文作者对河北唐山赵各庄剖面的北庵庄组(原称下马家沟组)和马家沟组(原称上马家沟组)较系统地采集了牙形刺样品,通过对所获之牙形刺标本和安太库等(1983)在同一剖面所描述的牙形刺动物群仔细研究,得到一些新的认识、分带及对比意见。唐山赵各庄剖面不仅具有丰富各异的地方型分子,如Aurilobodus leptosomatus,Loxodus dissectus,Tangshanodus tangshanensis,Rhipidognathus maggolensis,R.laiwuensis,Triangulodus changshanensis和Erraticodon tangshanensis外,尚有一些分布广泛和具地层意义的重要分子,如Histiodella holodentata,Eoplacognathus pseudoplanus和Eoplacognathus suecicus等。根据这些重要分子和牙形刺分带原则,本文对安太庠等(1983)的牙形刺分带重新修订,即将北庵庄组的Aurilobodus leptosomatus-Loxodus dissectus带和Tangshanodus tangshanensis带修正为Histiodella holodentata-Tangshanodus tangshanensis带,并由下而上可划分为Rhipidognathus maggolensis和R.laiwuensis两地方型亚带;将马家沟组的Eoplacognathus suecicus-Acontiodus?linxiensis带和Plectodina onychodonta带修正为Eoplacognathus suecicus带,并由下而上又可划分为Acontiodus?linxiensis和Plectodina onychodonta两个地方型牙形刺亚带。因此,唐山地区的北庵庄组产牙形刺的部分和马家沟组可与华南的牯牛潭组和新疆塔里木盆地的大湾沟组对比,归入奥陶纪达瑞威尔阶(Darriwilian),从而改变了过去将北庵庄组产牙形刺的层位,即所谓的Aurilobodus leptosomatus-Loxodus dissectus带和Tangshanodus tangshanensis带与华南区大坪阶(Dapingian)的大湾组相对比的认识。

The present study is based on samples from the Beianzhuang and Machiakou formations at Zha- ogezhuang, Tangshan in Hebei. The rich conodont faunas of these units were previously studied by An et al. （1983）. Our samples were very productive and yielded numerous specimens of not only the same endemic species as reported by An et al. （1983） such as Aurilobodus leptosomatus , Loxodus dissectus , Tangshanodus tangshanensis , Nhipidognathus rnaggolensis , R. laiwuensis, Triangulodus changshanensis , and Erraticodon tangshanensis, but also more widely distributed species, such as Histiodella holodentata, Eoplacognathus pseudoplanus and Eoplacognathus suecicus. An et al. （1983） subdivided the Ordovician Lower Machiakou Formation （= Beianzhuang Formation of this paper） into the Aurilobodus leptoso- matus-Loxodus dissectus Zone and the Tangshanodus tangshanensis Zone, and the Upper Machiakou Formation （= Machiakou Formation of this paper） into the Eoplacognathus suecicus-Acontiodus ？ linxiensis Zone and the Plectodina onychodonta Zone. They correlated the Lower and Upper Machiakou formations with the Dawan Formation （Dapingian） and Kuniutan Formation （Darriwilian） of South China, respectively. Until now this conodont biostratigraphy and associated correlations have been generally used by Chinese geologists. The first occurrences and ranges of Aurilobodus leptosomatus , Loxodus dissectus , Tangshanodus tangshanensis , and Histiodella holodentata （= Histiodella infrequensa An in the paper of An et al. , 1983） are nearly the same in our study sec- tion （Text-fig. 1） and as shown by An et al. （1983, text-fig. 7）. However, it is impossible tosubdivide the Beianzhuang Formation into the Au- rilobodus leptosornatus-Loxodus dissectus Zone and the Tangshanodus tangshanensis Zone based on the first occurrences and ranges of zonal species. Also, it is very difficult to correlate these zones with those of other areas because of the striking provincial faunal differences. Based on the present restudy of our conodonts collected from these for- mations combined with a detailed analysis of the conodont species ranges shown by An et al. （1983, text-fig. 7）, the present authors propose a revision of the previous conodont zonation. Hence, the Aurilobodus leptosomatus-Loxodus dissectus Zone and the Tangshanodus tangshanensis Zone of An et al. （1983） in the Beianzhuang Formation are proposed to be replaced by the Histiodella holo- dentata-Tangshanodus tangshanensis Zone, and the Eoplacognathus suecicus-Acontiodus ？ linxien- sis Zone and the Plectodina onychodonta Zone of An et al. （1983） in the Machiakou Formation are proposed to be replaced by the the Eoplacognathus suecicus Zone. All of these zones are referred to the Darriwilian of the Middle Ordovician and can be correlated with the contemporary faunas of South China, the Tarim Basin of Xinjiang, Balto- Scandia and North American Midcontinent. The currently revised the HistiodeHa holodentata-Tangshanodus tangshanensis Zone is not as the same as the Tangshanodus tangshanensis Zone of An et al. （1983）. It includes the Aurilobodus leptosomatus- Loxodus dissectus Zone and the Tangshanodus tangshanensis Zone of An et al. （1983）, but has been subdivided into the Rhipidognathus laiwuensis and the R. rnaggolensis subzones. The Eoplacognathus suecicus Zone can also be subdivided into the Acontiodus？ linxiensis and the Plectodina ony-chodonta subzones.