河北唐山下奥陶统牙形刺生物地层的新认识

本文作者在安太庠等(1983)研究过的唐山赵各庄剖面,对下奥陶统冶里组和亮甲山组较系统地采集了牙形刺样品,通过对所获牙形刺标本和安太库等(1983)所描述的牙形刺动物群仔细研究后,对包括分带和对比意见有新的认识。安太庠等(1983)把自寒武系凤山组顶部至下奥陶统亮甲山组这段地层,自下而上建立了Cordylodus proavus,Utahconus beimadaoensis-Monocostodus sevierensis,Cordylodus rotundatus-"Acodus"oneotensis,Scolopodus quadraplicatus-S.opimus,Scalpellodus tersus,Serratognathus bilobatus,S.extensus和Paraserratognathus paltodiformis等8带,其中Cordylodus proavus带为寒武系顶,Utahconus beimadaoensis-Monocostodus sevierensis至Scolopodus quadraplicatus-S.opimus等带为下奥陶统冶里组,其余则为下奥陶统亮甲山组,寒武系-奥陶系界线置于Utahconus beimadaoensis-Monocostodus sevierensis带之底。这一牙形刺序列和寒武系-奥陶系界线划分方案在国内被广泛引用至今(王志浩等,2011)。本文通过补充采集到的牙形刺标本和安太庠等(1983)报道的材料,根据一些重要分子在地层中的分布及公认的牙形刺分带原则,对安太庠等(1983)的牙形刺分带方案重新修订,自下而上修订为Cordylodus proavus,C.lindstromi,Iapetognathus jilinensis,Cordylodus angulatus,Chosonodina herfurthi-Rossodus manitouensis,Glyptoconus quadraplicatus,Scalpellodus tersus-Triangulodus aff.bifidus,Serratognathus bilobatus,S.extensus和Paraserratognathus obesus等10带,寒武系-奥陶系界线则置于Iapetognathus jilinensis带之底。根据这一修订后的牙形刺序列,寒武系顶至下奥陶统冶里组部分的牙形刺带可与国内、外相同层位的牙形刺带进行精细对比,而亮甲山组的牙形刺虽有明显的地方性特点,但除其顶部因更具地方性而不能直接对比外,其中下部仍可与华南区下奥陶统红花园组相对比。

NEW CONODONT DATA FROM THE LOWER ORDOVICIAN OF TANGSHAN,HEBEI PROVINCE,NORTH CHINA

The present study is based on samples from the Lower Ordovician Yehli and Liangchiashan formations at Zhaogezhuang,Tangshan,Hubei Province.The rich conodont faunas of these units were previously studied by An et al.(1983).Also our samples were very productive and yielded numerous specimens.An et al.(1983)subdivided the Yehli Formation into the Utahconus beimadaoensis-Monocostodus sevierensis,Cordylodus rotundatus-"Acodus"oneotensis and Scolopodus quadraplicatus-S.opimus zones,and the Liangchiashan Formation into the Scalpellodus tersus,Serratognatus bilobatus,S.extensus and Paraserratognathus paltodiformis zones.They put the Cambrian-Ordovician boundary at the base of the Utahconus beimadaoensis-Monocostodus sevierensis Zone.Based on the present restudy of conodonts collected by us from these formations combined with a detailed analysis of the conodont species ranges shown by An et al.(1983,text-fig.3),the present authors propose a revision of the previous conodont zonation.Hence,the interval from the Utahconus beimadaoensis-Monocostodus sevierensis Zone to the Scolopodus quadraplicatus-S.opimus Zone of An et al.(1983)in the Yehli Formation is classified into the Cordylodus lindstromi, Iapetognathus jilinensis,Chosonodina herfurthi-Rossodus manitouensis and Glyptoconus quadraplicatus zones.The Scalpellodus tersus Zone and the Paraserratognathus paltodiformi Zone of An et al.(1983)in the Liangchiashan Formation are referred to the Scalpellodus tersus-Triangulodus aff.bifidus,and the Paraserratognathus obesus Zone,respectively.We propose that the Cambrian-Ordovician boundary is placed at the base of the Iapetognathus jilinensis Zone.The revised conodont zonation of the Yehli Formation is widely applicable to other regions in China and can be correlated with zonations in some other areas in the world.Based on conodonts,the lower part of the Liangchiashan Formation can be correlated with the Honghuayuan Formation in South China but this is not the case with its upper part,probably reflecting depositional environment differences(See Table 2).In the late Floian Stage,there was a rise in sea level in South China whereas the sea remained very shallow in the Tangshan area as suggested by the deposition in the latter region of light-gray,thick-bedded dolomite and dolomitic limestone with chert nodules.