摘要
东秦岭中部晚奥陶世和早志留世地层分布较广,化石较丰富,尤其是腕足类,分为寺岗组、石燕河组、刘家坡组和张湾组。曾庆銮等(1993)根据腕足类及其群落的更替,把石燕河组和刘家坡组归於早志留世,因而引起较大争论。本文据岩性将寺岗组和石燕河组分别改称为石燕河组下段和上段,并据腕足类化石认为石燕河组和刘家坡组应归於晚奥陶世、张湾组为早志留世;另据上述地层生物群落的特征及群落的更替,认为从石燕河组到刘家坡组,以及刘家坡组至张湾组恰好反映了全球冰期引起的晚奥陶世海退和早志留世冰期结束引起的海侵,故本区奥陶系-志留系界线宜划在刘家坡组和张湾组之间。
In the middle part of the Eastern Qinling region,the Upper Ordovician and Lower Silurian strata are widespread and comparatively fossiliferous,especially rich in brachiopods.They were divided into the Sigang,Shiyanhe and Liujiapo Formations of the Upper Ordovician and Zhangwan Formation of the Lower Silurian(Yan et al.,1983; Liu et al.,1991).The Shiyanhe and Liujiapo Formations have been rccently assigned to the Silurian based on the brachiopods and their community changeover(Zeng et al., 1993).In the present paper,the Sigang and Shiyanhe Formations are assigned to the Lower and Upper Members of the Shiyanhe Formation respectively based on the litho- logical characters. In fact the brachiopods from the Shiyanhe to Liujiapo Formations are evidently dis- tinguished from those of the Zhangwan Formation.In the former,most of brachiopods are'Nalivkinia',which was considered to be only restricted in the Silurian,but now it also appears in the Late Ordovician,Sowerbyella is widespread in the Middle and Late Ordovician in the world.Other brachiopods are rather few,but they are also very impor- tant in the determination of stratigraphic age,for example,Mimella,Dinorthis,Rostri- cellula and Zygospira only occurring in the Middle-Late Ordovician;Altaethyrella and Latirhynchia appearing only in the Late Ordovician.Besides,Skenidioides,Salopina,Ae- giria and Triplesia can range from the Ordovician to Silurian.Up to now no brachiopod genera are only confined to the Silurian,and on the contrary,Eospirifer and Lep- tostrophia of the Zhangwan Formation are the typical Silurian genera. The brachiopods of the Lower Member of the Shiyanhe Formation can be assigned to Sowerbyella-'Catazyga'community,corresponding to the Benthic Assemblage 3, 'Nalivkinia'-Agetolitescommunity from the Upper Member of the Shiyanhe Formation is closed to BA_3,?Isorthis-Salopina community of the Liujiapo Formation should be re- ferred to BA_(1-2),the brachiopods of the Zhangwan Formation can be called Eospirifer community,corresponding to BA_(2-3).The community changeover from Upper Ordovici- cian to Lower Silurian in this part ,i.e.from the Sowerbyella-'Catazyga'to ?Isorthis- Salopina community and from?Isorthis-Salopina community to Eospirifer community may reflect a great regression of Late Ordovician and a great transgression of Early Sil- urian in the world respectively.To sum up,the boundary between the Ordovician and the Silurian in this part should be placed at the top of the Liujiapo Formation based on the brachiopods and their community changeover.
出处
《地层学杂志》
CSCD
北大核心
1996年第3期165-174,共10页
Journal of Stratigraphy
基金
本成果得到中国科学院古生物与古人类学科基础研究特别支持费的资助
编号:9201。
关键词
东秦岭
奥陶纪
志留纪
地层
腕足动物群
Eastern Qinling
Upper Ordovieian
Lower Silurian
biocommunity changeover
