贵阳乌当奥陶系湄潭组下段笔石带的划分

EARLY ORDOVICIAN GRAPTOLITE ZONATION OF THE LOWER MEMBER OF THE MEITAN FORMATION,WUDANG AREA OF GUIYANG,GUIZHOU PROVINCE,CHINA

位于贵阳北东向的乌当处于黔中古陆南缘,是黔北沉积区和黔南沉积区的交汇区。该区奥陶系湄潭组下段(O1m1)发育,主要由灰绿、紫红色泥页岩及浅灰、浅红色中厚层粉砂岩组成。大洼、小谷农两条湄潭组下段剖面均富含笔石、腕足类、三叶虫、棘皮动物及软体动物等多门类化石。其中笔石动物8属31种,包括3个相似种和5个未定种。总体分异度不高,大多数为上扬子区的常见属种,由下而上可划分为4个笔石带:Acrograptus fili-formis带、Didymograptellus eobifidus带、Corymbograptus deflexus带(?)及Azygograptus suecicus带,并能与东南地区同期地层的笔石带对比,填补了黔中古陆以南地区湄潭组下段笔石地层研究的空白。

The Central Guizhou Oldland was emergent during the Early to Middle Ordovician in an area which today comprises central Guizhou and eastern Yunnan.Previously,little has been known about the Ordovician Graptolite fanua from the seas surrounding this land.However,the Wudang area,near to Guiyang,is one of the few localities close to the southern margin of this Oldland and is also located at the interface between the north and the south Guizhou areas of deposition.The Meitan Formation – of Ordovician age – is well exposed in that area.Here we report on a study of the Graptolites from the faunistically diverse and richly fossiliferous Lower Member of the Meitan Formation from Wudang.So far we have found 31 species and 8 genera of incertae sedis.The Graptolite fauna has a relatively low diversity and most of the species are common in the Upper Yangtze Region.Based on their occurrences,four Graptolite zones were detected.In ascending order these are Acrograptus filiformis Zone,Didymograptellus eobifidus Zone,Corymbograptus deflexus Zone（？）,and the Azygograptus suecicus Zone,all of which correspond well with those recognized previously from other regions of southern China.