贵州遵义高桥奥陶系湄潭组下部的笔石

EARLY AND MIDDLE ORDOVICIAN GRAPTOLITES FROM THE MEITAN FORMATION IN ZUNYI, GUIZHOU, CHINA

对贵州遵义高桥大角寺剖面的奥陶系湄潭组下段和中段笔石的研究结果表明,湄潭组下段从下而上可以识别出Didymograptellus eobifidus带、Corymbograptellus deflexus带和Azygograptus suecicus带;中段含有一层笔石,缺少特征属种,根据其总体面貌,可能相当于Expansograptus hirundo带。文章描述其中11属16种:Pseudophyllograptus angustifolius(Hall),Phyllograptus claviger(Monsen),P.annaHall,Didymograptelluseobifidus(Chen and Xia),D.protoindentus(Monsen),Acrograptus kurcki(T rnquist),Corymbograptus vacil-lans(Tullberg,1880),C.deflexus(Elles and Wood),Expansograptus extensus(Hall),E.nitidus(Hall),Xi-phograptus lofuensis(Lee),X.cf.lofuensis(Lee),Azygograptus suecicusMoberg,Dictyonemasp.,Den-drograptussp.,Inocaulis plumulosusHall。这一笔石动物群分异度较低,大多数为上扬子区的常见属种,代表当时黔中古陆北缘近岸海洋环境中的笔石动物群面貌。

It has been widely accepted that the central Guizhou and east Yunnan regions were emergent during the Ordovician, and called the Central Guizbou Oldland （or Dian-Qian Oldland）. Surrounding this land, plenty of Ordovician graptolite-bearing sections have been found, while still little is known about the graptolite fauna from these sections located close to the oldland. Herein we conduct a study of the graptolites from the Meitan Formation at Dajiaosi section of Gaoqiao Town, Zunyi, Guizhou, which was regarded to be among the closest to the north margin of the oldland. From the Lower Member of Meitan Formation, three graptolite zones in ascending order： the Didymograptellus eobi fidus Biozone, the Corymbograptus de flexus Biozone and the Azygograptus suecicus Biozone, are identified, and for the Middle Member that contains some horizontally extending forms of graptolites but no index species, Expansograptus hirundo Zone （？）, is adopted provisionally. The present paper describes 16 species assigned to 11 genera of this fauna, which include Pseudophyllograptus angustifolius （ Hall ）, Phyllograptus claviger Monsen, P. anna Hall, Didymograptellus eobi fidus （Chen and Xia）, D. protoindentus （ Monsen）, Acrograptus kurcki （ Toernquist ）, Corymbograptus vacillans （Tullberg）, C. deflecrus （Elles and Wood）, Expansograptus extensus （ Hall）, E. nitidus （Hall）, Xiphograptus lo fuensis （Lee）, X. cf. lofuensis （ Lee ）, Azygograptus suecicus Moberg, Dictyonema sp., Dendrograptus sp., Inocaulls plumulosus Hall. This is a low-diversity graptolite fauna, and most of the species are common in Upper Yangtze Region, suggesting that the graptolite diversity became significantly lower towards the Central Guizhou Oldland.