论上扬子区南部中奥陶统大沙坝组

中奥陶统上部大沙坝组的命名地点在四川长宁双河附近 ,岩性以泥质粉砂岩、粉砂质泥岩为主 ,夹钙质结核和灰岩透镜体 ,分布在川东南的长宁—兴文—叙永—古蔺一带 ,其中笔石和壳相化石经常交互出现 ,均比较丰富 ;笔石动物属于上层下垂对笔石动物群 ( Didymograptus artus Fauna) ,据此 ,大沙坝组的地层时代可定为中奥陶世达瑞威尔期 (相当于兰维恩期 ) ;腕足动物可称作 Saucrorthis- Parisorthis动物群。大沙坝组的岩性和其中所含的动物群均明显有别于在黔北和黔东北广泛分布的、大致同期的以钙质泥岩和钙质粉砂岩为主的十字铺组。四川北部平武县水柏乡的中元古代大沙坝组 ,由于命名在后 ,应予以废弃。

REMARKS ON THE ORDOVICIAN DASHABA FORMATION OF SOUTH CHINA

The upper Middle Ordovician Dashaba Formation was named at Shuanghe of Changning, Sichuan Province, SW China by Mu En-zhi and others in 1978. It is mainly composed of argillaceous siltstone and silty mudstone, and distributed in Changning, Xingwen, Xuyong and Gulin counties of southeastern Sichuan. Graptolites and shelly fossils, both comparatively abundant, alternately occur in the strata. Based on the graptolites belonging to the Didymograptus artus Fauna of the upper layer, the age of the Dashaba Formation can be considered as Middle Ordovician Darriwilian (equivalent to Llanvirn). Brachiopods can be called the Saucrorthis-Parisorthis Fauna. The lithological character and faunas of the Dashaba Formation are both significantly distinguished from that of the Darriwilian Shihtzupu Formation, which is mainly composed of calcareous mudstone and calcareous siltstone and widely distributed in northern and northeastern Guizhou. The Mesoproterozoic Dashaba Formation established at Shuibai of Pingwu, northern Sichuan by Chen Jia-di in 1983 is a junior homonym and should be abandoned.