浅论华南扬子区下、中奥陶统紫台组

华南下、中奥陶统紫台组命名地点在贵州印江紫台。自命名以来,对紫台组作为岩石地层单位的有效性长期存有争议。资料总结和大范围野外调查发现,紫台组具有独特的岩相和生物相,它以紫红色泥灰岩、瘤状泥灰岩为特征(夹少量黄绿色泥页岩);以独特的三叶虫和头足类动物群为特色;并具有稳定的空间分布范围,出露在现今的黔东北、渝东南、湘西北、鄂西南以及皖南等地区,在古地理上大体沿扬子台地的东南缘呈带状分布。作为与湄潭组(上扬子地区)、大湾组(中下扬子地区)基本同期的沉积,紫台组是一个有效的岩石地层单元。

BRIEF DISCUSSION ON THE LOWER TO MIDDLE ORDOVICIAN ZITAI FORMATION IN THE YANGTZE REGION, SOUTH CHINA

The Lower to Middle Ordovician Zitai Formation is named by Mu et al. (in Nanjing Institute of Geology and Palaeontology, 1974) at Heshui, Yinjiang County, Guizhou Province. Since its establishment,there has been a long debate about its validity as a lithostratigraphical unit. By reviewing all relevant literatures and field investigations on the vast area of the Yangtze Platform, the authors find that the Zitai Formation has unique lithofacies and biofacies. It consists of a sequence of purple red, nodular argillaceous limestone intercalated with a few yellowish-green shale beds. The Zitai Formation has a certain spatial distribution from northeastern Guizhou, southeastern Chongqing, northwestern Hunan, southwestern Hubei to southern Anhui. It is palaeogeographically distributed along the southeastern margin of the Yangtze Platform. The Zitai Formation also yields characteristic trilobite and nautiloid faunas. As a rock unit roughly contemporaneous to the Meitan Formation (Upper Yangtze Platform) and the Dawan Formation (Middle to Lower Yangtze Platform), the Zitai Formation is a valid lithostratigraphic unit representing a package of Middle Ordovician rocks in South China.