中国奥陶系■江阶、石口阶的候选层型剖面——湖南祁东双家口剖面(Ⅰ)

<正> 一、绪言 (一)中国奥陶系江阶、石口阶的由来及存在的问题 我国奥陶系年代地层分类有二个。一个是以笔石相为主的分类(穆恩之1974;张文堂、李积金等,1982),一个是以介壳相为主的分类(赖才根等,1982)。笔石相的分类自上而下为:五峰阶,石口阶,澣江阶,胡乐阶,宁国阶及新厂阶。在这六个阶中,江阶、石口阶是以魏秀喆、肖承协等(1966)建立的江组、石口组为依据而建的(穆恩之1974)。分布

A candidate stratotype section of Hanjiagan and Shikouan stages (Ordovi- cian) --The Shuangjiakou section of Qidong, Hunan

A continuous section of Hanjiang and Shikou stages of Ordovician is well exposed at Shuangjiakou of Qidong County, S. Hunan. Over a hundred thousand specimens of grapt-olites were obtained during this field work. Shuangjiakou Formation (bed 256-140) was established by the authors (1979) with the type section located at Shuangjiakou of Qidong. It overlies conformably the Baimachong Formation and underlies conformably the Chengbu Formation. This formation is lithologically divided into three parts, the lower part, black thin-bedded carbonaceous shale intercalated with black siliceous rocks and bedded chert, the middle part, siliceous shale with mid-beJded chert (bed 205-202) with a Fe-Mn bed (bed 201), and the upper part, black siliceous rocks with thin-bedded siliceous shale, weathering greyish white in colour. Four fossil zones may be distinguished: 5. Climacograptus spi-niferus zone (lower part), 4. Dicranograptus clingani zone, 3. Climacograptus wilsoni zone, 2. Climacograptus bicornis zone and 1, Dicellograptus divaricatus rigidus zone, Chengbu Formation (bed 139-1) was estabished by the authors (1979), with its type locality at Daliang-tou, Chengbu county, S. Hunan, It overlies conformably the Shuangjiakou Formation and underlies conformably the Tianmashan Formation. This formation is lithologically divided into two parts, the lower part, composed of black laminated carbonaceous striped mudstone intercalated with a few thin-bedded siliceous shale, and the upper part, consisting of black moderately thick-bedded striped mudstone, weathering yellow and intercalated with a few yellowish-green mudstone (bed 51-47). Three fossil zones may be recognized: 7. Dicellograptus johnstrupi zone, 6. Climacograptus pygmaeus zone and 5.Climacograptus spiniferus zone (upper part).