地层学杂志

十多年来全球和我国奥陶系和下志留统下部年代地层研究取得很大进展 ,在前人所积累的大量成果和资料的基础上 ,结合近年来的研究 ,厘定了我国奥陶系和下志留统下部年代地层单位系统 ,建议分别以吉林浑江大阳岔 (或江西武宁或湖北宜昌黄花场 )、湖北宜昌黄花场、浙江常山黄泥塘、新疆柯坪大湾沟和湖北远安苟家垭剖面为界线层型 ,将中国奥陶系自下而上划分为 :新厂阶 (或特马豆克阶 )、道保湾阶、大湾阶、达瑞维尔阶、艾家山阶和钱塘江阶。为了客观反应我国奥陶系的特点 ,同时便于与全球年代地层单位界线对比 ,建议将上述各个阶底界的的界线依次划在 R.taojiangensis( =R.praeparabola) / H .simplex、 O.communis、 B.triangularis/ T.laevis、 U.austrodentatus、 N .gracilis和 N ankinolithus带的底部 ;中下奥陶统和中上奥陶统之间的界线分别定在道保湾阶 /大湾阶和达瑞维尔阶 /艾家山阶之间。建议下志留统最下部两个年代地层单位——龙马溪阶和大中坝阶——以湖北宜昌王家湾剖面为界线层型 ,分别以 A.ascensus和 D .triangulatus的首现为划分标志。对每一个阶的命名、含义、界线层型剖面的岩性特征、界线划分的生物标志、与全球相关年代地层单位层型和点 ( GSSP)的对比 ,以及与过去曾经建议或使用过

The present paper introduced and discussed in detail the advances in the Ordovician to earliest Silurian chronostratigraphic study at home and abroad. On the basis of the materials accumulated by many geologists for a long time and a recent study the Ordovician and earliest Silurian subdivision of series and stages are re revised in China. It is suggested that the Ordovician of China is subdivided in ascending order into the Xinchang (or Tremadocian,or Yichangian), Daobaowan, Dawanian, Darriwillian, Aijiashan, and Qiantangjiang Stages with Wuning section of Jiangxi (or Dayangcha section of Hunjiang, Jilin, or Huanghuachang section of Yichang, Hubei), Huanghuachang section of Yichang, Huangnitang section of Changshan, Zhejiang, Dawangou section of Keping, Xinjiang, and Goujiaya section of Yuanan, Hubei as boundary stratotypes respectively. In order to reflect the regional characteristics of the Ordovician of China and also to be easy to correlate with the global Ordovician boundaries of series and stages, the bases of 6 stages mentioned above are successively fixed at the base of the R. taojiangensis (= R. praeparabola) / H .simplex Zone, O. communis Zone, R. triangularis/ leavis Zone, U. auastrodentatus Zone, N. gracilis Zone and Nankinolithus Zone. The Middle Lower Ordovician Series and the Middle Upper Ordovican Series boundaries are separately defined between the Daobaowan Stage and Dawan Stage and between the Darriwillian Stage and Aijiashan Stage. The basal boundaries of the lowest Silurian two stages, named the Longmaxi and Dazhongba Stages, are drawn by the FAD of the A. ascensus and that D. triangulatus respectively with the the Wangjiawan section as their boundary stratotypes. It is proposed that the Huanghuachang and Wangjiawan sections as global stratotype and point for the base of the Middle Ordovician Series and for the Ordovician Silurian boundary respectively with the FAD of triangularis and ascensus as their boundary marks. Meanwhile the name and implication of each stage suggested above and its lithological feature, boundary biomark in the stratotype section are described. Their relationships with the global relevant choronostratigraphic boundaries decided or not decided yet, and with the series and stage, that was ever used or proposed in China, are discussed or explained. The main lithostratigraphic units, which are equivalents in age to each stage revised or proposed, but distributed in different facies regions of China, are successively pointed out. More importantly, our study has further confirmed that the Huanghuachang section and the Wangjiawan section fulfill the requirements for the GSSPs of the Lower Middle Ordovician boundary and the Ordovician Silurian boundary revised. The first appearance of Baltoniodus triangularis (Lindstrm, 1995), at 2.6m (the base of Hod 19) above the base of the Dawan Formation, is proposed as conodont icon for the definition of the Middle Ordovician base at the proposed Huanghuachang boundary stratotype and the first appearance of Tripodus laevis Bradshow, 1969, at the level (Hod 17), 0.2m above the boundary biozone, can be taken as a reasonable proxy for the boundary correlation. At the Wangjiawan section the first appearance of Akidogratus ascensus Davies, 0.32m above the base of the Longmaxi Formation is proposed as the biohorizon for the Silurian base revised. The biohorizone at the proposed two sections can be recognized and correlated globally with high precision. It is suggested that these problems need further study concernity the name of the Xinchang Stage and its stratotype section, the high precision correlation and stratotype section of the Qiantanjiang Stages, and the name of second stage (Dazhongba Stage) of the Silurian.