奥陶系上统赫南特阶全球层型剖面和点位的建立

A FINAL REPORT ON THE GLOBAL STRATOTYPE SECTION AND POINT(GSSP) FOR THE HIRNANTIAN STAGE (UPPER ORDOVICIAN)

赫南特阶(上奥陶统)的全球界线层型剖面和点位确立在中国宜昌王家湾北剖面观音桥层底界之下0.39m处。该剖面位于中国湖北省宜昌市以北42km处的王家湾村,经纬度为30°58′56″N、111°25′10″E。赫南特阶底界的GSSP点位以笔石Normalograptusextraordinarius的首次出现层位(FAD)为标志。碳同位素在此层位显示的正漂移以及N.ojsuensis的首现可作为第二标志。赫南特阶始于自Diceratograptusmirus亚带开始的全球生物大灭绝事件的主幕之后。王家湾北剖面出露完整,沉积和生物序列连续,笔石和壳相动物化石丰富并保存良好,王家湾北、王家湾南和王家湾小河边剖面都发育了合适的沉积相和生物相并具有广泛对比的潜力。该地区地质构造简单,岩石未经历较强的变质作用,且交通便利。王家湾小河边剖面尤其适合进行化学地层学研究和同位素年龄的测定。建立赫南特阶底界的全球界线层型剖面和点位的提案报告于2004年10月被国际地层委员会奥陶系分会通过,经补充完善后,于2006年2月被国际地层委员会通过,同年5月被国际地科联正式批准。

The Global Boundary Stratotype Section and Point （GSSP） for the base of the Hirnantian Stage （Upper Ordovician） is defined at a point 0. 39m below the base of the Kuanyinchiao Bed in the Wangjiawan North Section. The section is located near Wangjiawan village, 42km north of Yichang city （western Hubei, China）, at 30°58＇56＂N and 111°25＇10＂E on the east flank of the Huangling Anticline. The GSSP level coincides with the first appearance of the graptolite species Normalograptus extraordinarius （Sobolevskaya）. Secondary markers include the onset of a positive carbon-isotope excursion and a slightly earlier first appearance of Normalograptus ojsuensis （Koren ＆ Mikhailova）. The beginning of the Hirnantian was followed by a global episode of major extinction event, which happened in the Diceratograptus mirus Subbiozone. The Wangjia-wan North Section possesses continuity of sedimentation and biozonation with completeness of exposures, abundant and well-preserved graptolites and shelly fossils. The Wangjiawan North and South sections as well as the Wangjiawan Riverside section possess favorable facies and widespread correlation potential, free from structure complication, metamorphism and other alteration and good accessibility. The Wangjiawan Riverside section in particular possesses amenability to isotopic age determination. The proposal approved by the International Subcommission on Ordovician Stratigraphy in Oc tober of 2004, by International Commission on Stratigraphy in February of 2006, and ratified by International Union of Geological Sciences in May of 2006.