摘要
湖北宜昌黄花场剖面是我国奥陶系大湾阶的界线层型剖面,高精度生物地层以及岩石、层序地层和碳同位素地球化学的综合研究表明,该剖面完全符合关于建立全球下奥陶统-中奥陶统界线(即奥陶系第三个尚待正式命名的阶)界线层型剖面和点位(GSSP)的条件。建议以该剖面牙形石Baltoniodus?triangularis的首现层位,作为划分全球下奥陶统-中奥陶统界线的GSSP。所建议的GSSP交通方便,位于距宜昌市北北东22km的公路旁,地层出露完美无缺,界线生物层位于大湾组下段SHod-16层底部,距大湾组底部10.57m;界线上下发育了B.?triangu-laris的完整演化序列,并伴生有极好的Baltoniodus、Trapezognathus、Periodon和Microzarkodina等牙形石属种演化序列来作为佐证;界线之上0.2m所出现的世界广布的Microzarkodina abellum可作为划分此界线的辅助标志。界线生物层位于低位海侵序列之中,界线之下0.6m处所显示的从高位到低位的层序转换面和碳同位素最大偏移,与全球此时所发生低位事件密切相关,可作为识别该界线的物理和化学标志。所建议的界线生物层与笔石Azygograptus suecicus生物带上下组合之间的界线接近,与几丁虫Belonechitinacf.henryi生物带底界几乎一致,因此,所建议的界线易于在全球,无论浅水碳酸盐相、还是深水笔石相,识别和进行精确对比。同样的生物组合序列和层序及碳同位素异常亦在黄花场剖面之北5km的陈家河(即大坪)剖面得到了验证.
Integrated high-resolution bio-, litho-, sequence-stratigraphic and carbon isotopic geochemical studies suggest that the Huanghuachang section near Yichang, southern China meets the requirements a Global Stratotype Section and point （GSSP） for the base of the Middle Ordovician Series and the yet-to-benamed third stage of the Ordovician System （or lower stage of Middle Ordovician Series）. The definition of the base of the Middle Ordovician is proposed to he the first appearance datum （FAD） of Baltoniodus？ triangularis at the base of Bed （SHod） 16, 10.57m above the base of the Dawan Formation in the measured Huanghuachang section. The proposed GSSP is well exposed at the rodeside, NE 22. km from Yichang City and easy accessibility. Microzarkodina flabellum first appears at 0.2m above the boundary hio-horinzon and can he taken as a reasonable proxy for the boundary. The conodont succession at the section is complete across the Lower to Middle Ordovician series boundary and several excellent phylogenetic lineages of Baltoniodus, Trapezognathus, Periodon, and Microzarkodina are represented. The proposed boundary approximates the boundary between the lower and upper intervals of the Azygograptus suecicus graptolite Biozone, and nearly coincides with the base of the Belonechitina cf. henryi chitinozoan Biozone. It is within a transgressive sequence. The worldwide lowstand event indicated by the sequence change and the related carbon isotopic maxium positive excursion, occurred 0.6m below of the level with FAD of thd B. ？ triangularis, in the Huanghuachang section are auxiliary physical and geochemical global marker. The pro- posed boundary horizon can be recognized and correlated globally with high precision in both relatively shallow-water carbonate facies as well as in deep-water graptolite facies.
The same faunal succession is also recorded from the Chenjiahe （formerly Daping） section, 5 km to the north of the Huanghuachang section.
出处
《地层学杂志》
CSCD
北大核心
2005年第B11期467-489,i0003-i0008,共29页
Journal of Stratigraphy
基金
中国地质调查局(No.200113900072)
国家自然基金委员会(No.40272020)
全国地层委员会和德国科学交流中心(DAAD)资助项目成果之一。
