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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.
The Huanghuachang section in Yichang, Hubei Province, is the boundary layer profile of the Ordovician Dawanian Formation in China. A comprehensive study of high-precision bio-formations, rocks, sequence stratigraphy and carbon isotope geochemistry shows that the profile fully conforms to the standard for establishing the Lower Ordovician - The conditions for the boundary layer profile and point (GSSP) of the Middle Ordovician boundary line (the third order of the Ordovician to be formally named). It is suggested that the first episode of the Baltoniodus? Triangularis in this section be used as the GSSP for the division of the Lower Ordovician-Middle Ordovician boundary in the world. The proposed GSSP is conveniently located beside a 22km north-east Yichang Highway with perfect strata exposed. The boundary bio-layer is located at the bottom of the SHod-16 layer in the lower part of the Dawan Formation and is 10.57m away from the bottom of the Dawan Formation. The complete evolutionary sequence of B. tiangu-laris is accompanied by an excellent series of evolutionary sequences of the genus Baltoniodus, Trapezognathus, Periodon and Microzarkodina, etc. The world-wide Microzarkodina abellum can be used as a secondary marker to demarcate this line. The boundary bio-layer is located in the low-level transgressive sequence. The sequence of translocation and carbon isotope shifts from high to low at 0.6m below the boundary line is closely related to the global low-frequency event at this time, Boundary of physical and chemical signs. The proposed boundary creature is close to the line between the top and bottom assemblages of the Azygograptus suecicus biozone and nearly identical to the Belzonchitinacf.henryi biozone bottoms, so the proposed boundary is easy to find globally, regardless of whether the shallow water carbonate Phase, or deep-water pen phase, identify and accurately compare. The same sequence and sequence of biological assemblages and carbon isotope abnormalities were also verified in the Chenjiahe (ie Daping) profile 5 km north of the Huanghua field profile.