全球寒武-奥陶系界线划分对比的标志和纽带——中国北方小阳桥剖面的启示

Signs and Ties of the Global Cambrian-Ordovician Boundary Division and Correlation:Implicationsfrom the Xiaoyangqiao Section,North China

根据对中国小阳桥剖面高精度生物、层序、化学和磁性地层学综合研究的新成果及其与绿岬全球界线层型剖面(GSSP)的详细对比,进一步厘定了加拿大绿岬“金钉子”剖面所定义的奥陶系底界划分与对比的标志,并指出:(1)与所定义的全球标准奥陶系底界相当的界线位于小阳桥剖面块状叠层石灰岩之上19.9 m处的薄层纹层灰岩中(BD-24层的底部),即Cordylodus intermedius牙形石带的上部,其上1 m则出现世界上最早的浮游笔石Rhabdinopora proparabola,其上1.5 m则是牙形石Cordylodus lindstromi的首现层位;(2)所定义的奥陶系底界位于两次全球海平面升降事件(全球巴萨尔屋(Basal House)低位和弓头虫(Acerocare)低位事件)之间;(3)奥陶纪初期最大的δ_(13)C_(carb)同位素正偏移出现在Rhabdinopora parabola笔石带的底部;(4)Cordylodus intermedius牙形石生物带内记录的、被称之为Hirsutodontus simplex峰(=HSS,简单多刺牙形石峰)的显著δ_(13)C_(carb)碳同位素正偏移和稀土元素(REE)异常,均位于这两个剖面寒武-奥陶系界线之下。以上所有这些事件记录均可作为全球不同大陆和相区识别与划分对比寒武系-奥陶系界线的标志。

Based on the new results of high-precision and integrated research of biological-,sequence-,chemo-and magnetic stratigraphy from the Xiaoyangqiao section in China and its detailed comparison with the Green Point GSSP section,the signs of the division and correlation of the base of the Ordovician defined in the Green Point section of Canada was further revised.It is suggested that(1)The equivalent boundary to the global standard Ordovician base defined in the Green Point GSSP is located in 19.9 m above the laminated limestone(i.e.the bottom of the BD-24 bed)underlying the characteristic massive stromatolitic limestone of the Xiaoyangqiao section,i.e.,the upper part of the Cordylodus intermedius conodont zone,1 m below the level with the earliest planktonic graptolite Rhabdinopora proparabola,and 1.5 m below the base of the Cordylodus lindstromi Zone;(2)the Cambrian-Ordovician boundary defined is located between two global sea level rise and fall events,namely the global Basal House lowstand and the Acerocare lowstand event;(3)the Early Ordovician largestδ_(13)C_(carb) isotope positive peak appears at the bottom of the Rhabdinopora parabola graptolite zone;(4)The significantδ_(13)C_(carb) carbon isotope positive shift,known as Hirsutodontus simplex peak(=HSS),and the rare earth element(REE)anomaly recorded inside the Cordylodus intermedius Zone,are directly located below the CambrianOrdovician boundary of these two sections.All of these events record above could be used as proxies for identification and subdivision and correlation of the Cambrian-Ordovician boundary between different continents and facies worldwide.