从层序地层学观点论奥陶—志留系界线划分

ON THE ORDOVICIAN-SILURIAN BOUNDARY AT THE VIEWPOINT OF SEQUENCE STRATIGRAPHY

层序地层学研究表明,奥陶—志留纪之交的冰成海退事件在全球范围内形成了一个Ⅱ型层序界面。与上扬子地台奥陶—志留系交界处观音桥层相同层位的沉积,在世界各主要大陆上均具有基本一致的沉积背景和“浅海凝缩作用”特征,同属奥陶纪最晚期—志留纪最早期可进行全球对比的Ⅱ型三级层序的陆架边缘体系域。该陆架边缘体系域的顶面,即该Ⅱ型层序的初始海泛面,恰好与笔石 Glyptograptus persculptus带的首次出现相吻合,与已确认的奥陶—志留纪交界处的重大生物演化事件、沉积地球化学事件等也完全一致,因此全球奥陶—志留系界线应选择在笔石G. persculptus带之底。由于上述初始海泛面同时也是笔石G.persculptus带在世界各地首次出现的最低位置,因此它可作为这一全球界线识别性和操作性俱佳的天然标志。

On the basis of the sequences stratigraphy at the southeastern margin of Yangtze platform and the correlation with the similar strata on the other continents, the SMST of the third order type Ⅱsequence containing the Ordovician - Silurian Boundary, e.g., the so-called Guanyinqiao Bed and other synchro-depositions with typical features derived from the 'shallow marine condensation process', has been distinguished around the world. On account of the FFS above the SMST coinciding with the global terminal Ordovician mass extinction event and the geochemical anormaly event, it is suggested therefore, just like the position many other Chinese scholars recommended long before, the FAD of the Glyptograptus persculptus zone which appears at the FFS above-mentioned would be better to be admitted as the GSSP of Ordovician-Silurian Boundary instead of the FAD of Parakidograptus acuminatuszone. With the eye-catching physical surface and the well-correlated global distribution, the new potential GSSP marked by the FFS, which is the lowermost position of the FAD of the G. persculptus zone around the world, would be much more operable than the former one.