全球奥陶系底界的“金钉子”问题及我国特马豆克阶(Tremadocian)的划分与对比

On the Problem of Global Ordovician Lower Boundary “Golden Spike”and Tremadocian Subdivision and Correlation in China

对全球寒武系与奥陶系界线层型剖面和点位(GSSP)——西纽芬兰绿岬(Green Point)剖面和我国吉林白山(原浑江)大阳岔小洋桥原全球寒武系与奥陶系界线候选剖面的对比研究表明,绿岬"金钉子"剖面所指定的界线生物标志——波动古大西洋牙形石(Iapetognathus fluctivagus)并不存在于所指定的界线生物层和点位上,而且在分布和分类上是均存在争议的物种,更遗憾的是,所展示的大部分牙形石和笔石标本图片以及碳氧同位素异常数据也并非来自该"金钉子"剖面,因而完全违背了选择金钉子剖面的原则和要求,有必要进行重新的评估。小洋桥寒武系与奥陶系界线剖面交通方便、环境优美,寒武系与奥陶系界线地层系由一套深水且未变质的黑色、黄绿色页岩与灰色薄层瘤状灰岩所构成的韵律沉积组成,并保存了完整的具有广泛代表性牙形石和笔石序列,在详细研究的34 m界线间隔中,自下而上保存了完整的Cambrostodus、Codylodus proavus、C.intermedius、C.lindstromi和C.angulatus等5牙形石生物带,在C.intermedius带上部至C.angulatus带之间,还依次出现3层笔石,归属于重新厘定的Rhabdinopora parabola和Anisograptus matanensis等2个笔石带,此外还有大量三叶虫和介形类化石共生,结合高精度同位素地球化学的研究,笔者等建议,以全球广泛分布的牙形石C.intermedius的首现,取代Iapetognathus fluctivagus,作为寒武系与奥陶系界线划分对比的标志,其层位与原来所指定的界线层几乎一致或接近,界线之上所记录的最大碳同位素异常和首次出现的最早的浮游正笔石——R.parabola(含R.praeparabola),可作为该界线划分的辅助标志。

Correlative studies of the Cambrian and Ordovician global boundary stratotype section and point （GSSP） -western Newfoundland Green Point section with the former GSSP candidate for the Cambrian and Ordovi- cian boundary, Xiaoyangqiao section in Dayangcha, Baishan （formerly Hunjiang）, Jilin, China indicate that the conodont species Iapetognathus fluctivagus is not present in the specified boundary layer and point at Green Point section （ i. e. Bed 23） ,and it is a disputed species both in distribution and taxonomy. The most of the conodont and graptolite specimens as well as carbon and oxygen isotopic data submitted in the original proposal, unfortu- nately, are not from the ＂Golden Spike＂ section. This is totally incompatible with the principles and requirement for selecting GSSP and necessary for reassessment. The Xiaoyangqiao Cambrian-Ordovician boundary section is easy accessibility and well-exposed along beautiful rivulet. The Cambrian-Ordovician boundary interval is com- prised of unmetamorphic deep water black, grey and green shale intercalated with thin-bedded gray nodular lime- stone, bearing complete and well represented conodont and graptolite succession from uppermost Cambrian to low- est Ordovician. In 34 m interval, 5 conodont biozones can be recognized in ascending order： the Cambrostodus , Codylodus proavus , C. intermedius , C. lindstromi and C angu]atus and in turn three graptolite-bearing beds, re- ferred to 2 graptolite biozones, the Rhabdinopora parabola biozone with R. ＂praeparabola＂ and the Anisograptu~ matanensis biozone from the upper C. intennedius to C. angulatus conodont biozones. There are also a plenty of trilobite coexisting with ostracoda fossils. Combining high-resolution carbon and oxygen isotope studies the present authors suggest that the Cambrian-Ordovician boundary would be better to be defined by the first appearance （FAD） of widespread conodont C. intermedius, instead of the FAD of the controversial and rare Iapetognathus fluctivagus as boundary biomarker for the base of the Ordovician System. This boundary proposed, however, is close-if not identical-to the level specified at the GSSP section and easily recognized and correlated anywhere in the world. The maximum carbon isotopic excursion recorded above the boundary layer and appearance of the earliest planktonic graptolites--R, parabola （ including R. ＂praeparabola＂, can be make as an auxiliary signs for determination of the boundary.