扬子区中奥陶统大湾阶底界精细生物地层分带与对比

ELABORATE BIOSTRATIGRAPHIC SUBDIVISION AND CORRELATION OF THE BASAL DAWAN STAGE(MIDDLE ORDOVICIAN) IN YANGTZE PLATFORM, CHINA

系统研究湖北宜昌黄花场大湾组下段和重庆城口翁溪沟湄潭组下部笔石、牙形石和几丁虫生物分带及其对比关系。精细的生物地层学研究结果证明 ,在我国大湾阶底界界线层型剖面上的生物群组合存在冷水型分子与暖水型分子共生的特点 ,以牙形石Baltoniodustriangularis首现所确定的中奥陶统底界较Tripoduslaevis的首现低 0 .2m ,位于笔石Didymograptusprotobifidus/Corymbograptusdeflexus带或几丁虫Conochitinalangei/C .brevis带中部。中奥陶统底界上述牙形石、笔石和几丁虫的对比关系不受沉积相限制 ,在重庆城口翁溪沟含笔石碎屑岩相的湄潭组下部同样存在。

The work on the definition of global Series and Stages of the Ordovician is now focusing on the selection of GSSPs for the base of Middle Ordovician Series. The Whiterock Narrows section in the Monitor Range was first recommended as the boundary stratotype for the base of the global Middle Ordovician Series with the boundary placed at the first appearance of T. laevis. However, the age for the boundary level at this section is younger than expected previously and is correlated with the Yapeenian to the lowest Darriwillian, but not with the Isograptus victoria lunatus Zone. Albanesi and Ortega (2001) proposed the Niqivil section, Argentina as the global stratotype for the base of the Middle Ordovician with the FAD of Protoprioniodus aranda or of simultaneous Texania heligma as alternative index species for the boundary biomark. However, our recent research demonstrates the first appearance of this species in the middle of Upper O. evae Biozone at the Wengxigou section of Chengkou, indicating the FAD of P. aranda there is slightly lower than that reported in Argentina.