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