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The base of the Middle Ordovician(i.e.Dapingian Stage) has been defined at the first appearance datum(FAD) of conodont Baltoniodusl triangularis at Huanghuachang,Yichang,China, but the precise correlation of the boundary to regions of other facies remain to be resolved.Herein we review the biostratigraphy and chemostratigraphy of the Huanghuachang Global Standard Stratotype-Section and Point(GSSP) section,and present our latest stratigraphic work on the nearby Chenjiahe section in Yichang,and the Hengtang Quarry section,Jiangshan,Zhejiang,which is regarded as representative of slope facies.The conodont and graptoiite biostratigraphy as well as chemostratigraphy of the Chenjiahe section indicate that the base of the Middle Ordovician also falls within the graptoiite Azygograptus suecicus Zone,and coincides with a high or maximumδ~(13)C value within a minor positive carbon isotope excursion,suggesting that the base boundary can be readily recognized across the entire Yangtze Gorges area.The integrated graptoiite and conodont biostratigraphy and chemostratigraphy of the Hengtang section,Jiangshan,indicates that the basal boundary probably falls within the graptoiite Isograptus caduceus imitatus Zone that overlies the Azygograptus suecicus Zone,and coincides with a remarkable drop ofδ~(13)C.This difference indicates that a multi-disciplinary approach is critical to identify the base boundary in those regions where the Baltoniodusl triangularis is absent.
The base of the Middle Ordovician (ieDapingian Stage) has been defined at the first appearance datum (FAD) of conodont Baltoniodusl triangularis at Huanghuachang, Yichang, China, but the precise correlation of the boundary to regions of other facies remain to be resolved. Herein we review the biostratigraphy and chemostratigraphy of the Huanghuachang Global Standard Stratotype-Section and Point (GSSP) section, and present our latest stratigraphic work on the nearby Chenjiahe section in Yichang, and the Hengtang Quarry section, Jiangshan, Zhejiang, which is deemed as representative of slope facies. The conodont and graptoiite biostratigraphy as well as chemostratigraphy of the Chenjiahe section indicate that the base of the Middle Ordovician also falls within the graptoiite Azygograptus suecicus Zone, and coincides with a high or maximum δ ~ (13) C value within a minor positive carbon isotope excursion, suggesting that the base boundary can be readily recognized across the entire Yangtze Gorges area The integrated graptoiite and conodont biostratigraphy and chemostratigraphy of the Hengtang section, Jiangshan, indicates that the basal boundary probably falls within the graptoiite Isograptus caduceus imitatus Zone that overlies the Azygograptus suecicus Zone, and coincides with a remarkable drop of δ ~ (13) C. This difference indicates that a multi-disciplinary approach is critical to identify the base boundary in those regions where the Baltoniodusl triangularis is absent.