MIDDLE ORDOVICIAN ACRITARCHS FROM THE GLOBAL STRATOTYPE SECTION OF HUANGNITANG IN CHANGSHAN, ZHEJIANG, CHINA
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Abstract:
The Huangnitang section-located in the Changshan County of Zhejiang Province- includes the Global StratotypeSection and Point (GSSP) of the upper Middle Ordovician Darriwilian Stage. Abundant, though not especially well preservedacritarchs have been obtained from the Ninguo Formation, specifically from the biostratigraphic interval comprising theAzygograptus suecicus and Exigraptus clavus graptolite zones (lower Middle Ordovician: Dawanian) and the succeedingDarriwilian Arienigraptus zhejiangensis and lower part of the Undulograptus sinicus graptolite subzones. The acritarchassemblage of the Ninguo Formation, thus sampled, consists of 26 genera, 41 known species, and 12 unnamed species; itcontains a paucity of Perigondwanan "cold water" forms. The assemblage is divisible into two successive biostratigraphicunits, based on acritarch first appearances and coinciding generally with the Azygograptus suecicus and Undulograptusaustrodentatus graptolite zones respectively. Taking into account the known acritarch distributions and the inferredpalaeogeography of South China, a cool oceanic current from Perigondwana affected the upper Yangtze region, but, to a muchlesser degree, the lower Yangtze region and the Jiangshan-Changshan-Yushan (JCY) area.
