MIDDLE ORDOVICIAN ACRITARCHS FROM THE GLOBAL STRATOTYPE SECTION OF HUANGNITANG IN CHANGSHAN, ZHEJIANG, CHINA

The Huangnitang section-located in the Changshan County of Zhejiang Province- includes the Global Stratotype Section and Point (GSSP) of the upper Middle Ordovician Darriwilian Stage. Abundant, though not especially well preserved acritarchs have been obtained from the Ninguo Formation, specifically from the biostratigraphic interval comprising the Azygograptus suecicus and Exigraptus clavus graptolite zones (lower Middle Ordovician: Dawanian) and the succeeding Darriwilian Arienigraptus zhejiangensis and lower part of the Undulograptus sinicus graptolite subzones. The acritarch assemblage of the Ninguo Formation, thus sampled, consists of 26 genera, 41 known species, and 12 unnamed species; it contains a paucity of Perigondwanan cold water forms. The assemblage is divisible into two successive biostratigraphic units, based on acritarch first appearances and coinciding generally with the Azygograptus suecicus and Undulograptus austrodentatus graptolite zones respectively. Taking into account the known acritarch distributions and the inferred palaeogeography of South China, a cool oceanic current from Perigondwana affected the upper Yangtze region, but, to a much lesser degree, the lower Yangtze region and the Jiangshan-Changshan-Yushan ( JCY) area.