The base of the Darriwilian Stage is defined in a section through the Ningkuo Shale exposed at Huangnitang, Changshan County, Zhejiang Province, southeast China, where it coincides with the first appearance datum of the graptolite Undulograptus austrodentatus.
The Huangnitang Section is located along an unpaved road on the riverside near the village of Huangnitang, 3.5km southwest of the Changshan County Town of Zhejiang Province, southeast China. The section is accessible by vehicle (in fair weather) from the town of Erduqiao, 500m east of Huangnitang.
The Huangnitang Section comprises mainly the Ningkuo Formation. The Ningkuo Formation is dominantly a well-sorted, fine-grained, thinly and evenly laminated black shale.
First appearance of graptolite Undulograptus austrodentatus at the base of Bed AEP 184 of the Huangnitang Section, which is 22m below the top of the Ningkuo Formation. The first appearance of Undulograptus austrodentatus also defines the base of the Undulograptus austrodentatus Biozone. This biozone is dbvided in a lower Arienigraptus zhejiangensis Subzone and an upper Undulograptus sinicus Subzone.
Arienigraptus zhejiangensis (=Pseudisograptus angel Jenkins) first occurs in Bed AEP 182, 0.5m below the FAD of Undulograptus austrodentatus.
Graptolite FAD Undulograptus austrodentatus
The base of the Undulograptus austrodentatus graptolite biozone lies just above the base of the Microzarkodina parva conodont biozone in the North Atlantic and within the upper part of the Histiodella altifrons Biozone in the North American mid-continent conodont zonation. The base of the Darriwilian is coeval with a level in the Fennian Stage of the upper Arenig of Wales.
Mitchell, C. E., Chen Xu, Bergstroem, S. M., Zhang Yuan-dong, Wang Zhi-hao, Webby, B. D., and Finney, S. C., 1997. Definition of a global stratotype for the Darriwilian Stage of the Ordovician System. Episodes 20/3, p. 158-166.