Definition of a global boundary stratotype for the Darriwilian Stage of the Ordovician System

The International Commission on Stratigraphic Nomenclature and the IUGS Executive Committee have recently approved selection of a global stratotype section and point defining the base of the second stage of the Middle Ordovician Series. This stage is named the Darriwilian Stage after the Australian regional stage of the same name, with which it shares identical definition and scope. The base of the Darriwilian Stage is fixed at the base of the Undulograptus austrodentatus Biozone in a section through the Ningkuo Shale exposed at Huangnitang, Changshan County, Zhejiang Province, southeast China. This boundary is defined by the first appearance datum of the U. austrodentatus within a densely sampled succession of graptolite first occurrences that facilitate precise global correlation of this boundary. Conodont species are also present within this section and indicate that the base of the Darriwilian is likely to lie within the lower part of the Microzarkodina parva Biozone of the North Atlantic conodont zonal succession. Correlation of this boundary with well studied successions elsewhere indicates that it is coeval with a level in the mid Whiterockian in North America, and in the lower Fennian Stage of the upper Arenig of Wales. The base of the Darriwilian is about 467.5±3 Ma.