The Xiaoyangqiao section, Dayangcha, North China: the new global Auxiliary Boundary Stratotype Section and Point (ASSP) for the base of the Ordovician System

In 2019 the Sub-Commission on the Ordovician System approved the Xiaoyangqiao section, North China as a new ASSP section for the base of the Ordovician System. The sedimentary succession of the section is exposed in a natural outcrop near the Dayangcha Village at a position of 42 degrees 3'24 '' N, 126 degrees 42'21 '' E. It has a well-preserved, abun-dant and diverse fossil record across the boundary with key markers (conodonts and graptolites), which provide improved intercontinental correlation of the Cambrian- Ordovician boundary. The appearance of the first plank -tonic graptolites is immediately below the base of the Cordy-lodus lindstromi Conodont Zone. Other fossils, including acritarchs, brachiopods and trilobites are also present in the Xiaoyangqiao ASSP section. Non-biotic secondary global markers near the base of the Ordovician System include a positive carbon isotopic excursion with the maximum peak (named HSS) below the boundary, a prominent unnamed negative peak immediately below the boundary and a prominent carbon isotope excursion with positive peaks above the boundary. The latter excursion is associated with the appearance of the planktonic graptolites in the Ordo-vician. The strength of the Xiaoyangqiao ASSP section is the correlation between the conodonts and graptolites, correspondence of sea-level lowstands, and the matches of geochemical parameters.

Automated summary

The Xiaoyangqiao section in North China was approved as a new ASSP section for the base of the Ordovician System by the Sub-Commission on the Ordovician System in 2019. This section features a well-preserved, abundant and diverse fossil record, including key markers like conodonts and graptolites, aiding in improved intercontinental correlation of the Cambrian-Ordovician boundary.