Advances in research on the GSSP candidate section for Cambrian Stage 10 (Furongian Series) at Wa'ergang, Hunan, South China.

Approved by a vote in 2005, the International Commission on Cambrian Stratigraphy (ISCS) had chosen a single horizon to define the base of the uppermost Cambrian Stage (Stage 10) at the FAD of cosmopolitan agnostoid species Lotagnostus americanus or another fossil in a comparable stratigraphic position. In following this decision, two sections have been proposed as potential GSSP candidates for the boundary, the Wa’ergang section in South China and the Khose-Nelege section in Siberia. The Wa’ergang section, which is well documented and richly fossiliferous, lies in northwestern Taoyuan County, Hunan Province, South China. It consists of a thick Cambrian-Ordovician carbonate succession in slope facies. In the section, the proposed GSSP horizon is precisely identified at 29.26 m above the base of the Shenjiawan Formation, coinciding with the FAD of Lotagnostus americanus. Detailed field study on the L. americanus-bearing interval reveals this horizon is almost identical with the lowest occurrences of two intercontinentally distributed polymerid trilobites Hedinaspis regalis and Charchaqia norini. Conondont faunas are recorded from the Waergang section, based on detailed sampling in the interval spanning the proposed GSSP horizon. The faunas contain a number of cosmopolitan species such as Proconodontus tenuiserra¬tus, P. muelleri, P. serratus, and Eoconodontus notchpeakensis, enabling to correlate the conodont succession. The yield of E. notchpeakensis is important as the lowest occurrence of the species was proposed as another horizon defining the base of Stage 10. To date, the Wa’ergang section is probably the only section in the world, which yields both Lotagnostus americanus and E. notchpeakensis and shows the great difference in stratigraphic level between their lowest occurrences. Four conodont zones are recognized in the Shenjiawan Formation. The conodont biostratigraphy of Wa’ergang section reveals that base of the P. posterocostatus Zone can be directly correlated to the proposed GSSP horizon, whereas the lowest occurrence of E. notchpeakensis is not in a comparable stratigraphic position with FAD of L. americanus but at a much higher level that is very close to the base of Ordovician. Recent isotope studies showed many negative rather than a single distinguished carbon isotope excursion (i.e. TOCE or HERB) in the uppermost stage of Cambrian elsewhere in the world. The high-resolution carbon isotope analyses for the Shenjiawan Formation in the Wa’ergang section reveals that there are two negative δ13C excursions (named as N1 and N2) preceding the HERB/ TOCE (N3) event, among which the N1 negative δ13C excursion occurs just above the proposed GSSP horizon or close to the base of conodont Proconodontus posterocostatus Zone. It represents the first significant carbon isotope excursion event following the SPICE positive excursion and can be well correlated to that in the Australian and Argentinian sections and probably that in the Laurentian section. The combination of the isotope excursion with the FADs of the globally distributed agnsotoid and conodont species and the widely distributed polymerid species as well allows a confident recognition of the base of Cambrian Stage 10 on a global scale.