Cloudina aggregates from the uppermost Dengying Formation, Three Gorges area, South China, and stratigraphical implications
Résumé
Numerous issues remain concerning palaeobiology, palaeoecology, and biostratigraphy of Cloudina, an iconic tubular organism of the Ediacaran period. In this study, accumulations of Cloudina are described from the topmost bed of the Dengying Formation in the Baiguotang section of the eastern Three Gorges area, South China. The massive occurrence of Cloudina refutes any possibility of setting the Ediacaran-Cambrian boundary (E-CB) beneath the lithological transition between Dengying and Yanjiahe formations in this area. The age of the Cloudina aggregates is younger than 540.6 Ma based on a concordia 206 Pb/ 238 U age of 540.6 ± 3.4 Ma obtained from underlying upper part of the Baimatuo Member (Baiguotang section), which is consistent with the 539 Ma E-CB reported from southern Namibia. Combining fossil evidence provided by Asteridium-Heliosphaeridium-Comasphaeridium acritarchs and Anabarites trisulcatus-Protohertzina anabarica small shelly fossil assemblages and the Basal Cambrian Carbon isotope Excursion (BACE), we tentatively place the E-C boundary within the lower part of the Yanjiahe Formation, close to the BACE excursion located in the lower part of the Yanjiahe Formation in the southern Huangling Anticline.