SIMS U-Pb Zircon Geochronological and Carbon Isotope Chemostratigraphic Constraints on the Ediacaran-Cambrian Boundary Succession in the Three Gorges Area, South China

Abstract

The Ediacaran-Cambrian succession in South China records dramatic biological, oceanic and geochemical changes, but it is not well constrained geochronologically. This study reports a new SIMS U-Pb date of 543.4±3.5 Ma (MSWD=1.2) from a tuffaceous layer in the Zhoujiaao Section, and carbonate C-O isotopes in both Zhoujiaao and Sixi sections, Three Gorges area. This tuffaceous layer is present in the upper Dengying Formation (i.e., the Baimatuo Member) which is characterized by a stable δ13Ccarb plateau and the beginning of a negative δ13Ccarb shift near its upper boundary. In accordance with the existing biostratigraphic and chemostratigraphic data, this new date corroborates that the upper boundary of the Dengying Formation in South China is approximately equivalent to the Ediacaran-Cambrian boundary (ca. 541 Ma). This age also provides the minimum age of the last appearance of the Shibantan biota in the Three Gorges area, indicating that the terminal Ediacaran index fossils (e.g., Cloudina, Sinotubulites) are not reliable stratigraphic markers for further subdivision of the uppermost Ediacaran.