New U–Pb age from the basal Niutitang Formation in South China: Implications for diachronous development and condensation of stratigraphic units across the Yangtze platform at the Ediacaran–Cambrian transition

Abstract

The black shale dominated Niutitang Formation covers a large portion of the early Cambrian Yangtze platform in South China. The base of this unit has been traditionally taken as the Ediacaran–Cambrian boundary (ca. 542Ma). Recent radiometric ages from the basal Niutitang Formation and its correlative units, however, are significantly younger than 542Ma and raise questions about the diachrony of stratigraphic units across the Yangtze platform at the Ediacaran–Cambrian transition. Here we report a new U–Pb sensitive high-resolution ion microprobe (SHRIMP) age of 522.7±4.9Ma from a tuffaceous bed of the basal Niutitang Formation in a deep-water section in Taoying, Guizhou Province of South China. This age, in combination with existing ages and biostratigraphic data, suggests that the base of the Niutitang Formation is of Tommotian or upper Meishucunian age and the Ediacaran–Cambrian boundary is most likely within the Liuchapo Formation. The depositional age of the unusual metal-enriched (Ni–Mo–PGE) sulfide deposits, which are approximately 1m above the ca. 523Ma tuffaceous bed, should be early Tommotian (upper Meishucunian) rather than early Nemakit-Daldynian (lower Meishucunian). Regional correlation confirms the diachronous development and condensation of stratigraphic units across the early Cambrian Yangtze platform and provides important information for understanding the paleogeography and paleoceanographic events at the Ediacaran–Cambrian transition in South China.