Integrated study of the Doushantuo Formation in northwestern Hunan Province: Implications for Ediacaran chemostratigraphy and biostratigraphy in South China

Abstract

The Ediacaran Doushantuo Formation in South China documents profound biological and environmental changes after the Neoproterozoic global glaciations. However, since the geological records of these changes are scattered in various lithofacies, the establishment of the co-evolutionary pattern of life and environment largely relies on regional stratigraphic correlation. A negative δ13C excursion in the upper Doushantuo Formation in the shelf-margin facies has been correlated with different δ13C chemostratigraphic features in the Doushantuo Formation in the shelf-lagoon facies: either with EN3 (widely regarded as equivalent to the Shuram excursion in Oman) or with EN2, which occur respectively in the upper and middle Doushantuo Formation in the Yangtze Gorges area of South China. Here we report new litho-, chemo-, and biostratigraphic data of the Douoshantuo Formation at the Caojunba section in shelf-margin facies, where the lithostratigraphic sequence and carbon isotopic profile resemble those at other shelf-margin sections, e.g., the Yangjiaping and Zhongling sections. Diverse microfossils including nine genera and eighteen species of acanthomorphic acritarchs are found from the upper Doushantuo Formation at Caojunba, which represent to date the first report of the upper Doushantuo acritarch assemblage from the shelf-margin facies in South China. The newly recovered microfossil assemblage includes Hocosphaeridium anozos, H. scaberfacium, Tanarium conoideum, and clustered leiospheres, and these taxa support the stratigraphic correlation of upper Doushantuo Formation between shelf-margin and shelf-lagoon facies. This correlation indicates that the interval with negative δ13C values in the upper Doushantuo Formation of shelf-margin facies is likely equivalent to EN3. This correlation also suggests that the δ13C profiles of the EN3/Shuram excursion interval in different facies in South China have various expressions (e.g., stratigraphic range, magnitude), making its regional and continental correlations more challenging. On the other hand, the discovery of acritarch assemblage characteristic of the upper Doushantuo Formation in the Yangtze Gorges area from shelf-margin facies confirms their potential as an independent stratigraphic tool in the subdivision and correlation of Ediacaran strata.