|
Chuanming Zhou, Nanjing Institute of Geology and Palaeontology (China)
Shuhai Xiao, Virginia Polytechnic Institute and State University (United States)
Zhe Chen, Nanjing Institute of Geology and Palaeontology (China)
Xunlai Yuan, Nanjing Institute of Geology and Palaeontology (China)
|
|
The Ediacaran System in South China was deposited in a southeast-facing (present orientation) passive margin on the Yangtze Block, which was located near Australia or India in late Neoproterozoic. It overlies ∼635 Ma glacial deposits of the Nantuo Formation and underlies basal Cambrian cherts and phosphorites, and consists of the Doushantuo and Dengying formations. Doushantuo lithologies are dominated by organic-rich, fine-grained siliciclastics, especially in the basinal settings on the southeast Yangtze Block, whereas carbonates and phosphorites occur in shallow-water platform facies in the northwestern.
The overlying Dengying Formation has more uniform lithologies on the shallow-water platforms and is characterized by thick successions of dolostone with minor siliceous shales and mudstones, whereas it is dominated by black cherts in basinal environments.
Detailed palaeontological investigations in the past 30 years have revealed fossil assemblages including abundant large acanthomorphic acritarchs (Doushantuo- Pertatataka acritarchs), micro-algae, tubular metazoans, animal embryos, questionable bilaterian, and Burgess-shale type carbonaceous compressions (mainly macro-algae) in the Doushantuo Formation of South China.
Biostiatigraphic studies indicate that Doushantuo-Pertatataka acritarchs appear shortly after the termination of Nantuo glaciation (∼635 Ma), and disappear before the rise of large Ediacara organisms, at a horizon with a pronounced negative δ13C excursion in the uppermost Doushantuo Formation. Despite the discrepancy between South China/northern India vs. South Australia, biostratigraphy of Doushantuo-Pertatataka acritarchs may be the only useful biostratigraphic tools in the early Ediacaran, before the rise of classical Ediacara fossils and biomineralizing animals.
Unlike the Doushantuo Formation, animal fossils dominate the late Ediacaran Dengying Formation (∼551-542 Ma) in South China. Up to now, Ediacara macrofossils, trace fossils, and tubular shelly fossils such as Cloudina and Sinotubulites have been discovered in the Dengying Formation. These fossils, particularly Cloudina, occur in many upper Ediacaran successions around the world.
Based on the different fossil assemblages in the early (characterized by acritarchs, micro-algae, and micrometazoans) and late (characterized by macro-metazoans and biomineralizing fossils) Ediacaran, we can subdivide the Ediacaran System into two or more Series. One of the golden spikes might be nailed at the last appearance horizon of Doushantuo-Pertatataka acritarchs. This horizon is also near the pronounced negative δ13C excursion in the uppermost Doushantuo Formation and near one of the best dated Ediacaran ash beds (∼551 Ma). The first appearance horizons of Doushantuo-Pertatataka acritarchs (e.g., the FAD of Tianzhushania spinosa) and the shelly fossil Cloudina may also be served as additional markers at the Series or Stage level.
|
