Integrated Ediacaran (Sinian) chronostratigraphy of South China

Abstract

The Ediacaran Period was a critical time in the history of life and Earth. Understanding the complex interactions between geological and biological events during this interval requires high-resolution chronostratigraphy. Thanks to its uniquely favorable palaeogeographic setting, South China is an ideal region for constructing a framework for global chronostratigraphic correlation of Ediacaran strata. Here, continuous Ediacaran successions contain both siliciclastic and carbonate rocks that range from shallow to deep marine and contain rich fossil assemblages showing a variety of preservation styles. As part of the present study, 62 reference sections distributed throughout the Yangtze Platform were investigated. Analyses of the sedimentary facies and sequence stratigraphy of more than 40 sections revealed four major sequences in the shallow marine facies. Three major sequence boundaries occur respectively in the middle and upper parts of the Doushantuo Formation and in the middle part of the Dengying Formation. Subdivision and correlation of the Ediacaran successions of the Yangtze Platform based on sequence stratigraphy is congruent with the carbon isotope chemostratigraphy of 12 reference sections (1075 samples). The sequence boundary (SB2) in the middle part of the Doushantuo Formation is suggested here as the Lower (Xiadongian)/Upper (Yangtzean) series boundary of the Ediacaran which coincides with several globally recognizable events, namely 1) the end of the Neoproterozoic glaciations, 2) the first appearance of animals, 3) the sudden increase in the diversity of acanthomorphic acritarchs, and 4) a negative δ 13C excursion. The Ediacaran System of South China may be subdivided into five stages. Furthermore, the criteria for defining these stages may be applicable throughout the world. Also, the composite carbon isotope profile of the Ediacaran of South China can be used as a global reference for chronostratigraphic subdivision and correlation. In addition to the globally recognizable, negative δ 13C excursion in the cap carbonate at the base of the Ediacaran, the negative δ 13C excursion in the uppermost Doushantuo/Wonoka/Shuram interval represents another, globally correlatable major excursion. The positive δ 13C excursion immediately following the uppermost Doushantuo excursion and ending at 548 Ma is also useful for global correlation. Current Ediacaran chronostratigraphy and global correlations indicate that the upper Doushantuo/Wonoka/Shuram negative δ 13C excursion ranges from ca. 560 Ma to 551 Ma, and therefore coincides with the evolutionary renovation of life. Feedback between evolutionary innovation and geochemical cycling was the key mechanism for this unique and most strongly negative δ 13C excursion in earth history. Our chronostratigraphic correlations also imply that the earliest known metazoan biotas, namely the Weng'an and characteristic Ediacaran biotas, do not constitute two temporally separate evolutionary lineages, as proposed previously, but represent two parallel evolutionary pathways preceding the Cambrian explosion of animal life.