SEQUENCE AND CHEMOSTRATIGRAPHY OF THE TERMINAL NEOPROTEROZOIC (EDIACARAN) YANGTZE PLATFORM

The terminal Neoproterozoic (Ediacaran Period, ca. 635–542 Ma) Yangtze platform in south China are composed of up to 1000-m-thick siliciclastic and carbonate rocks assigned to Doushantuo and Dengying formations that contain abundant large acanthomorph acritarch, multicellular algae, and diverse Ediacara fossils. The siliciclastic to carbonate strata are divisible into seven intervals bounded by regional stratigraphic discontinuities, among which three are interpreted as karstic unconformities (sequence boundaries) on the basis of (1) paleokarstic depressions with mappable relief; (2) subaerial dissolution and weathering products (breccias and calcrete) filling vertical fissures, dikes, cavities, and shallow depressions in underlying carbonate rocks; (3) small-scale evidence for subaerial exposure at an erosion surface; and (4) oolitic/intraclastic grainstone and siltstone-wackestone facies overlying the surfaces contain reworked components of paleokarstic material, including sporadically distributed lag deposits and breccias with partially rounded fragments. These karstic unconformities, however, appear exclusively in the Dengying Formation that, according to the recent age constraints, was deposited during a time period of less than 8 million years. An increase of accommodation space during deposition of the Dengying Formation is required and, considering the passive margin setting of the Yangtze platform during the terminal Neoproterozoic, a tectonic event near 550 Ma is implied. Isotopic analysis from multiple sections across the two platform to basin transects show considerable variability in both absolute δ¹³C values and trends. Some negative δ¹³C 'excursions' such as those at the cap carbonate level and near the Doushantuo/Dengying boundary may have global significance, while others may reflect local phenomena and/or diagenetic artifacts. Comparing with the global stratigraphic record, a more detailed Ediacaran chronostratigraphy is emerging, but radiometric ages are still the key for the division and correlation of the Ediacaran strata.