Paper No. 4
Presentation Time: 2:30 PM
Quantitative Paleontology of Ediacaran (635-550 Ma) Microfossil Assemblages In the Doushantuo Formation: Implications for Biostratigraphic Applications In South China
The Doushantuo acanthomorphic acritarchs (635-551 Ma) are large morphologically complex organic-walled microfossils broadly constrained between the Marinoan glaciation and the first appearance of Ediacara Biota. They are potential biostratigraphic tools for subdivision and correlation of the Ediacaran System in China. However, regional discrepancies between their gradual appearance in the Yangtze Gorges area, just above the Marinoan-age Nantuo diamictite (635 Ma), and their respective sudden appearance above a middle Doushantuo sequence boundary in Weng'an, ~750 km to the southwest, imply a preservational bias and/or ecological control. In order to resolve this inconsistency, we provide a biostratigraphic framework in China utilizing high-resolution paleontological data from the Doushantuo Formation along a north-south trending profile in the Yangtze Gorges region. Six sections over 100 km apart were incorporated into this study, and >25,000 individual fossils from early-diagenetic cherts were counted to flesh out temporal and spatial distribution of prokaryote, multicellular algae, leosphaerid, and acanthomorphic acritarch groups. Preliminary paleontological data suggest that abundance is highly variable for all fossil groups in all sections. However, total diversity (including prokaryotes) decreases spatially from 34 species in the deep basin (Sixi, Jiulongwan, and Sifantang) to 28 species in shallow water sections (Wangfengang and Xiaofenghe); acritarch diversity decreases from 16 to 10 species, respectively. Acanthomorphic acritarchs in deep water successions first appear shortly after the Marinoan glaciation, five meters above an ash layer dated at 632 Ma, and diversify throughout the lower half of the Doushantuo. However, in shallow subtidal successions updip first appearance of acritarchs occur higher in the section. In the most proximal section at Zhancunping, acritarchs are rare and occur stratigraphically above a sequence boundary in the middle Doushantuo. In all sections, acritarchs are most commonly associated with laminated, clean to argillaceous cherty dolomite, suggesting that the Doushantuo acritarchs could be restricted to deeper shelf environments.