DOUSHANTUO-PERTATATAKA ACRITARCHS IN EDIACARAN SUCCESSIONS OF SOUTH CHINA: PRESERVATIONAL BIAS OR ECOLOGICAL CONTROL?

Doushantuo-Pertatataka acritarchs, broadly constrained between the Marinoan glaciation and the first appearance of the Ediacara Biota, are potential biostratigraphic tools for the subdivision and correlation of the Ediacaran System. However, there appears to be an age discrepancy between their gradual appearance in South China, just above the Marinoan-age Nantuo diamictite (635 Ma), and their respective sudden radiation above the Acraman ejecta layer, which lies about 1000 meters above Marinoan-age diamictites in Australia. This implies a preservational bias and/or ecological control. In order to resolve this inconsistency, a high-resolution biostratigraphic, taphonomic, and paleoenvironmental framework in South China must be established to effectively compare the two regions.

We present preliminary high-resolution biostratigraphic and facies data from the 635-551 Ma Doushantuo Formation in Yangtze Gorges region, South China, to better elucidate dominant factors controlling preservation as well as temporal and spatial distribution of the Doushantuo-Pertatataka acritarchs. The Doushantuo Formation in the Yangtze Gorges area provides an excellent template to test a preservational bias or ecological signal because it represents the most complete and fossiliferous succession in China. The lower Doushantuo Formation consists of interbedded carbonaceous shale and cherty argillaceous dolomite, interpreted as deep-water (below wave base) intrashelf deposits. Doushantuo-Pertatataka acritarchs preserved in early diagenetic cherts dominate the fossil assemblage. Their abundance decreases as the succession shallows upward into dolomitic phosphatic and peloidal grainstones in the upper Doushantuo Formation; they only occur rarely in bedded chert horizons in deeper shelf environments. In contrast, in situ and ripped-up microbial mats, cyanobacteria fragments, algae, and Leiosphaeridia, are common in bedded chert horizons of the upper Doushantuo Formation. This suggests that there might be a facies dependence of the Doushantuo-Pertatataka acritarchs in South China – they may be restricted to deeper shelf facies. This interpretation implies that detailed facies analysis is required when Doushantuo-Pertatataka acritarchs are applied as biostratigraphic tools.