Acritarchs and late Neoproterozoic correlations

Category Paleontology and Stratigraphy
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Willman, Sebastian
Holding Date 21 September 2008

The Neoproterozoic was a time of major environmental change, including global glaciations during the Cryogenian Period. The Ediacaran Period (~35-542 Ma ago), on the other hand, is a period with greenhouse-type conditions. These warmer conditions caused major shifts in ocean geochemistry, including oxygenation of the oceans, and evolution of the marine biosphere. The Ediacaran fossils document a most important evolutionary event that includes the origin and diversification of a variety of microorganisms and metazoans. Among them, organic-walled microbiota of unknown affinities, referred to as acritarchs, forms an integral part of the biosphere. Acritarchs seem to change from long-ranging and showing little geographic partitioning throughout the Mesoproterozoic, to the taxa in the Ediacaran showing rapid turnovers, accelerated evolution and geographic partitioning. This diversification of microbiota is preserved in Ediacaran-age rock successions worldwide, especially in the Centralian Superbasin in Australia but also in Siberia, China and Baltica.
Increased interest in Neoproterozoic oil plays has reinforced the need for robust correlation of this period of time. A problem in Neoproterozoic subdivision has been the lack of biostratigraphic control, and although Neoproterozoic biostratigraphy is increasingly well-understood it is a developing field. The methods for correlation of the Neoproterozoic successions currently applied include chemostratigraphy, sedimentology, event stratigraphy (impacts, glacial episodes, volcanic eruptions and the alike), magnetostratigraphy, and the subject of this presentation: Ediacaran biostratigraphy using acritarchs.
Microfossils were collected from unevenly sampled intervals in the Giles 1, Murnaroo 1, Lake Maurice West 1, WWD 1, Observatory Hill 1, and Munta 1 drillcores. Samples were selected from lithologies that are suitable for palynological preservation (i.e., unoxidized mudstones, shales, and carbonaceous rocks). Examination of the fossils using transmitted light microscopy provides further evidence for the Ediacaran acritarch diversification previously documented. Many acritarch taxa are stratigraphically constrained and the patterns observed here match patterns first reported by Grey in 2005. Acritarchs are well-preserved and diverse, change over short stratigraphic intervals, and allow the recognition of the previously established zones by use of certain acanthomorphic species. The presence of common species and taxonomic similarities between entire assemblages from Australia, Siberia, Baltica and South China provide a means for global correlation of the Ediacaran System using palynology. Portions of the Ediacaran System can be confidently correlated using discrete species distributed across various palaeocontinents. This suggests that acritarchs may be used for biostratigraphic analysis and correlation because other fossils are too scarce, too geographically restricted, or too difficult to interpret.