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Subdividing the Ediacaran of Australia using biostratigraphy
Geological Survey of Western Australia, Department of Industry and Resources.
Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Palaeobiology. Paleobiologi.
Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Palaeobiology. Paleobiologi.
Mineral Resources Tasmania, Australia.
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2005 (English)In: Central Australian Basins Symposium: Petroleum and Mineral Potential, 2005Conference paper (Refereed)
Abstract [en]

A new Global Stratotype and Section (GSSP) for the terminal Neoproterozoic, the Ediacaran Period and System, has been ratified by the International Union of Geological Sciences (IUGS), but problems of subdivision and correlation remain. Hydrocarbon and mineral exploration in the Officer, Amadeus, and Georgina basins, and the Adelaide Rift Complex has resulted in the development of palynological (mainly acritarch) correlations using range charts, based on >1000 samples from >30 drillholes sampled about every 10 m. As in the Cryogenian, biostratigraphic correlations, based on palynology and stromatolite biostratigraphy, are feasible, and results are consistent with correlations based on carbon isotope curves established using splits of palynology samples.

So far, zonation is only possible for the lower and middle Ediacaran in Australia. Upper Ediacaran lithologies are generally unsuitable for palynomorph preservation and assemblages appear to be highly impoverished. However, a distinctive assemblage of large acanthomorph acritarchs, with highly complex morphologies and short stratigraphic ranges, characterises the middle Ediacaran. They are ideal candidates for biostratigraphy and this interval can be correlated with a high degree of confidence. Assemblages from Baltica and the East European Platform suggest that palynological zonation of the upper Ediacaran may be possible, despite species reduction and a return to simple morphologies. Moreover, the upper Ediacaran contains the Ediacara fauna, which may also be a suitable tool for correlation.

Stromatolites indicate Australia-wide correlation at certain levels of the Ediacaran. Incipient columns of Elleria minuta, characteristic of the Amadeus Basin (Marinoan-equivalent) cap dolomite, were identified in a 50 cm-thick dolomite horizon above a diamictite, in Empress-1/1A in Western Australia. Tungussia julia is widespread and appears to be facies independent. It occurs in shallow-water carbonates of the Julie Formation (Amadeus Basin), Wonoka Formation (Adelaide Rift Complex), Elkera Formation (Georgina Basin), and Wilari Dolomite Member of the Tanana Formation (eastern Officer Basin), and is present in the periglacial Egan Formation in the Kimberley area. Relative stratigraphy indicates that the Egan Formation is considerably younger than the Elatina Formation (Marinoan glaciation). The Egan glaciation took place at about 560 Ma, only a short time before the appearance of the first bilaterian trace fossils.

Palynomorph assemblages are sparse during and between the Sturtian and Marinoan glaciations (~700–600 Ma) and samples immediately above the Marinoan glaciation are barren. Post-glacial benthic mats and leiospheres quickly re-established and flourished, as sea level and temperatures rose, but there is no obvious post-glacial species diversification, and no evidence of invasion by extremophiles from hot-spring refugia as envisaged in Snowball Earth predictions. Only a handful of species survived, but pre-glacial species appear to be identical to post-glacial species. Specimen numbers increased rapidly as sea-level rose, but so far, no new taxa have been identified below the Acraman impact ejecta layer.

Above the Acraman impact layer, during a second sea-level rise, there is a striking change in the palynoflora, when >50 species of large acanthomorph acritarchs, belonging to several new genera, first appear and diversify rapidly. They differ significantly from older taxa and in some aspects resemble dinocysts. At least four zones have been recognised, based mainly on assemblages from continuous core in the eastern Officer Basin (Munta-1, Observatory Hill-1, Lake Maurice West-1, and Birksgate-1), the Adelaide Rift Complex succession (SCYW-1a, WWD-1 and MJ-1) and the Amadeus Basin (Wallara-1 and Rodinga-4). More detailed studies are in progress on distributions in Lake Maurice West-1, Observatory Hill-1, Murnaroo-1, and Giles-1. These drillholes are of particular significance because the precise position of the ejecta layer is known in each.

Although the acanthomorph assemblage was recognised previously in Murnaroo-1, systematic sampling was not carried out and the position of the ejecta layer was not known. More refined sampling and the discovery of the ejecta layer at 279.55 m has now confirmed observations from other drillholes that the earliest appearance of the acanthomorphs is above the ejecta layer and that diversification was rapid, with 10 species already present, less than 50 m above the ejecta layer. Studies continue in an attempt to locate the earliest appearance of acanthomorphs. Preliminary examination of samples from Giles-1, where the ejecta layer was found at 554.90 m, confirms the acritarch distribution pattern. Stable isotope studies are also providing significant data about the effect of the Acraman impact on the biosphere.

Several key acanthomorph species are present elsewhere in the world, including Svalbard, Norway, Siberia, and China, raising the possibility of global correlation. In particular, the Australian assemblage has several taxa in common with a succession in an area in eastern Siberia that contains one of the giant Neoproterozoic gas fields. Further work is required to define the ranges of key species outside Australia, so the scheme can be extended globally.

At present, contradictions arise when correlations are attempted with the Doushantuo Formation in China. In part, this reflects the lack of methodical stratigraphic sampling in the Chinese succession and the disparity in thickness between the Chinese succession (<200 m) and the Australian succession (>2000 m). There are also discrepancies in the acritarch biostratigraphy and carbon isotope curves that raise issues about whether the Nantuo Tillite should be correlated with the type ‘Marinoan’ glaciation, the Elatina Formation, and these discrepancies have implications about how many glacial episodes happened in the Neoproterozoic. Recent dating on probable equivalents of the Elatina Formation in King Island and Tasmania suggest an age of 580 Ma. This is similar to the age of the Gaskiers and Squantum Tillites in western Canada, but considerably younger than recently obtained ages of 635 Ma on successions in southern Africa and the Nantuo Tillite in China.

Prospects for subdividing and correlating the Ediacaran using biostratigraphy are excellent, provided taxonomic ranges are properly documented. Biostratigraphic subdivisions can be integrated with other means of correlation to provide a rigorous means of global correlation.

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URN: urn:nbn:se:uu:diva-83166OAI: oai:DiVA.org:uu-83166DiVA: diva2:111073
Available from: 2006-10-23 Created: 2006-10-23

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