| 1. |
- Grey, Kathleen, et al.
(författare)
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Neoproterozoic subdivision in Australia
- 2005
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Ingår i: Central Australian Basins Symposium.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Neoproterozoic subdivision is well advanced in Australia using integrated results from lithostratigraphy, palynology, isotope chemostratigraphy and stromatolite biostratigraphy. Previously proposed Cryogenian correlations were tested by the drilling of GSWA Lancer-1 in the western Officer Basin and were found to be reliable. The consistency of the results allows the succession to be tied to limited geochronological ages from the Adelaide Rift Complex. Data are still sparse on the interval between the Sturtian and Marinoan glaciations, but good correlations exist in the Ediacaran.The base of the cap carbonate in Enorama Creek in the Adelaide Rift Complex has been ratified as the Global Stratotype Section and Point (GSSP) for the newly defined Ediacaran System and Period. The Flinders Ranges have a near-continuous section from the Marinoan glaciation to the Cambrian boundary, especially around the Brachina and Bunyeroo gorges. Lithostratigraphy, and local and regional correlations are well established, including links between the type sections, measured sections throughout the Adelaide Rift Complex and continuously cored drillhole sections on the Stuart Shelf, 80–100 km to the northeast, and to the Officer and Amadeus basins. An integrated approach, using lithostratigraphy; stratigraphic markers (glacial episodes, the time-synchronous Acraman impact ejecta layer, canyon cutting); carbon isotope chemostratigraphy; sequence stratigraphy; seismic interpretation; stromatolite biostratigraphy; and the first appearance of bilaterians and associated trace fossils, has proved successful.Acritarchs are acid-insoluble fossils of single-celled, phytoplanktonic green algae and are ideal for zonation. A rigorous sampling program began in 1991 to establish an acritarch biostratigraphy throughout Australian Neoproterozoic basins, based on palynological studies of continuously cored drillholes. Extensive field sampling of sections in the Flinders Ranges was unsuccessful, partly because the rift succession is too thermally mature for palynomorphs to be preserved, and partly because organic material has been leached from a deeply weathered profile. No identifiable acritarchs were recovered from this area, or from outcrops in other basins. However, Stuart Shelf drillholes contain well-preserved palynomorphs, although preservation is patchy in parts of the succession. Nevertheless, there are enough data for correlation with excellently preserved assemblages from the Officer and Amadeus basins. Georgina Basin preservation is too poor for useful analysis at present. Data from the Officer and Amadeus basins demonstrate the potential for biostratigraphic zonation in the lower and middle Ediacaran. Biostratigraphic correlation parallels correlations based on organic carbon isotope curves.Middle Ediacaran acritarch assemblages are extraordinarily diverse taxonomically, morphologically complex, and show typical patterns of secular diversity. These characteristics are ideal for the development of a zonal scheme, with levels of precision equivalent to the Phanerozoic record. The zones are independent of taphonomic and palaeoenvironmental influences, and they are demonstrably independent of lithology, lithostratigraphy, and sequence stratigraphy. Using composite sections, two palynofloras, the Ediacaran Leiosphere Palynoflora (ELP) and the Ediacaran Complex Acanthomorph Palynoflora (ECAP), have been recognised. The ECAP has been subdivided into four zones. The scheme is preliminary, but represents a significant advance in Neoproterozoic biostratigraphic studies, and should provide an important tool for future stratigraphic correlation. Additional studies are in progress to refine the correlations by examining undocumented parts of the succession, especially from Murnaroo-1 and Giles-1. The presence of certain acritarch species in probably coeval successions in Australia, Siberia, China, and northern Europe, suggests that the proposed zonation has good potential for global application, and that biostratigraphic principles and methodology can be applied to the Neoproterozoic.
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| 2. |
- Grey, Kathleen, et al.
(författare)
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Subdividing the Ediacaran of Australia using biostratigraphy
- 2005
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Ingår i: Central Australian Basins Symposium.
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Konferensbidrag (refereegranskat)abstract
- 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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| 3. |
- Grey, Kathleen, et al.
(författare)
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Taphonomy of Ediacaran acritarchs from Australia: significance for taxonomy and biostratigraphy
- 2009
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Ingår i: Palaios. - : Society for Sedimentary Geology. - 0883-1351 .- 1938-5323. ; 24:3-4, s. 239-256
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Tidskriftsartikel (refereegranskat)abstract
- A diverse assemblage of Australian Ediacaran (late Neoproterozoic) acritarchs from the Centralian Superbasin and Adelaide Rift Complex demonstrates a range of taphonomic degradation. Recognition of taphonomic variants is critical for taxonomic studies and biostratigraphic interpretation. Taphonomic features observed include compression features, folding and tearing of vesicle walls, pitting, perforation, abrasion, exfoliation, shrinking, twisting, splitting, curling, shredding, pyritization, particle entrapment, and thermal maturation effects. The physical and chemical structure of the vesicle wall is instrumental in determining the degree of taphonomic damage. Consistent associations allow Identification of degradation series that incorporate previously described individual species and provide a framework for taxonomic revision. Taphonomic associations may also characterize taphofacies, providing an additional tool for basin analysis.
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| 4. |
- Hill, Andrew C., et al.
(författare)
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New records of Ediacaran Acraman ejecta in drillholes from the Stuart Shelf and Officer Basin, South Australia
- 2007
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Ingår i: Meteoritics and Planetary Science. - 1086-9379 .- 1945-5100. ; 42:11, s. 1883-1891
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Tidskriftsartikel (refereegranskat)abstract
- New occurrences of the Acraman impact ejecta layer were recently discovered in two South Australian drillholes, SCYW-79 1a (Stuart Shelf) and Munta 1 (Officer Basin) using lithostratigraphy, acritarch biostratigraphy, carbon isotope stratigraphy, and biomarker anomalies to predict the stratigraphic position. The ejecta layer is conspicuous because it consists of pink, sand-sized, angular fragments of volcanic rock distributed along the bedding plane surface of green marine siltstone. In SCYW-79 1a it forms a layer 5 mm thick; in Munta 1 the ejecta layer is thin and discontinuous because of its distance (similar to 550 km) from the impact structure. Palynological, biomarker, and carbon isotope anomalies can now be shown to coincide with the ejecta layer in SCYW-79 1a and Munta 1 suggesting the Acraman impact event may have had far reaching influences on the rapidly evolving Ediacaran biological and geochemical cycles.
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| 5. |
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| 6. |
- Willman, Sebastian, et al.
(författare)
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Acritarchs in the Ediacaran seas
- 2005
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Ingår i: The Palaeontological Association. ; , s. 64-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The terminal Neoproterozoic radiation of planktonic photosynthetic microbiota (acritarchs) is one of the most significant evolutionary events of the time, including diversification of prokaryotic cyanobacteria and eukaryotic green and brown algae, the appearance of thecoamoebaens and subsequently metazoans (the Ediacara fauna). The Ediacaran radiation of phytoplankton is recognizable by the first appearance of more than fifty new species of large ornamented acritarchs in a short interval of time at ca. 570 Ma. This radiation event occurred after the Snowball Earth conditions returned to a kind of "normal" environmental stasis, and it may be interpreted as a recovery diversification of phytoplankton after a major biotic extinction caused by the global glaciation.The appearance of numerous, morphologically innovative and large acritarch taxa may also be connected with the Acraman impact event in South Australia, suggested recently by Grey et al. (2003), as a biotic recovery after the catastrophic environmental disturbance caused by the giant bolide. The latter hypothesis has to be tested, however, because a few individual taxa of ornamented acritarchs may have actually appeared below the ejecta layer, which is difficult to recognize with certainty in some borehole successions.The Ediacaran acritarch records are from Australia (the Officer and Amadeus Basins), China and Siberia, showing a worldwide distribution in a relatively short interval of time (ca. 20 Ma; Grey, 2004, in press). The greatest taxonomic diversity is known from Australia (ibidem), and the present study is focused on the investigation of Ediacaran microbiota in greater detail and from different stratigraphic levels, their palaeobiology and affinities, mode of life and reproduction cycle. The new assemblage of organic-walled microfossils from the Murnaroo 1 borehole comprises filamentous cyanobacteria, and ornamented and spheroidal acritarchs.The Ediacaran successions in Australia have been well documented in terms of lithostratigraphy, depositional settings and structural geology. The sedimentation proceeded in two different depositional regimes, recognized today in a series of sub-basins. One of them is the Officer Basin, comprised of complex intracratonic, east-west trending troughs and sub-basins extending from Western Australia to South Australia. The studied Murnaroo 1 borehole is also located there. The sediments accumulated in tidal, sub- and intertidal shelf conditions, and the predominantly mudstone lithology from which the samples were collected, is ideal for palynological processing and preservation of microfossils. The lack of macrofossils in the successions rendered efforts and advances in acritarch biostratigraphy since the 1980's, which helped to reveal a complex history of the Officer Basin. The discovery of two distinct palynofloras, an older leiosphere-dominated flora (ELP) and a younger acanthomorph-dominated flora (ECAP), is suggested to be largely environmentally independent (Grey, 2004, in press) in terms of the observed lithology and sedimentological sequences. However, the possible coupling between the Marinoan glaciation, the Acraman impact and the radical change in the palynofloras was inferred (ibidem) and this will be examined with the new data available from the Murnaroo 1 borehole. Previously, the Murnaroo succession was only studied preliminarily.In my communication, I will discuss the stratigraphic sequence of appearances of various species and their relationship to the environmental conditions, the Acraman impact event and the changes associated with the global glaciations.GREY, K., WALTER, M.R. and CALVER, C.R. (2003) Neoproterozoic biotic diversification: "Snowball Earth" or aftermath of the Acraman impact? Geology 31, p. 459-462.GREY, K., (2004, in press) Ediacarian Palynology of Australia. Australasian Association of Palaeontologists, Memoirs.
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| 7. |
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| 8. |
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| 9. |
- Willman, Sebastian, et al.
(författare)
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Taphonomic analysis of Ediacaran acritarchs and its importance for taxonomy,biostratigraphy and global correlation
- 2008
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this study we analysed the taphonomic degradation history of a diverse assemblage of Ediacaran(late Neoproterozoic) acritarchs from the Centralian Superbasin and Adelaide Rift Complex inAustralia. Taphonomic features observed include compression features, folding and tearing ofvesicle walls, pitting, perforation, abrasion, exfoliation, shrinking, twisting, splitting, curling,shredding, pyritization, particle entrapment, and thermal maturation effects. The physical andchemical structure of the vesicle wall determines the degree of taphonomic damage. Consistentassociations allowed identification of degradation series which incorporate previously describedindividual species and provide a framework for taxonomic revision. Recognition of taphonomicvariants is an important first step in systematic studies, and tracking degradational pathways forparticular species resulted in more precise taxonomic identification. Other biostratigraphicallyuseful fossils are uncommon in the Neoproterozoic which means that considerable reliance willbe placed on acritarch biostratigraphy for future global correlations. It is vital, therefore, that thesignificance of taphonomic degradation, for both taxonomy and palaeoenvironmental analysis, isgiven adequate recognition. Identification of taphonomic variants is critical for taxonomic studiesand must be considered before making biostratigraphic subdivision of the Ediacaran System.
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