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Precise U-pb age constrains on the ediacaran biota in podolia, east european platform, Ukraine

Abstract : the Neoproterozoic era was characterized by rapidly changing paleogeography, global climate changes and especially by the rise and fall of the ediacaran macro-biota. the correlation between disparate ediacaran fossil-bearing localities and the tentative reconstruction of their paleoenvironmental and paleogeographic contexts are usually complicated by the lack of precise and accurate age data. For this reason, Neoproterozoic sedimentary sections associating ediacaran biota fossils and fresh volcanic material are especially valuable for radioisotopic dating. our research in the podolya Basin, southwestern Ukraine, revealed the presence of four Neoproterozoic volcanic ash deposits (potassium-bentonite layers) within ediacaran fossil-bearing siliciclastic rocks of the Mohyliv-podilskyi Group. We used zircon U-pb LA-ICpMs and CA-ID-tIMs methods to date two of those layers. the results indicate that a diverse assemblage of body and trace Ediacaran fossils occurred as early as 556.78 ± 0.18 million years (Ma) ago. By combining morphological evidence and new age determinations, we suggest a closer paleobiogeographical relationship between the Ukrainian ediacaran assemblage and the Avalon paleocontinent than previously estimated. The Neoproterozoic Era corresponds to a period of global changes related to the breakup of the supercontinent Rodinia and to protracted global glacial events 1. In terms of biological evolution, it is associated with deep innovations likely related to the so-called 'second great oxygenation event' (NOE) 2 , and is marked by the rise and fall of the Ediacaran biota 3-8. As revealed by over thirty sites inventoried worldwide 6,9-14 , the soft body imprints of the Ediacaran macro-organisms have been preserved in various marine environments and related deposits, such as carbonate rocks 15-17 , turbidites and volcanoclastic successions 18,19 , as well as siliciclastic deposits 20-22. Several species forming the Ediacaran biota-e.g., Charnia (575-545 Ma), Dickinsonia (560-541 Ma), Onegia (558-543 Ma), Rangea (558-545 Ma), Palaeopascichnus and Tribrachidium (558-541 Ma)-are long-lived taxa without substantial morphological change 10 , and their presence/absence thus does not represent a useful indicator for reliable biostratigraphical assessment. Additionally, the common lack of datable ash beds interlayered with the sedimentary sequences is the major obstacle for geochronological correlations between different Ediacaran fossil bearing sections 6,8,13. In most contexts, the only way Ediacaran biostratigraphy could be appropriately placed into reliable chronological order is by high-precision radioisotopic dating of zircons from the products of large explosive volcanic eruptions such as ash, tuff or ignimbrite interlayered within Ediacaran fossil bearing strata 19,23-25. In some Proterozoic terrains, ash deposits are altered and transformed into bentonite, whose chemical composition and mineralogy depend on the alteration processes and diagenetic history 26. In southwestern Ukraine, the siliciclastic deposits of the Mohyliv-Podilskyi Group outcropping in the Podolya Basin have revealed an abundant Ediacaran macrofauna 22 , but the preservation conditions of the fossil assemblages do not systematically grant secure biostratigraphic correlations at macro-regional scale across different sedimentary basins. For now, only one bentonite bed has been described in the Yarishyvska Formation 27. However, the only available date for this context of 553 Ma 28 is from the tuffaceous level without related information on its stratigraphical position as well as petrological description. Therefore, with the aim of constraining the
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Y Soldatenko, A. El Albani, M Ruzina, C. Fontaine, V Nesterovsky, et al.. Precise U-pb age constrains on the ediacaran biota in podolia, east european platform, Ukraine. Scientific Reports, Nature Publishing Group, 2019, 9, pp.1675. ⟨10.1038/s41598-018-38448-9⟩. ⟨hal-02529286⟩

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