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Using volcaniclastic rocks to provide depositional ages : what can (and cannot) be inferred from U-Pb/Zircon dating ? Examples from south East Asia terrestrial sections

Abstract : Volcaniclastic sediments are commonly used to provide constraints on the depositional age of sedimentary rocks by the mean, for example, of U-Pb geochronology on zircon. These time constraints allow for correlations between distant basins and are critical to assess the contemporaneity between major events, such as mass extinctions. In this case, the spatial and temporal patterns of extinction and recovery provide essential information about the mechanisms leading to the extinctions. However, assessing the temporal patterns of extinction and recovery using U-Pb/zircon geochronology on volcaniclastic rocks requires that volcanic activity is coeval with sedimentation. With the exception of “primary” volcaniclastic rocks (e.g., pyroclastic flows), rare in the sedimentary record, all the other “secondary” volcaniclastic rocks are made of volcaniclasts that have been reworked to various extents. In ancient deposits, the distinction between primary and secondary (reworked) volcaniclasts is notoriously difficult, hindering a direct access to the conclusion that volcanism and sedimentation were contemporaneous. The use of secondary volcaniclastic rocks for dating purposes thus often results in obtaining maximum depositional ages that may be significantly older than sedimentation, but the assessment of the contemporaneity between volcanism and sedimentation is often overlooked, if not simply ignored. To assess the contemporaneity between volcanism and sedimentation in ancient volcaniclastic series, we propose a simple methodology that requires a large number of samples collected along continuous sedimentary piles. This method relies on the comparison of (i) the relative stratigraphic positions of the investigated samples, (ii) their maximum depositional age and (iii) the analytical precision of the measured ages. If the maximum depositional ages get systematically younger together with the position of the samples in the stratigraphic column, the correlation between absolute ages and stratigraphy is very likely to demonstrate that some volcaniclasts were produced contemporaneously with sedimentation. The differences in maximum depositional ages from two successive samples in a pile, and, in some cases, the uncertainties obtained from the U-Pb ages can then be used as crude estimates of the ‘lag time’, defined as the difference between the age of the volcaniclast production and the depositional age of its host strata. Applied to various Permian and Triassic terrestrial sections located in South East Asia, this methodology allows to valid, reject or evidence the impossibility to demonstrate the contemporaneity between volcanism and sedimentation. Such assessments contribute to relativize the strength on which earlier interpretations about synchronism of major events, such as the Permian-Triassic mass extinction, are based
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Conference papers
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Submitted on : Tuesday, January 31, 2017 - 11:40:12 AM
Last modification on : Thursday, March 5, 2020 - 11:56:11 AM

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  • HAL Id : insu-01450684, version 1

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Camille Rossignol, Sylvie Bourquin, Erwan Hallot, Marc Poujol, Marie-Pierre Dabard, et al.. Using volcaniclastic rocks to provide depositional ages : what can (and cannot) be inferred from U-Pb/Zircon dating ? Examples from south East Asia terrestrial sections. 32nd IAS meeting of Sedimentology, International Association of Sedimantologists, May 2016, Marrakech, Morocco. ⟨insu-01450684⟩

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