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    Refining the chronostratigraphy of the Karoo Basin, South Africa: Magnetostratigraphic constraints support an early Permian age for the Ecca Group

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    Authors
    Belica, M.
    Tohver, E.
    Poyatos-Moré, M.
    Flint, S.
    Parra-Avila, L.
    Lanci, L.
    Denyszyn, S.
    Pisarevskiy, Sergei
    Date
    2017
    Type
    Journal Article
    
    Metadata
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    Citation
    Belica, M. and Tohver, E. and Poyatos-Moré, M. and Flint, S. and Parra-Avila, L. and Lanci, L. and Denyszyn, S. et al. 2017. Refining the chronostratigraphy of the Karoo Basin, South Africa: Magnetostratigraphic constraints support an early Permian age for the Ecca Group. Geophysical Journal International. 211 (3): pp. 1354-1374.
    Source Title
    Geophysical Journal International
    DOI
    10.1093/gji/ggx344
    ISSN
    0956-540X
    School
    School of Earth and Planetary Sciences (EPS)
    URI
    http://hdl.handle.net/20.500.11937/72980
    Collection
    • Curtin Research Publications
    Abstract

    © The Authors 2017. The Beaufort Group of the Karoo Basin, South Africa provides an important chrono- and biostratigraphic record of vertebrate turnovers that have been attributed to the end-Permian mass extinction events at ca. 252 and 260 Ma. However, an unresolved controversy exists over the age of the Beaufort Group due to a large data set of published U-Pb SHRIMP (Sensitive High Resolution Ion Microprobe) zircon results that indicate a ca. 274-250 Ma age range for deposition of the underlying Ecca Group. This age range requires the application of a highly diachronous sedimentation model to the Karoo Basin stratigraphy and is not supported by published palaeontologic and palynologic data. This study tested the strength of these U- Pb isotopic data sets using a magnetostratigraphic approach. Here, we present a composite ~1500 m section through a large part of the Ecca Group from the Tanqua depocentre, located in the southwestern segment of the Karoo Basin. After the removal of two normal polarity overprints, a likely primary magnetic signal was isolated at temperatures above 450 °C. This section is restricted to a reverse polarity, indicating that it formed during the Kiaman Reverse Superchron (ca. 318-265 Ma), a distinctive magnetostratigraphic marker for early-middle Permian rocks. The Ecca Group has a corresponding palaeomagnetic pole at 40.8°S, 77.4°E (A95= 5.5°). U-Pb SHRIMP ages on zircons are presented here for comparison with prior isotopic studies of the Ecca Group. A weighted mean U-Pb age of 269.5 ± 1.2 Ma was determined from a volcanic ash bed located in the uppermost Tierberg Formation sampled from the O + R1 research core. The age is interpreted here as a minimum constraint due to a proposed Pb-loss event that has likely influenced a number of published results. A comparison with the Geomagnetic Polarity Time Scale as well as published U-Pb TIMS ages from the overlying Beaufort Group supports a ca. 290-265 Ma age for deposition of the Ecca Group.

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