Magnetostratigraphy across the Triassic-Jurassic boundary in the main Karoo Basin |
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| Institution: | 1. Department of Geological Sciences, University of Cape Town, Cape Town, South Africa;2. Department of Geology, University of Johannesburg, Johannesburg, South Africa;3. Fundación Conjunto Paleontológico de Teruel-Dinópolis, Teruel, Spain;1. Dpt. de Mineralogia, Petrologia i Geologia Aplicada, Facultat de Geologia, Universitat de Barcelona, Martí i Franquès, s/n, 08028 Barcelona, Spain;2. Dpto. de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad de Granada, Avda. Fuentenueva, s/n, 18071 Granada, Spain;3. Instituto Andaluz de Ciencias de la tierra, CSIC-Universidad de Granada, Avda. de las Palmeras, 4, 18100 Armilla, Granada, Spain;4. Dpt. Enginyeria Civil i Ambiental, Universitat Politècnica de Catalunya, Jordi Girona, 31, 08034 Barcelona, Spain;1. Univ Lyon, Université Claude Bernard Lyon 1, ENS de Lyon, CNRS, UMR 5276 LGL-TPE, F-69622 Villeurbanne, France;2. Sorbonne Universités, UPMC Université Paris 06, UMR 7193, ISTeP, 75005 Paris, France;3. MARE-Marine and Environmental Sciences Centre, Faculty of Sciences and Technology, Department of Earth Sciences, University of Coimbra, Rua Sílvio Lima, 3030-790 Coimbra, Portugal;4. Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark;1. Paleomagnetic Laboratory ‘Fort Hoofddijk’, Department of Earth Sciences, Utrecht University, Netherlands;2. Stratigraphy/Paleontology, Department of Earth Sciences, Utrecht University, Netherlands;3. Nordic Centre for Earth Evolution, University of Copenhagen, Denmark;1. Department of Earth Sciences “Ardito Desio”, University of Milano, via Mangiagalli 34, 20133 Milano, Italy;2. Department of Geosciences, University of Padova, via Gradenigo 6, 35131 Padova, Italy;3. Institute of Geosciences and Earth Resources, Consiglio Nazionale delle Ricerche (CNR), via Gradenigo 6, 35131 Padova, Italy;4. Department of Physics and Earth Sciences, University of Ferrara, via Saragat 1, 44122 Ferrara, Italy;5. Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854, USA;6. Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA |
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| Abstract: | The end-Triassic mass extinction and the transition and explosive diversification of fauna over the Triassic-Jurassic boundary is poorly understood and poorly represented in the rock record of the Southern Hemisphere. This is despite the rich diversity in both body and trace fossils of Triassic-Jurassic age in southern Africa, which is not found in coeval Northern Hemisphere localities. We report here the first palaeomagnetic polarity zonation of the Upper Triassic-Lower Jurassic continental red bed succession (Elliot Formation; Stormberg Group) in southern Africa. The results from 10 partially overlapping sections, with a composite thickness of ~ 280 m, provide a magnetic polarity chronology of the main Karoo Basin in South Africa and Lesotho. Palaeomagnetic analyses reveal that heating samples to between 150 °C and ~ 300 °C removes the secondary, moderately inclined (~ 48°) normal-polarity component of remanent magnetization. This component overlaps with the present-day field and is comparable to the overprint direction expected from Lower Jurassic Karoo dolerite intrusions. In contrast, a likely primary, high unblocking temperature component, of dual polarity, consistently is of steeper inclination (~ 63°). This characteristic remanence passes the reversals test, except where means are based on small sample populations. There are only two resulting polarity zones for the ~ 200 m thick lower Elliot Formation (LEF) with potential for a thin 3rd magnetozone in the uppermost part. The upper Elliot Formation (UEF), in contrast, which was sampled over a thickness of ~ 80 m, has five polarity zones. The failure of the reversal test for the UEF and combined Elliot Formation (LEF + UEF) indicates that the normal polarity samples may be biased by a younger overprint of either the Jurassic normal polarity of the Karoo Large Igneous Province or present day field. The separate poles calculated for the four sites in the LEF and ten sites in the UEF overlap with the Late Triassic and Early to Middle Jurassic Gondwana poles, respectively. The combined Elliot Formation and UEF pole positions are better constrained than the LEF and therefore considered more reliable. Overall the LEF shows considerable overlap with the Late Triassic Apparent Polar Wander Paths (APWP) poles. |
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