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[dinosaur] Triassic-Jurassic boundary in Karoo Basin magnetic polarity chronology




Ben Creisler
bcreisler@gmail.com



A new paper:




Lara Sciscio, Michiel de Kock, Emese Bordy & Fabien Knoll (2017)
Magnetostratigraphy over the Triassic-Jurassic boundary in the main Karoo Basin.
Gondwana Research (advance online publication)
doi:  https://doi.org/10.1016/j.gr.2017.07.009
http://www.sciencedirect.com/science/article/pii/S1342937X16302593


Highlights

Primary magnetizations are preserved in the continental red beds of the Elliot Formation in the main Karoo Basin.
Global magnetostratigraphic correlations and palaeopole calculations support a Late Triassic-Early Jurassic age for the Elliot Formation.
An age range of ~ 213Ma – ~ 195 for the lower member of the Elliot Formation and ~ 195 - ~ 190 Ma for the upper member of Elliot Formation is estimated.

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 ~ 80 m thickness, 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 lower 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.