Magnetostratigraphic constraints on the age of the lower Beaufort Group, western Karoo basin, South Africa, and a critical analysis of existing U‐Pb geochronological data

Abstract

We studied three partially overlapping sections with a composite thickness of ∼600 m in the upper Permian fluvial siltstones and fine‐grained sandstones of the Abrahamskraal Formation, the basal unit of the Beaufort Group, in the Karoo Basin of Western Cape Province, South Africa. Paleomagnetic analysis reveals three components of Natural Remanent Magnetization (NRM). Heating to ∼180°C removes a remanent magnetization parallel to the present‐day field, which is interpreted as a viscous overprint. An intermediate unblocking temperature component is removed by heating to 450°C; this direction is always of normal polarity and is identical to a regional overprint imparted during the Early Jurassic emplacement of the Karoo Large Igneous Province. A high‐temperature component isolated above 450°C is of dual polarity and is interpreted as primary on the basis of a positive reversals test. The virtual geomagnetic pole position for the Abrahamskraal Formation computed from the average high‐temperature characteristic remanent magnetization direction is in agreement with the late Permian directions for stable Gondwana and with previous results from the lowermost Abrahamskraal Formation and Waterford Formation at the Ouberg Pass section. The predominantly normal polarity of this magnetization is in agreement with either a middle‐late Lopingian age (ca. 254–256 Ma) or a late Guadalupian age (ca. 262 Ma) according to the global geomagnetic polarity time scale. We integrate these new results with existing magnetostratigraphic, biostratigraphic, and geochronological results from the Karoo Basin, with particular emphasis on the controversy over zircon age data reported from the underlying Ecca Group.

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

“…Here we provide a more straightforward explanation for this apparent age inversion: that the zircon grains from ash beds in the middle Ecca Group have been affected by Pb-loss, the degree of which is difficult to assess at the resolution of the SHRIMP method for samples <500 Ma (Tohver et al, 2015). For example, for Phanerozoic samples, subsequent loss of radiogenic Pb will cause analyses to plot in an array between the magmatic age and the age of disturbance along a line subparallel to the Concordia curve, and with uncertainty ellipses still overlapping with Concordia, so the degree of Pb-loss is potentially impossible to resolve (Tohver et al, 2015).…”

“…Previous paleomagnetic work in the Tanqua depocentre revealed up to three paleomagnetic components, including a low-temperature (≤ 175°C) recently-acquired VRM and a higher temperature (≤ 475°C) remagnetization that may be related to Jurassic Large Igneous Province (LIP) emplacement (Lanci et al, 2013;Tohver et al, 2015). Reverse polarity ChRM data reported from the overlying Abrakamskraal Formation in the Beaufort Group (Fig.…”

“…For example, for Phanerozoic samples, subsequent loss of radiogenic Pb will cause analyses to plot in an array between the magmatic age and the age of disturbance along a line subparallel to the Concordia curve, and with uncertainty ellipses still overlapping with Concordia, so the degree of Pb-loss is potentially impossible to resolve (Tohver et al, 2015). The reproducibility of consistently younger ages from the underlying Ecca Group and older ages from the overlying Beaufort Group, along the southern margin of the Karoo Basin, can be explained by an episode of Pb-loss.…”

“…Though Karoo Basin paleomagnetic directions were originally considered to be representative of a complete remagnetization (demagnetization to 350°C) due to a negative fold test in the Cape Fold Belt (Ballard et al, 1986), a number of paleomagnetic studies have since been published which resolved primary magnetization components suitable for comparison with the GPTS (De Kock and Kirschvink, 2004a;Gastaldo et al, 2015;Lanci et al, 2013;Tohver et al, 2015;Ward et al, 2005). The Cape Orogeny (ca.…”

A geomagnetic polarity timescale for the Permian, calibrated to stage boundaries

“…691 692 The 'brief-GU1r' option relies on the large wealth of data from the Russian sections through the 693 Urzhumian, and requires that the U-Pb SHRIMP ages from the Abrahamskraal Fm are too young, 694 probably impacted by lead loss (e.g. Tohver et al 2015). It also indicates the Kapteinskraalian LVF 695 extends into the Kungurian, counter to vertebrate workers hypotheses.…”