An attempt to constrain the age, duration, and eruptive history of the Karoo flood basalt: Naude's Nek section (South Africa)

Abstract

[1] We have carried out paleomagnetic sampling of a ∼750 m sequence of the Karoo large igneous province (Naude's Nek Pass, South Africa). K-Ar dating (Cassignol-Gillot) has been performed on four samples from the 650 m upper unit (mean age 179.2 ± 1.8 Ma) and a sample from the lower unit (184.8 ± 2.6 Ma). A succession of two phases of volcanism is suggested. The lower 25 flows (115 m thick) have recorded a reversed polarity; the next 23 flows (135 m thick) are transitional and contribute a detailed record of the "Van Zijl" (1962) Jurassic reversal. The upper 38 flows (500 m thick) have normal polarity. Directional groups (DGs) of lava flows with quasi-identical remanence directions indicate eruption durations too short to have recorded geomagnetic secular variation and hence are interpreted as single eruptive events. Altogether, 19 DGs and 10 sheet lobes yield a sequence of 29 distinct directions. This could correspond to a total eruptive activity shorter than 3000 years, less than one per mil of the total duration over which the section was emplaced. We obtain a new paleomagnetic pole for South Africa at ∼180 Ma (l = 75.2°N, ' = 276.4°E, A 95 = 5.8°, N = 19), which is consistent with earlier reports.

Emplacement of inflated Pāhoehoe flows in the Naude’s Nek Pass, Lesotho remnant, Karoo continental flood basalt province: use of flow-lobe tumuli in understanding flood basalt emplacement

“…The sequence has been the subject of numerous studies relating to petrology, geochemical stratigraphy, palaeomagnetism, and duration of emplacement (e.g. van Zijl et al 1962;Cox et al 1967;Marsh and Eales 1984;Marsh et al 1997;Duncan et al 1997;Jourdan et al 2007;Moulin et al 2011;Moulin et al 2017). In contrast, physical volcanological aspects of the Lesotho remnant and the Karoo province as a whole have received little attention with the few studies being focussed on phreatomagmatic complexes (near Sterkspruit, Fig.…”

“…Some are extensive (extending along flow contacts beyond the exposure of the outcrop > 50 m) but others form only small pockets in undulating flow tops. It is likely that the boles are weathered flow tops or palaeosols that formed in-situ, or in small depressions where thin sedimentary deposits accumulated (Moulin et al 2011).…”

“…4). They are dominantly fine-grained, quartz-rich sandstones and minor siltstones with variable amounts of clays and volcaniclastic clasts (plagioclase, pyroxene, devitrified glass; Moulin et al 2011). Many of the sandstones resemble Clarens Formation (Fig.…”

“…1 and 2). This study was conducted in conjunction with a detailed palaeomagnetic investigation (see Moulin et al 2011). In terms of the geochemical stratigraphy (Fig.…”

Emplacement characteristics, time scales, and volcanic gas release rates of continental flood basalt eruptions on Earth

“…Overall, this is a refl ection of similar eruption conditions and magma types in most provinces. A certain amount of volcanological knowledge exists about the subaerial volcanism that occurred in continental fl ood basalt provinces (e.g., the Deccan province; Bondre et al, 2004), but this does not extend to the intrusive components, except in a couple of cases, i.e., the Karoo-Ferrar province (Svensen et al, 2012;Moulin et al, 2011;Elliot et al, 1999) and the Siberian province (e.g., Svensen et al, 2009;Sobolev et al, 2011).…”