On the Age, Duration and Eruptive History of the Karoo Flood Basalt: new results from the Oxbow-Moteng Pass Sections (South Africa)

The Karoo traps in southern Africa have been linked to the (relatively small) Pliensbachian-Toarcian extinctions. In an attempt to understand why the extinction was far less severe than the one at the KT boundary (related to the Deccan traps and Chicxulub impact), we have undertaken a large sampling program for mainly paleomagnetism and geochronology. A first 800 m thick section of the traps at Naude's Nek (NN), near the southern border of Lesotho, has been published (Moulin et al, 2011). We have next investigated the Moteng Pass and Oxbow sections some 200 km to the North, where lava thickness reaches almost 1500 m. Our age determinations (40K-40Ar Cassignol-Gillot and 40Ar/39Ar techniques) show that the whole lava pile was emplaced in a relatively short time around the Pliensbachian-Toarcian boundary. Detailed flow-by-flow magnetostratigraphy shows that the eruptive sequences can be divided into several volcanic pulses and interbedded individual lava flows. Based on archeomagnetic estimates of the velocity of secular variation, we estimate that total eruption time may have been as short as a few thousand years. However, we cannot determine the amount of time elapsed between successive volcanic pulses, which is of course required to fully reconstruct the eruptive sequence. Combined with previously published sections covering much of Lesotho (NN, Bushmen's Pass - Prévot et al, 2003 -, Mafika Lisiu and Sani Pass - Kosterov and Perrin, 1996), these new results allow the correlation of numerous directional groups and allow one to partly reconstruct and constrain the eruptive history of the Drakensberg group of the Karoo LIP.