MAGNETOSTRATIGRAPHY AND DATING OF THE NAUDE'S NEK SECTION (KAROO TRAPS, SOUTH AFRICA): ONGOING ANALYSIS OF THE ERUPTIVE SEQUENCE

A strong connexion between large igneous provinces and mass extinctions is supported by an increasing database of age determinations. The impact of recent (much smaller) eruptions on climate is largely due to SO2 injection into the stratosphere. Successful climate modelling of the environmental effects of LIP eruptions requires high-resolution timing of volcanism, i.e. knowledge of numbers, volumes and durations of peak episodes. Chenet et al have studied the Deccan traps, which correlate with the major KT crisis, combining geochronology (K-Ar), paleomagnetism (secular variation), volcanology (flow types) and analysis of alteration levels between flows (red boles); they have shown that emplacement occurred as a small number of discrete, very large and short-lived volcanic pulses. But not all LIPs, however apparently similar, are correlated with major crises. For instance, the Karoo traps of South Africa, dated at about 180 Ma, erupted close to the relatively small end-Pliensbachian extinction. We therefore decided to resume the same kind of analysis we had undertaken in the Deccan in the Karoo. We began this new study with the Naude's Nek section (the lower 700m of the traps), located in South Africa near the southern border of Lesotho. Our first age determinations (40K-40Ar Cassignol-Gillot technique) yield ages of 181.3 ± 1.8 Ma, in good agreement with previous studies (Jourdan et al., 2007). Detailed flow by flow magnetostratigraphy (site-mean directions based on thermal demagnetization) shows that the eruptive sequence can be divided into several volcanic pulses having each likely lasted less than ~100 years. Particular attention has been focused on the remarkably detailed record of the single reversal recorded in the sequence and identified by van Zijl 50 years ago: 136m of lava, erupted over less than a few thousand years. In order to strengthen our understanding of the eruptive sequence (the record of the eruptive sequence during a reversal as well as the role of the Karoo traps in the Pliensbachian crisis), we are now investigating other thicker sections (Moteng Pass and Oxbow) further to the North in Lesotho, where the Karoo outcrop is the thickest (almost 1500 m). Results available at the time of the meeting will be presented.