From !khure

Project A4: Large igneous provinces, impacts and climate change

French pi: F. Fluteau (with V. Courtillot)
South African pi: G. Marsh


Project Participants


V. Courtillot, F. Fluteau, J. Besse, X. Quidelleur, G. Delpech, C Jaupart, M. Gérard, H. Bouquerel

South Africa:

J. Marsh, M. Watkeys, M. Klausen, A. Duncan, plus one or two Msc or PhD students to be determined

Project Summary

The project focuses on detailed flow-by-flow sampling for palaeomagnetic secular variation studies on two well-characterized Karoo volcanic sequences (a) The low-Ti Oxbow sequence of northern Lesotho, and (b) the high-Ti Olifants River sequence of northern Lebombo. Palaeomagnetic measurements will be done at IPGP and will be complemented by K/Ar and 40Ar/39Ar dating (IPGP) and additional geochemical studies (South Africa) where necessary. The project goal is to construct a refined model for the emplacement of the Karoo volcanic sequences with particular emphasis on volumes and duration of eruptive episodes (in particular identification of short large pulses) and their impact on climate modifications with implications for the end-Pliensbachian extinction. This will be compared to the case of the Deccan volcanics and KT mass extinction.


Figure 1. Correlation of mass extinction events with very large mafic magma eruptions

There is considerable debate on the causes of the biological mass extinction events recognized in the Phanerozoic fossil record. There is consensus that extinctions reflect events of dramatic climate change but the underlying causes of the change remain controversial. Correlations between the mass extinctions and large volume volcanic eruptions (Fig. 1 from Courtillot & Renne, 2003) together with known temperature/climate modification effects of some historical eruptions such as Pinatubo (1991-1992) and Laki (1783-1784) are powerful arguments for a volcanic cause of many of the extinction events. To strengthen and test this hypothesis, large volcanic events have to be investigated in detail to establish (a) the precise timing of the volcanism, (b) volume-duration relationship during the volcanic event and (c) climate change modelling. Detailed results from one such study (Chenet et al, in press) on the Deccan Flood basaltic sequence, which correlates with the large extinction event at the K-T boundary, have shown that flow-by-flow palaeomagnetic sampling of well-characterized stratigraphic sequences allow geomagnetic secular variation data to be recovered from the lava flows. The secular variation can be used as a time proxy which has allowed considerable refinement of the details of the emplacement history of the volcanic sequence, in particular volume-duration relationships. Together with data from red boles intercalated in the lava sequence, results indicate that the Deccan was emplaced as a small number of discrete large-volume short-lived pulses without significant quiescence between them. Gas emissions (largely SO2) of each pulse are of the same order as those proposed for the Chixculub impact, the rival hypothesis for the K-T extinction. These data are strongly suggestive of Deccan volcanism having a significant impact on Earth's climate.

Figure2:Distribution of Karoo volcanic outcrops, the two main mafic magma systems, and the location of proposed study sections.

The coincidental eruption of the Deccan sequence with the occurrence of the Chixculub impact indicates that it is crucial to validate the hypothesis arising from the Deccan results of Chenet et al.(in press) with a similar study in another large continental flood basalt sequence. It is proposed to do so with the Karoo flood basalt sequence of southern Africa (Fig. 2). Outcrop remnants of the Karoo volcanic sequence (largely mafic), as well as intrusions, occur over an area >2.5 * 106 km2 with lava sequences exceeding 1 km in thickness in many places. These eruptions tapped two major mafic magma systems - a low-Ti system in the south and a high-Ti system in the north. Eruptive volumes are of similar magnitude to those of the Deccan (total  2x106 km3). Modern age dating (largely 40Ar/39Ar) indicate that the volcanic sequence was emplaced over a short time at about 180-183 Ma (Duncan et al, 1997; Jourdan et al., 2004) which broadly correlates with the extinction event at the Pliensbachian-Toarcian boundary. Detailed geochemical studies have established a well-defined geochemical stratigraphy for all major lava remnants and correlations between them (Fig. 3 - Marsh et al., 1997; Sweeney et al, 1994; Klausen et al., 2005 and Duncan & Marsh, unpubl.). A palaeomagnetic reversal stratigraphy has been well-defined for the Lesotho remnant, and tentatively defined in the Lebombo sequence (Hargraves, et al.,1997). As a first step of the prposed program, we have started a detailed flow-by flow sampling for secular variation studies similar to those of Chenet et al.(in press) in the Deccan in the Naude‟s Nek section of southern Lesotho in November 2006 and samples are currently being processed. Thus the Karoo Province has been sufficiently well-characterized for a focussed palaeomagnetic study to yield meaningful results.

Detailed Research Proposal and Motivation

To carry out detailed flow-by-flow sampling of two other, complementary sections: (a) the Oxbow section of northern Lesotho and (b) the Olifants River section of the northern Lebombo

The Oxbow section

This section lies along the paved road between Butha-Buthe and Mokhotlong in Lesotho. The volcanic sequence is continuously exposed in the Moteng and Mahlasela passes and is some 1500 metres thick. Geochemical and palaeomagnetic data for the lavas has been published by Marsh et al.(1997) and Hargraves et al.(1997). A single palaeomagnetic reversal occurs within this section and has been correlated to the single reversal found in all other sections through the Lesotho volcanic sequence, including Naude‟s Nek. Similar consistent correlations also exist in the lava flow geochemical stratigraphy. This will assist in correlating the detailed geomagnetic secular variation results in the two sections and establish minimum volume/duration relationships for the construction of the low-Ti Lesotho magma system. Precise timing of the eruptions will be addressed through combined K/Ar and 40Ar/39Ar dating

The Olifants River Section

Figure 3: Stratigraphic correlations established with geochemistry of lava flows. Palaeomagnetic reversal stratigraphy from Hargraves et al. (1997)

This section comprises an estimated 5 km sequence of the mafic lavas and a further thickness of overlying Jozini Rhyolites of the Lebombo Group as well as numerous cross-cutting dykes. This section is exposed along the banks of the Olifants River in the Kruger National Park and is accessible from a tourist road that follows the river. The mafic rocks in the section include flows of the basal Mashikiri Formation (nephelinites), the overlying Letaba Formation (picrites) and several basaltic types of the Sabie River Formation. The entire sequence is representative of the high-Ti magma system whose geochemical stratigraphy has been well characterized (Klausen et al.,2005 and unpublished) and correlations established with the volcanic sequence of central and southern Lebombo, with the sequence in the Tuli syncline and northern Botswana, as well as with the low-Ti sequence in Lesotho via an intercalation of flows of one of the high-Ti basaltic chemical types with the low-Ti flows in the Springbok Flats remnant (Fig. 3 modified from Marsh et al., 1997). In addition palaeomagnetic data are available from which a tentative reversal stratigraphy has been constructed. The Olifants River section presents the best locality for a detailed palaeomagnetic and K-Ar and Ar-Ar studies to complement those in the low-Ti Lesotho sections. The results should refine the proposed correlations between the low-Ti and high-Ti volcanic sequences, address the important issue of a time difference in the emplacement of the two systems and hence allow construction of a refined volume-duration model for the whole Karoo province. Such a model is essential to understand the impact of the Karoo volcanism on climate at about 180 Ma.


  • Chenet, A-L., Fluteau, F., Courtillot, V., Gérard, M. & Subbarao, K.V.(in press, 2007) Reconstructing the eruptive history of the Deccan traps: (I) Constraints from palaeomagnetic secular variation and red bole formation in a 1200m-thick section from the Mahabaleshwar escarpment. J. Geophys. Res.
  • Chenet, A.L., Quidelleur, X., Fluteau, F., Courtillot, V., and Bajpai, S., .(in press, 2007) 40K-40Ar Dating of the Main Deccan Large Igneous Province: Further Evidence of KTB Age and Short Duration, Earth Planet. Sci. Lett.
  • Courtillot, V.E. & Renne, P.R.(2003) On the age of flood basalt events. C.R. Geosciences, 335, 113-140.
  • Duncan, R.A., Hooper, P.R. Rhacek, J., Marsh, J.S. & Duncan, A.R.(1997) The timing and duration of the Karoo igneous event, southern Gondwana. J. Geophys. Res., 102, 18127-18138.
  • Hargraves, R.B, Rehacek, J. & Hooper, P.R. (1997) Palaeomagnetism of the Karoo igneous rocks in southern Africa. S.Afr.J.Geol.,100,195-212.
  • Jourdan, F., Féraud, G., Bertrand, H., Kampunzu, A.B. Tshoso, G., Watkeys, M.K., & le Gall,B.(2005) The Karoo large igneous province: brevity, origin, and relation with mass extinction questioned by new 40Ar/39Ar age data. Geology, 33, 745-748.
  • Klausen, M.B., Marsh, J.S. & Watkeys, M.K.(2005) geochemical variation in the Karoo basalt-rhyolite lava sequence along the northern Lebombo monocline (Olifants River Section). Geol.. Soc. S. Afr. GEO2005 Programme and Abstracts volume.
  • Marsh, J.S., Hooper, P.R. Rehacek, J., Duncan, R.A. & Duncan, A.R.(1997) Stratigraphy and age of Karoo basalts of Lesotho and implications for correlation within the Karoo igneous province. American Geophysical Union, Geophysical Monograph 100, 247-272.
  • Sweeney, R.J., Duncan, A.R. & Erlank, A.J.(1994) Geochemistry and petrogenesis of Central Lebombo basalts of the Karoo igneous province. J. Petrology, 35, 95-125.