SEARCHING FOR IMPACT FRAGMENTS ACROSS THE EIFELIAN-GIVETIAN BOUNDARY

The middle to late Devonian stratigraphic interval is marked by several extinction horizons. Climatic and oceanographic changes, anoxia, flood basalt volcanism or impacts events have been suggested as potential causes. Recently, possible impact markers (shocked quartz and microspherules) were reported in the middle Devonian, at the Eifelian-Givetian boundary (EGB), in Jebel Mech Irdane, Morocco (Ellwood et al., 2003), where the GSSP boundary is placed. Based on magnetic susceptibility a putative ejecta layer is proposed just below the conodont zone Polygnathus hemiansatus, marking the base of the Givetian. This level is also characterized an enigmatic shift in delta 13C of -9 per mil. Unfortunately, no Ir analyses support the impact origin of this layer, and the identification of shock metamorphism is questionable (Racki and Koeberl, 2004). To test this impact - extinction link, sections across the Eifelian-Givetian transition are currently being studied in the Dinant Synclinorium (south of Belgium). Based on paleogeography the two locations were not very distant in the middle Devonian. An impact event of the magnitude postulated by Ellwood et al, - that is capable of causing the extinction of 40% of the marine species - should clearly be recorded in what is now Southern Belgium. Seven shale or clay beds can be considered as potential equivalent to the Jebel Mech Irdane impact layer. Some beds are probably highly altered bentonite. Others show enrichment in chalcophile elements, in agreement with the geochemical anomalies in As, V reported at the Moroccan site. They are interpreted as indicative of low oxygen conditions. These layers contain quartz, pyrite and Fe-rich grains grains. Under the SEM, the latter could be equivalent to the impact microspherules of Ellwood et al. but they are clearly not of impact origin. So far none of the 7 layers has yielded clear impact evidence such as shocked minerals, microtektites/microkrystites, or high PGE levels. The shift in delta13C has yet to be identified. At this point, there is no confirmation of an impact event across the EGB in the Dinant Synclinorium. This would indicate that if the Jebel Mech Irdane layer is indeed of impact origin, which remains to be confirmed, its is a local event of a magnitude probably insufficient to trigger a major mass extinction.

Ref: Ellwood et al., (2003) Science, 300 p. 1734. Racki and Koeberl, (2004) Science 303, p. 471b.