NI-RICH SPINEL AT THE CRETACEOUS-TERTIARY BOUNDARY OF EL KEF, TUNISIA

Ni-rich spinel, a mineral formed by fusion and oxidation in the atmosp here of meteoritic material, has been found throughout the world at th e Cretaceous-Tertiary (KIT) boundary, supporting the view that a cosmi c event did occur at the end of the Cretaceous. Here, we report on the stratigraphic distribution of this mineral in the stratotype for the KIT boundary at El Kef, Tunisia. This site is appropriate for the deta iled stratigraphical study of the KIT cosmic and biologic catastrophes as it has a high sed:mentation rate did is barely affected by reworki ng. We observed that the stratigraphic distribution of spinel is confi ned to a 1-3 mm rust colored layer that coincides with a dramatic decr ease of the carbonate fraction. This observation clearly shows that th e cosmic event and the biologic crisis took place abruptly and develop ed rapidly, in less than 100 years, consistent with the hypothesis of a large asteroid or cometary impact triggering the mass extinction at the end of the Cretaceous. Chemical analyses of spinel from El Kef rev eal that it differs from spinel from other sites, even close to El Kef , suggesting the accretion of several objects. This result can be expl ained by the fragmentation of the bolide, either before the impact (co met break-up) or upon the impact (oblique impact), with in both cases, dispersion of the debris all over the Earth.