Abstract

Ni-rich spinel, a mineral formed by fusion and oxidation in the atmosphere of meteoritic material, has been found throughout the world at the Cretaceous-Tertiary (K/T) boundary, supporting the view that a cosmic event did occur at the end of the Cretaceous. Here, we report on the stratigraphic distribution of this mineral in the stratotype for the K/T boundary at El Kef, Tunisia. This site is appropriate for the detailed stratigraphical study of the K/T cosmic and biologic catastrophes as it has a high sedimentation rate and is barely affected by reworking. We observed that the stratigraphic distribution of spinel is confined to a 1-3 mm rust colored layer that coincides with a dramatic decrease of the carbonate fraction. This observation clearly shows that the cosmic event and the biologic crisis took place abruptly and developed 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 reveal that it differs from spinel from other sites, even close to El Kef, suggesting the accretion of several objects. This result can be explained by the fragmentation of the bolide, either before the impact (comet break-up) or upon the impact (oblique impact), with in both cases, dispersion of the debris all over the Earth.

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