THE STRATIGRAPHIC DISTRIBUTION OF IRIDIUM AT THE CRETACEOUS-TERTIARY BOUNDARY OF EL KEF, TUNISIA

The Cretaceous-Tertiary (KIT) boundary is marked by the collision of t he Earth with an asteroid or a comet. This event produced high concent rations of iridium and other PGEs in K/T boundary sediments. Although delivered by a very brief event, Ir is observed over a sizeable thickn ess of sediments simulating a long duration phenomenon (several tens o f thousands of pears). In order to understand this point we have analy sed rocks from the WT section of El Kef (Tunisia), a site which offers ideal conditions (hi,oh deposition rare and low bioturbation) for a d etailed stratigraphic study. The Ir distribution shows a millimetre th ick pulse-like feature occurring in perfect coincidence with the begin ning of the biological crisis. This feature is superimposed to a diffu se component extending over about two metres of sediment from the uppe r Maastrichtian to the top of the Globigerina eugubina zone in the Dan ian. The pulse-like feature is associated with crystals of Ni-rich spi nel, a mineral derived from meteoritic material. This pulse, reminisce nt of the brevity of the K/ event, results from the prompt deposition of a large amount of debris of the projectile dispersed world-wide as a result of the impact event. As far as the diffuse component is conce rned, its duration agrees with the estimated residence time of Ir in s ea water. This component is explained by the transit through the ocean ic reservoir of the iridium associated with the finely divided fractio n of the projectile.