Sedimentological and diagenetical conditions of the impact level of the Cretaceous/Tertiary boundary in Tunisia: no anoxia required

The Ain Settara section (Kalaat-Senan area, Tunisia) shows a well-exposed a nd expanded Cretaceous/Tertiary (K/T) boundary that can be divided into sev eral units of contrasted lithologies. The bolide impact level is identified thanks to the occurrence of a peak of iridium and Ni-rich spinel abundance . The impact level is a jarosite nodule-rich horizon upon which the emphasi s is placed: the jarosite nodules, initially present as pyrite nodules, may be interpreted as reflecting reducing depositional conditions at the K/T b oundary. Trace metal geochemistry indicates that depositional conditions we re not reducing and that the nodules must not have formed during earliest d iagenesis. Consequently, water column-scaled anoxia cannot be put forward t o account for the mass extinctions observed at the K/T boundary at Ain Sett ara.