Dupuis, C., Steurbaut, E., Molina, E., Rauscher, R., Tribovillard, N., Ignacio Arenillas, Arz, J. A., Robaszynski, F., Caron, M., Robin, E., Rocchia, R., and Lefevre, I.
The Cretaceous-Palaeogene (K/P) boundary, until recently known as the "Cretaceous-Tertiary" or K/T boundary, is well exposed at Aïn Settara in the Kalaat Senan area (Central Tunisia), 50 km south of the El Kef section. Micropalaeontological and geochemical studies led to the identification of six main features tentatively named "events", which characterise the K/P boundary interval, and of which at least two (B and C) have global significance. The lowermost event A located at about 14 cm below the base of the Dark Boundary Clay is marked by a sudden increase in tiny bioturbations, by small nodules and a few macrofossils, a 50% drop in calcareous nannofossil abundance and an increase in Scytinascias (organic linings of foraminifera). It is thought to witness a slowdown in sedimentation. Event B is characterised by a burrowed surface, separating the ca 60-cm thick Dark Boundary Clay from the underlying Aïn Settara marls. It indicates an episode of nondeposition, just before a major change in lithology from marls to clays, corresponding to a major flooding. No substantial palaeontological changes have been recorded in relation to this event. Event C is characterised by maximum concentrations of Ir and Ni-rich spinels, which have been observed in platy nodules, similar to the level at El Kef (K/P boundary sensu ODIN, 1992). It coincides with a major extinction in planktonic foraminiferal species (71%) and a 60% drop in nannofossil abundance. The change in lithology (occurrence of small ripples and channel-like structures) recorded at event D, a few cm up-section, might be related to a locally recorded storm activity. Events E and F, which are situated higher up in the Dark Boundary Clay, are mainly determined by palaeontological changes (palynomorphs and nannofossils), probably resulting from small sea-level variations. The coincidence of the cosmic markers with the major biotic changes at event C pleads for the asteroid impact hypothesis. Their disjunction from the base of the Dark Boundary Clay shows that the change of lithology usually used to determine the K-P boundary is distinct from the major extinction (in the planktonic realm), classically referred to this boundary and linked to the presence of cosmic markers. These results argue the need for the revaluation of the K-P boundary GSSP at El Kef. It is suggested to redefine the K-P boundary at the level of coincidence of the major biotic changes and the cosmic markers.