THE CRETACEOUS-TERTIARY BOUNDARY STRATOTYPE SECTION AT EL KEF, TUNISIA - HOW CATASTROPHIC WAS THE MASS EXTINCTION

The Cretaceous/Tertiary (K/T) boundary stratotype section at El Kef, T unisia, represents the most complete and expanded sedimentary record a cross this important mass extinction horizon presently known. High res olution analysis of planktic foraminifera in two outcrops (El Kef I-st ratotype and El Kef II) along with comparisons between planktic and be nthic foraminifera, calcareous nannofossils, ostracods, pollen and spo res, and dinoflagellates indicate that major changes across the K/T bo undary are registered only in benthic and planktic foraminifera and ca lcareous nannofossils. Biotic changes in benthic foraminifera are uniq ue to Fl Kef and similarly shallow continental shelf sections and appe ar to be the result of a sea-level regression in the latest Maastricht ian followed by a sea-level rise across the K/T boundary that was acco mpanied by expansion of the local oxygen minimum zone (OMZ). Biotic ch anges in planktic foraminifera appear partly related to these conditio ns also, but in general reflect more global oceanographic changes. For instance, species extinctions are gradual and selective as observed i n K/T sections worldwide, rather than random and abrupt. Although ther e is a 69% decline in species richness between 25 cm below and 10 cm a bove the K/T boundary, only rare species disappeared. Their combined r elative abundance constitute less than 20% of the total population. Ab out 52% of these extinct taxa (8% of the population) are large, ornate , morphologically complex tropical-subtropical forms that lived at or below the thermocline. No planktic foraminifera from this depth range survived the K/T boundary event. All survivor tart were surface dwelle rs living within the photic zone. Their relative abundance (similar to 80%) dominates both Cretaceous and early Tertiary populations. These data indicate that the K/T biotic record in the shallow continental sh elf section at El Kef was significantly influenced by local conditions which, combined with the latest Maastrichtian sea-level regression an d subsequent sealevel rise, resulted in shallowing of the local OMZ re lative to the sea-surface. Shallowing of the local OMZ lead to the sel ective disappearance of benthic faunas and may have adversely affected the surviving photic zone dwellers. The selective nature of species e xtinctions, however, appear to be related partly to long-term global o ceanographic changes which were accelerated at the K/T boundary possib ly by a bolide impact.