The GSSP of the Danian stage, commonly referred to as the K/T boundary (where K stays for Kreide and T for Tertiaty), represents an extreme example of event stratigraphy. It separates the Mesozoic from the Cenozoic Erathemes, the Cretaceous from the Paleogene Systems, the Maastrichtian from the Danian Stages, with a numerical age of approximately 65 Ma. The K/T boundary coincides with the youngest of the "big five" extinction events that affected both the marine and terrestrial fauna with the disappearance of several fossil groups that flourished in the latest part of the Cretaceous such as Dinosaurs, Ammonites, Belemnites, Inoceramids, Rudistids, Globotruncanids, Heterohelicids.
The importance of the K/T boundary and the interpretation of its quite unusual peculiarities were among the major topics of the 21° IGC held in Copenhagen in 1960, with visit to the classical localities of the Danian stage, originally defined by Desor in 1847 and attributed to the Cretaceous. In 1964 detailed micropaleontological investigation on the pelagic Scaglia rossa formation of central Apennines revealed the sudden disappearance of the large, rich and diverse planktonic foraminiferal fauna followed by a repopulation of a small, dwarf and poorly diversified fauna, after the deposition of a millimetric boundary clay. Soon later the biostratigraphic succession was calibrated paleomagnetically. Further studies documented a marked iridium anomaly in the boundary clay, finally leading in 1980 to the interpretation of a bolide impact shared by Alvarez et al. (Gubbio section) and Smith and Hertogen (Caravaca section).
A working group was soon appointed by ICS to propose a GSSP for the K/T boundary. Four candidate sections: El Kef in Tunisia, Zumaya in Spain, Brazos in USA and Stevns Klint in Denmark were proposed in 1982. After years of intense, coordinated studies the section of El Kef was approved with a 80% majority, and ratified in 1991 by IUGS.
Meanwhile the exploration of the worlds oceans by the Deep Sea Drilling Project, extended the study areas from the continents to the oceans. The K/T boundary was first identified in a drillhole in 1971, in the Caribbean (DSDP Leg 15). In the following years it could be recognized in all the oceans, and located on magnetic anomaly 29R in the Indian Ocean. Targeted drillsites in the Gulf of Mexico helped to locate the astrobleme in the Chicxulub crater in the Yucatan peninsula. The bolide causing the devastating extinction had a diameter estimated 10 km. Tectites, tsunamiites, impactites were recovered in various localities in the Gulf of Mexico but far from the impact location no lithologic break is noticed and the boundary clay can well be overlooked falling within a monotonous succession within a single formation.
The validity of the GSSP at El Kef has been recently reiterated. Indeed, the event is so unique and so strong as to be identifiable in all the oceans, and in all the continents.