The GSSP for the base of the Danian was defined in the El Kef Section (Tunisia) and ratified by the IUGS in 1991. However, this GSSP was not officially published in a prestigious stratigraphical journal of wide distribution. Only a short note was published in Episodes by Cowie et al. (1989), in a report on activities of the ICS from 1984 to 1989, so that a detailed proposal was unknown to many scientists working on the K/Pg boundary. In the meantime, new sections in Mexico were found and controversial interpretations were proposed. To solve  problems, ICS has finally required ISPS to  publish the proposal. The chairman of ISPS (E. Molina) in collaboration with Tunisian colleagues visited the GSSP at El Kef for a second time, to put in place an artificial marker (“golden spike”), and  request Tunisian authorities to protect the site. A series of photographs to document the site was taken on that occasion, and used for the official publication (Molina et al. 2006).

In 2009, several auxiliary sections (Aïn Settara and Elles in Tunisia, Caravaca and Zumaya in Spain, Bidart in France, and El Mulato and Bochil in Mexico) were selected and described (Molina at el. 2009). These sections are the most continuous, expanded and representative of marine sedimentation in areas proximal and distal to the Chicxuiub meteorite impact site. In addition, these sections are classical, very well known, physically accessible, have been exhaustively studied and allow a very detailed global correlation. The correlatian criteria used were the meteorite impact evidence (Ir anomaly, Ni-rich spinel etc.) and the mass extinction of planktic micro- and nannofossils. Furthermore, it was proposed that the K/Pg boundary is marked exactiy by the moment of the meteorite impact, which implies that all the sediments generated by the impact belong to the Paleogene. While in distal areas of the impact site the K/Pg boundary coincides with a millimeter-thick rusty layer, in proximal areas the K/Pg boundary correlates to the base of a meter-thick Clastic Unit, including a thick calcareous breccia in the sections close to the impact crater.

References:

Molina, E., Alegret, L., Arenillas, I., Arz, JA., Gallala, N., Hardenbol, J., Von Salis, K., Steurbaut, E., Vandenbeghe N. & Zaghbib-Turki D. (2006). The Global Boundary Stratotype Section and Point for the base of the Danian Stage at El Kef, Tunisia: original definition and revision. Episodes 29, 263-273.

Molina E., Alegret L., Arenillas I., Arz J.A., Gallala N., Grajales M., Murillo-Muñetón, G. & Zaghbib D. (2009). The Global Boundary Stratotype Section and Point for the base of the Danian Stage: auxiliary sections and correlation. Episodes 32, 84-95.

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