This paper summarises our current knowledge about 21 sections across the K/T boundary from the Basque Country (western Pyrenees), all of them comprising intermediate-deep basinal facies. This study allowed us to establish that Sopelana III and Bidart are the best sections for analysing the extinction of the planktic foraminifers at the K/T boundary. Detailed analyses of planktic foraminifers from four new sections allow us to differentiate four biozones, one at the end of the Cretaceous and three at the beginning of the Tertiary. These analyses further show that 63 Upper Maastrichtian planktic foraminifers species reached the boundary where 33 species became extinct. The study also shows that some species decrease markedly in abundance in the last few metres of the Cretaceous prior to the extinction event which could be related to environmental changes at the end of the Maastrichtian. More than 50% of the planktic foraminifers, that is 33 species, became extinct at the end of the Cretaceous. However, most of the extinct species were rare and only about 20% of the total Cretaceous assemblages are involved in the extinction event. The 30 surviving species, that is less than 50% of the Cretaceous species, later disappear through the Pr. longiapertura and P. pseudobulloides biozones of the beginning of the Tertiary. Above the K/T boundary, samples are far poorer in planktic foraminifer specimens than those from the uppermost Maastrichtian and include 16 Tertiary species. Moreover, together with this extinction event there are impact markers (iridium and Ni-rich spinels), as well as a high concentration of soot at the beginning of the Danian at the Sopelana III section. This strengthens the hypothesis of a causal link between the impact and K/T extinctions.

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