Palaeoceanographic data provided by the foraminifers of the Barrika section (Santonian of the Rasque Arc)

Analyses carried out on benthonic foraminiferal assemblages of the Barrika section (Basque Arc) are part of a multidisciplinary research with the main subject of understanding the palaeoceanographic events that took place at the end of the Cretaceous within the Basque Basin. The Barrika section has been dated as uppermost early Santonian-upper Santonian by means of the planktonic species Dicarinella concavata and Dicarinella asymetrica. It is additionally confirmed by the detection of benthonic species such as Eponides concinna, Vemeuilina muensteri, Tritaxia capitosa, and Reussella szajnochae praecursor. A total of 130 benthonic species have been identified, which clearly correspond to deep marine water microfauna, except that from shallow waters at the top of the section. In this context, some of the aspects about the palaeoenvironmental variations detected in Barrika section are crucially provided by qualitative and quantitative changes of the foraminiferal assemblages, specific diversity changes of the microfauna, and both the rate between different kinds of benthonic wall structures, and the ratio between benthonics and planktonics. Thus, the dominant foraminiferal assemblages, the great percentage of agglutinated tests, and the high values of the oceanicity index suggest a cool well-oxygenated bathyal waters palaeoenvironment for most of the section. However, the top of the section is particularly marked by a relative sea-level fall, that could reach its maximum at the end of the Santonian, producing a dramatic change to an internal platform palaeoenvironment. This event may have probably been the response to the global regression occurred at 85 M.a