The Sopelana sea-cliff section exposes a continuous marl-limestone alternation from the Lower Maastrichtian
to the Danian, in the deep Basque Arc domain. The high-resolution analysis (% CaCO3) of the Lower-
Upper Maastrichtian couplets confirms a simple and regular layout versus a more complex behaviour of
the Danian couplets. The marl and limestone mean values of the Upper Maastrichtian point out a continental
debris enrichment. The early entry of deep, cold, oxygenated water produces a gradual (grey/ yellowgreenish/
red) change of coloration in the Lower Maastrichtian couplets. It is the beginning of the Cretaceous
oceanic red beds (CORB´s). The “bleaching” around macrofossil nucleus and small fractures in the red
facies shows a small diagenetic stage due to the influx of reducing fluid
