The Sopelana sea-cliff exposes marl-limestone couplets from the Upper Maastrichtian (MS1 and MS2
sections) and the Lower Eocene (EI section), in the deep Basque Arc domain. The high-resolution analysis
(% CaCO3) of the MS1 couplets confirms a complex behaviour with strong compositional contrast vs. a
more regular MS2 couplets. This may be due to a drastic environmental change (long humid and rainy
alternating to drier periods) towards dominant humid and rainy periods, with a minor seasonal nature and
high sedimentation rate (72 cm/20 ky). The deep, cold, oxygenated marine water produces a change of
coloration (grey to purple) during the sedimentation, in the Upper Maastrichtian couplets before diagenesis
activity. It is the beginning of the Cretaceous oceanic red beds (CORB´s
