Remarkably thick sequences of upper Cretaceous and lower Tertiary sediments outcrop on the northern coast of the Basque country. We have sampled the well exposed section along the Sopelana beach over 150 m, spanning roughly from middle Maastrichtian (71 Ma) to lower Paleocene (66 Ma), for both biostratigraphic and magnetostratigraphic studies. Thermal demagnetization of ∼ 400 specimens revealed both normal and reversed recent overprints unblocked below 200°C, and both normal and reversed characteristic directions at higher temperatures (200–450°C in the grey limestones; 350–550°C in the redder marls and limestones). Thermal and AF demagnetization and monitoring of weak field susceptibility are consistent with some form of (titano-) magnetite as the main carrier of magnetization. 259 demagnetization diagrams yielded two nearly antipodal clusters of directions, which are still polluted by some 20% remaining recent overprint. The overall mean direction in stratigraphic coordinates is D = 356°, I = 51° (α = 3°), consistent with what is expected for Eurasia at KTB time. Magnetic stratigraphy outlines a succession of eight polarity intervals and is rather straightforward, except for two highly complex zones (52 to 40 m, and 28 to 26 m below the KTB) where the magnetic polarity appears to flip at an unreasonable rate. Biostratigraphic check allows unambiguous assignment of several chrons, with the recognition of the Gansserina gansseri, Abathomphalus mayaroensis, “Globigerina” eugubina, Eoglobigerina edita ( = E. pseudobulloides) and E. trinidadensis zones. The magnetostratigraphic section therefore begins in chron 31R and ends in 29N. Further detailed study of the complex zones reveals that specimens there have a distinct magnetic behaviour: susceptibility increases beyond 400°C, indicating instability and mineralogical change, intensities are higher than elsewhere and more scattered, and a higher unblocking temperature/higher coercivity magnetic component is uncovered. This component, which is absent from the rest of the section, could be an early chemical remagnetization. Indeed the complex zones contain larger amounts of chlorite, a sign of more intense diagenesis. We have therefore discarded samples showing this anomalous behaviour, and have retained only the 223 samples where demagnetization was achieved by 450°C. The thicker, lower complex zone reduces to a reversed chron which we propose to correlate with 30R. We have identified an extra short reversed event within chron 30N, for which independent support is found in DSDP Sites 524 and 577a, and possibly in Gubbio. Finally, the high-sedimentation rate section at Sopelana (25 m/Ma in the Maastrichtian, 7 m/Ma in the Danian) displays one of the best resolved on-land magnetostratigraphies around KTB time. The two complex zones of early remagnetization form an “echo” which may have been generated some 100 kyr after deposition of the sediments. The KTB itself occurs approximately halfway to 3/5 up 29R in terms of time.