Detailed correlation and astronomical forcing within the Upper Maastrichtian succession in the Basque Basin

We have undertaken a comprehensive, integrated, cyclo-magnetostratigraphic analysis and study of the calcareous nannofossils of the Upper Maastrichtian hemipelagic succession in three sections of the Basque Basin (Zumaia, Sopelana and Hendaia). The sections were correlated at bed-by-bed scale through careful analysis of the lithological stacking pattern and significant sedimentary features. For spectral analysis we used an available high-resolution carbonate proxy record spanning 64 m of section below the K/Pg (Cretaceous/Palaeogene) boundary at Zumaia containing 72 precession-related limestone-marl couplets. The continuous wavelet spectrum helped to determine and visualize the orbital forcing at both the short (~100-ky) and long (405-ky) eccentricity band. We applied bandpass Gaussian filters to the carbonate record to extract the relevant periodicities and provide a cycle-numbering scheme starting at the K/Pg boundary. The full hierarchy of precession cycles and eccentricity-related bundles is then extended toward the base of the section in question, which contains a total of 33 short eccentricity-related bundles, thus spanning more than 3 Ma. The chron C31r/C31n boundary (estimated to occur at ~3.08 Ma below the K/Pg boundary) in the lower part of the succession was determined unambiguously in all three sections studied although the C30n/C29r reversal could not be determined due to a pervasive reverse magnetization acting on the purplish lithologies in the upper part of the succession. Relevant calcareous plankton bioevents could be accurately placed on the cyclo-magnetostratigraphic template. The cyclostratigraphic framework also allowed us to estimate the duration of previously defined sea-level-related 3rd-order depositional sequences in the basin, which appear to be strongly paced by the long-term 1.2 My obliquity amplitude modulating cycle. This is an outstanding feature in the Maastrichtian greenhouse period, during which continental ice sheets are expected to be either ephemeral or non-existent. This is a matter that deserves further attention