Cyclostratigraphy and astronomical tuning of the Late Maastrichtian at Zumaia (Basque country, Northern Spain)

The standard Geological Time Scale for the Cretaceous is still largely based on seafloor anomaly profiles combined with radio-isotopic tie-points. The astronomical tuned time scale with its much higher resolution and accuracy has recently been extended to the K/Pg-boundary and is being extended into the Cretaceous. To construct such a time scale for the Cretaceous, we selected the upper Maastrichtian of the Zumaia section in the Basque country (northern Spain) which contains a cyclic alternation of limestones and marls deposited in a hemipelagic setting. The Paleogene portion of the Zumaia section has previously been studied for a joint cyclostratigraphic-radioisotopic intercalibration of the age of the K/Pg boundary. Here we present a high-resolution cyclostratigraphic framework for the upper Maastrichtian (Latest Cretaceous) of the Zumaia section in the Basque country (northern Spain), with new biostratigraphic and magnetostratigraphic data. Bed-to-bed correlation with the nearby Sopelana section provides additional bio- and magnetostratigraphic constraints. The stacking pattern of the lithologies shows a hierarchy that reflects the combined influence of the orbital parameters of precession and eccentricity. This is confirmed by time series analyses of lithological and geochemical data, indicating a strong influence of eccentricity-modulated precession on latest Cretaceous climate. The expression of the 405-kyr eccentricity cycle serves as primary signal for astronomical tuning. We provide two tuning options depending on absolute K/Pg-boundary ages of 65.56 and 65.97 Ma. The logged part of the section encompasses nine and a half 405-kyr cycles in total and spans 3.9 Myr. The acquired cyclostratigraphic framework provides ages for characteristic planktonic foraminiferal events, magnetic reversals and carbon isotope excursions and resolves the late Maas- trichtian time scale in unprecedented detail with relative age uncertainties o100 kyr. The high resolution and large amplitude of shifts in d 13 C on the 405-kyr and 1.2-Myr scales allow for global correlation and may shed more light on the orbital pacing of Late Cretaceous climate.