Closing the Mid- Paleocene gap: toward a complete astronomically tuned Paleocene Epoch and Selandian and Thanetian GSSPs at Zumaia (Basque Basin, W Pyrenees)

Abstract

An integrated magneto-, bio- and cyclostratigraphic framework is presented for the Mid-Palaeocene interval from the (hemi) pelagic sea-cliff section of Zumaia in the Basque basin. The new ∼55 m long studied section expands about 3.5 Myr and closes the gap between previously published integrated studies in the section. The occurrence of magnetochron C26n is now documented, and its duration (complemented also by data from the Ibaeta section), and that for chrons C26r and C25r is estimated by counting precession related lithologic couplets assigned to have 21-kyr duration (C25r=∼1449 kyr, C26n=∼231 kyr, C26r=∼2877 kyr). Consequently, the Zumaia section now provides the first complete Palaeocene astronomically derived chronology, rendering this section a master reference section. Due to limitations in the orbital calculations and uncertainties in the radiometric dating method no robust tuning and absolute ages can be given for the moment. However, the FOs (First Occurrences) of key calcareous plankton species and the Mid Palaeocene Biotic Event (MPBE) are placed within the magnetostratigraphic and cyclostratigraphic template along the studied Mid-Palaeocene interval. In addition, the dataset provides the key elements for a proper settling of the Thanetian and Selandian Global Stratotype Section and Point (GSSPs), which is one of the primary objectives of the ICS (International Commission of Stratigraphy). We consider the base of chron C26n and the criteria associated to the lithostratigraphic change between the Danian Limestone Fm and the Itzurun marl Fm at Zumaia, as the respective delimiting points for the Thanetian and Selandian bases as recently agreed by the Paleocene Working Group of the International Subcommission of the Paleogene Stratigraphy of the ICS. Consequently, the duration of the Thanetian, Selandian and Danian component stages can be estimated at Zumaia to be about ∼3129 kyr, ∼2163 kyr and ∼4324 kyr respectively (see text for error considerations). However, the MPBE located 8 precession cycles below the base of C26n in correspondence to a short eccentricity maxima at Zumaia, could also serve as a guiding criteria to approximate or redefine the Thanetian base if this level demonstrated synchronous.