New chronostratigraphy of the Cretaceous–Paleogene boundary interval at Bidart (France)

The Cretaceous–Paleogene boundary section at Bidart (French basque coast) was prevented for a long time to be considered as a reference section for the K–T boundary due to a suspected hiatus comprising most of Chron C29r. new magneto- and biostratigraphic study of this section we discovered that the underlying primary magnetic polarity is not revealed in the vector directions but in a small, but consistent, intensity excursion during laboratory demagnetization trends. Magnetite and maghemite are the main carriers of remanence, with contribution of hematite to the magnetic mineralogy of the red-brown lowermost Danian and Maastrichtian marly limestones. Magnetite is the main magnetic mineral in the white to pink Danian limestones. The NRM of marls and marly limestones is characterized by: (1) a present-day field overprint, (2) a medium-temperature (300–400 °C) reversed component interpreted as a relic of the characteristic remanent magnetization (ChRM) carried by magnetite grains which are not completely oxidized into maghemite, and (3) a high temperature (up to 590 °C) normal component due to late diagenesis hematite growth. The NRM of Danian limestones is characterized by (1) a present-day field overprint and (2) a ChRM component of normal or reversed polarity. RM directions allow a new correlation of the magnetic polarity sequence with the Geomagnetic Polarity Time Scale (GPTS): Chron C29r corresponds to the Maastrichtian unit and extends up to 3.5 m above the K–T boundary, and Chron C29n occurs at the top of the section. There is no evidence for a hiatus at or below the K–T boundary, and the section does not extend up to Chron C28r as previously suggested. These new chronostratigraphic assignments are in good agreement with the previous biostratigraphic data and from the new biostratigraphic analyses based on calcareous nannofossils presented in this paper. udy demonstrates that the section of Bidart can be considered as a key section to study the K–T boundary event(s).