The Lutetian/Bartonian transition (middle Eocene) at the Oyambre section (northern Spain): Implications for standard chronostratigraphy

Abstract

With the aim of finding a potential Global Stratotype Section and Point (GSSP) for the base of the Bartonian Stage, an integrated stratigraphic study was carried out in the Acebosa Fm exposed at the Cape of Oyambre (Cantabria, Spain). While the Oyambre section does comply with the majority of the requirements of the International Commission on Stratigraphy, several shortcomings were also found, which apparently make it unsuitable for the GSSP for the base of the Bartonian Stage. An unexpected finding was that the chronostratigraphic calibration of the Oyambre planktonic foraminiferal events revealed major discrepancies with the standard timescale. The lowest occurrence of Globigerinatheka index was found in the upper part of Chron C20r, and not in the upper part of C20n as depicted in the standard chart, being dated at 43.68Ma and not at 42.64Ma. The lowest occurrences of Turborotalia pomeroli and Globigerinatheka euganea were found in the upper part of calcareous nannofossil Zone CP14a (40.92Ma), and not in the upper part of Zone CP13c (42.21Ma). Finally, the highest occurrences of Guembelitrioides nuttalli and Morozovelloides bandyi were also found in the upper part of Zone CP14a (40.68Ma), and not in the upper part of Zone CP13c (42.07Ma). The accuracy of these anomalous results was further substantiated with data from other areas, strongly suggesting that the standard Eocene chronostratigraphy was in need of amendment. The Oyambre findings, if proven significant at a global scale, would have two important chronostratigraphic outcomes. Firstly, the standard Eocene planktonic foraminiferal biostratigraphy would need revision, as there would be practically no space to accommodate zones E9 and E11. Secondly, an array of sound biostratigraphic markers would now be defined around Chron C19n, which has been considered the best marker for the Lutetian/Bartonian boundary, thus facilitating correlation of a potential GSSP.