Calcareous nannofossil biostratigraphy of an outcrop section of Aptian sediments of west-central Portugal (Lusitanian Basin)

Abstract. The present work is the first study of nannofossils from the Ponta Alta and Praia da Lagoa members of the Cresmina Formation and provides a new approach to establishing the age of these sediments. Nannofossil biostratigraphy improves previous age assignments: the Ponta Alta Member is placed within the latest Early Aptian, the passage between the Ponta Alta to the Praia da Lagoa members is assigned an early Late Aptian age and the Praia da Lagoa member is assigned an age not younger than the early Late Aptian. The lower half of the Rodísio Formation is assigned an age not younger than the Albian.

The integration of the nannofossil results with isotope stratigraphy provides support for the age attributions and calibration with more open-marine sections. The low abundance of nannoconids observed within the Ponta Alta Member coincides with the OAE 1a carbon isotope excursion and is possibly a manifestation of the ‘nannoconid crisis’ event. The subsequent increase in the abundance of nannoconids in the upper Ponta Alta Member coincides with the highest values of the δ¹³C isotope anomaly and may reflect a position above the ‘nannoconid crisis’ event in the early Late Aptian.