Calibration of Pliensbachian calcareous nannofossil events in two ammonite-controlled sections from northern Spain (Basque-Cantabrian area)

Common and diverse calcareous nannofossìl assemblages were found in two continuous and expanded successions, sedimented on hemipelagic carbonate ramp. Based on refined ammonite biostratigraphy closely spaced sampling and semiquantitative analysis, the main purpose of this paper is to calibrate against ammonire zones and subzones, the nannofossil composition chanqes and the identified biohorizons. In the study area, the first notable composition change is in the lower Carixian, with the appearance and abundance increase of the gents Similiscutwm, that represents a large amount of the assemblages up to the lower Domerian. Although, the appearance of the genera Blscwtum and B ussonius is eastly detectable around the Carixian/Domerian Zone boundary the other relevant Pliensbachian composition changes took place in the lower Domerian. It includes the sharp abundance increase of Calci'uascularls and the abundance appearance of the genus Lotharingiws, that dominate the upper Domerian assemblages, also characterized by the significant presence of the genus Biscutum. The set of easily recognizable and reliable events includes the First Occurrences (FOs) oÍ Similiscwtum aùtwm and SimiLiscutum cruciulus $anesoniZone), the FOs of Lotbaringiws barozii and Lotharingius haffii (Stokesi/Margaritatus Zone boundary), the First Common Occurrence (FCO) of Lotharingius hauJfii (Margaritatus/Spinatum Zone boundary) and, probably, the FCO of Calciuascularis jansae (Stokesi/Margaritatus Zone boundary). Instead, the FOs of Biscutum dwbium, Bussoniws prinsii (Davoei Zone), Biscutum novum, Biscutum grande and Biscutum finchii (Stokesi Zone) are slightly difficult to calibrate against ammonite zones