PLIENSBACHIAN CALCAREOUS NANNOFOSSILS OF THE SANTOTISSECTION (BASQUE-CANTABRIAN BASIN, N SPAIN)

摘要

The target of this paper is to study the succession of the Pliensbachian calcareous nannofossil assemblages recovered from the Santotis section (Northern Spain) in order to calibrate the biohorizons against the ammonite zones. In this section, one hundred nineteen marly and carbonate-dominated levels have been recognized and forty-one of them have been sampled. The semiquantitative analysis has been carried out on smear slides and reveals that the majority of the assemblages show a bad to moderate degree of preservation and the species abundance varies from rare to frequent. In the studied section, the most relevant Pliensbachian composition changes of the assemblages are the first appearances of the genera Similiscutum (Jamesoni Zone) and Lotharingius (Stokesi Zone). The assemblages further change due to the appearance of the species belong-ing to the genus Biscutum. The succession of the assemblages have allowed the identification of two main events, the FOs of Similiscutum cruciulus (Jamesoni Zone) and Lotharingius haled (Stokesi Zone) as well as four secondary events, the FOs of Biscutum novum (Davoei Zone), Biscutum grandis (Stokesi Zone), Biscutum finchii (Stokesi Zone) and Lotharingius barozii (Stokesi Zone). Assemblages and biohorizons have allowed the identification of the upper part of the NJ3 Crepidolithus crassus Zone, the complete NJ4 Similiscutum cruciulds Zone and the almost complete NJ5 Lotharingius hauflii Zone.