Calcareous nannofossil biostratigraphy of the Lower Jurassic in the Cantabrian Range (Northern Spain)

This paper provides a synthesis of the Upper Sinemurian-Lower Toarcian (Lower Jurassic) calcareous nannofossil biostratigraphy for the Cantabrian Range (Northern Spain), based on four sections from the Basque Cantabrian Basin and one section from Asturias. Data derive from previously published biostratigraphic schemes and unpublished data, based on results obtained from semi-quantitative analysis of more than 485 calcareous nannofossil assemblages. Calcareous nannofossil main and secondary events are calibrated to the ammonite zonations established for the studied areas. We propose 5 main events: the first occurrences (FOs) of Similiscutum cruciulus, Biscutum novum, Lotharingius hauffii, Lotharingius sigillatus and Carinolithus superbus, based on continuously recorded and common taxa, and 5 secondary events: the FOs of Biscutum grande, Biscutum finchii and Lotharingius barozii and the first common occurrences (FCOs) of Calcivascularis jansae and L. hauffii, on the basis of rare taxa with a discontinuous record. A new biostratigraphic scheme, consisting of 4 calcareous nannofossil Zones (CNZs; NJ3 Crepidolithus crassus, NJ4 Similiscutum cruciulus, NJ5 Lotharingius hauffii and NJ6 Carinolithus superbus), two new calcareous nannofossil Subzones (CNSzs; NJ4a Crepidolithus cantabriensis and NJ4b Biscutum novum) and one emended CNSz (NJ5a Biscutum finchii), is proposed here for the Lower Jurassic of the Cantabrian Range. The new biostratigraphic scheme is compared to schemes from NW Europe, Portugal and Italy/S France. The FOs of S. cruciulus and C. superbus are reproducible in all the compared areas. However, the FOs of L. hauffii and L. sigillatus seem to be diachronous events in the Boreal and Tethyan Realms, probably due to the absence of a refined and precise ammonite zonation, to differential preservation or to provincialism of the ammonite faunas. It is possible that the FO of L. hauffii recognized in Portugal and Italy/S France within the Spinatum ammonite Zone (AZ) could be correlated with the FCO of L. hauffii identified in Cantabrian Basin. Probably the Lotharingius species firstly appeared in the Boreal Realm, and later spread into the Tethyan Realm. This study confirms the potential of calcareous nannofossil biostratigraphy for dating Lower Jurassic sections and intra- and for inter-regional correlation.