Los nanofósiles calcáreos del Pliensbachiense de la sección de Tudanca (Cuenca Vasco-Cantábrica, España)

Abstract

This study deals with Pliensbachian calcareous nannofossils from the Tudanca section (Basque-Cantabrian Basin, Spain). In this section, two hundred fourteen marly and calcareous stratigraphic levels have been recognized and sixty five horizons have been sampled. A semiquantitative analysis has been undertaken on the sixty and five smear slides, showing most of them, a bad to moderate state of preservation and a rare to frequent total abundance of calcareous nannofossils. Two divisions, two classes, five orders, seven families, one subfamily, thirteen genera and twenty-six species, have been identified. The resulting data allow us to recognize a calcareous nannofossil detailed biostratigraphy calibrated with ammonite zones, and to highlight the first occurrence and the last occurrence of some of the most abundant species. The entire calcareous nannofossil zones NJ4 Similiscutum cruciulus and NJ5 Lotharingius hauffii, have been identified, as well as the upper part of NJ3 Crepidolithus crassus Zone. Some significant changes in the nannofossil assemblages have been recognised. The first change is the appearance of Similiscutum genera in the ammonite Jamesoni Zone. Other relevant composition change occurred in the Upper Pliensbachian, and includes the first occurrence of Biscutum genus, the increase in numbers of C. jansae, first appearance of the Lotharingius genera, and the decrease in numbers of the genera Similiscutum, Crucirhabdus, Crepidolithus and Parhabdolithus. Within the Spinatum Zone, the main change is the marked increase of L. hauffii. The main events in the Tudanca section are: 1. FO of S. cruciulus; 2. FO of L. hauffii. Secondary events are: 1. FO of B. novum; 2. FO of B. finchii; 3. FO of B. grandis; 4. FO of L. barozii; 5. FCO of C. jansae; 6. FCO of L. hauffii.