Benthic foraminiferal assemblages across the Aalenian–Bajocian boundary in the Iberian and Basque-Cantabrian basins (North Spain)

Abstract

The aim of this work is to describe the benthic foraminiferal assemblages recorded across the Aalenian–Bajocian boundary (Middle Jurassic) in two sections of the Iberian and Basque-Cantabrian basins (North Spain), well calibrated with the standard ammonite zonation, in order to establish a biostratigraphic scale based on the foraminiferal record, and to infer the palaeoecological conditions that affected their development. A total of 29 samples were collected in the studied sections, and the composition of the foraminiferal assemblages recorded throughout the studied stratigraphic interval was analysed using the stratigraphic distribution of the identified species, the quantitative data obtained through the estimation of relative abundances and the calculation of several diversity indexes. The studied assemblages are moderately abundant and diverse, and they are constituted by a total of 5435 specimens, corresponding to 5 suborders, 13 families, 25 genera and 51 species. Their composition displays close similarities to those already described for the Jurassic carbonate platforms of the Boreal Realm (Type-A), Boreal Atlantic subdivision. For the studied stratigraphic interval, it was recognised the Lenticulina quenstedti Foraminiferal Zone, in which the recorded assemblages developed in intermediate part of a marine platform environmental conditions, well oxygenated and with normal salinity. In the Aalenian–Bajocian transition, it was possible to identify a sharp decrease in the abundance and diversity of the foraminiferal assemblages that has also been recognised in other coeval basins of the Iberian Plate palaeomargin, thus representing a bioevent of the Northern Hemisphere.