--> Integrated biostratigraphy (Ammonoidea, Foraminiferida) of the Aalenian-Bajocian (Middle Jurassic) in Iberia: preliminary results

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Integrated biostratigraphy (Ammonoidea, Foraminiferida) of the Aalenian-Bajocian (Middle Jurassic) in Iberia: preliminary results


The Aalenian-Bajocian boundary (Middle Jurassic) is particularly well represented in several basins located around the Iberian Plate, namely in the Lusitanian Basin, where the Bajocian GSSP has been defined (Murtinheira section at Cape Mondego; Portugal). Despite their location in different paleogeographic contexts, the ammonite assemblages recorded in several reference sections in the Lusitanian Basin (Murtinheira and Serra da Boa Viagem II sections in the northern sector of the basin; Portugal), in the Basque-Cantabrian Basin (San Miguel de Aguayo and Pujayo sections; Northern Spain) and in the Iberian Cordillera (Muro de Aguas and Hontoria del Pinar sections; NE Spain) has enabled the establishment of a detailed biostratigraphic framework displaying supra-basinal value: the Concavum Biozone (Concavum and Limitatum subzones) for the upper Aalenian, and the Discites Biozone for the lower Bajocian.

From a compositional point of view, the foraminiferal assemblages recorded throughout the same sections are typical of the Jurassic carbonate platforms of the Northern Hemisphere, being the representatives of the Lagenina suborder the main components of the assemblages, and the Lenticulina genus the most abundant.

The taxonomic study of the foraminiferal assemblages has allowed the establishment of a biostratigraphic framework based on that microfossil group, well calibrated with accurate biostratigraphic charts based on ammonites: Lenticulina quenstedti (Gümbel) Biozone, ranging from the Bradfordensis Biozone (middle Aalenian) until the lower Discites Biozone, and Ramulina spandeli Paalzow Biozone, ranging from the lower Discites Biozone until the Laeviuscula Biozone.

The establishment of alternative zonal scales based on microfossils displays potential usefulness as dating tool of samples without macrofossils, namely in core samples. But bioevents are also important proxies, representing another additional biostratigraphical tool of regional rank. In this sense, it is worth noting the decrease in abundance and diversity of the foraminiferal assemblages recorded at the Limitatum Subzone - Discites Biozone transition (lower Bajocian), an important bioevent also recognized in other basins of the Northern Hemisphere.