--> High resolution biostratigraphy (Ammonoidea, Foraminiferida) of the Toarcian-Aalenian (Lower-Middle Jurassic) in Iberia: preliminary results

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High resolution biostratigraphy (Ammonoidea, Foraminiferida) of the Toarcian-Aalenian (Lower-Middle Jurassic) in Iberia: preliminary results


The biostratigraphic framework for the Toarcian-Aalenian (Lower-Middle Jurassic transition) boundary based on the ammonite assemblages recorded in several reference sections of the Iberian Peninsula is presented. They refer to the following sections: Murtinheira, São Gião, Maria Pares and Zambujal de Alcaria sections of the northern and central sector of the Lusitanian Basin (Portugal); Cillamayor, San Andrés, Camino, San Miguel de Aguayo, Pujayo, Tudanca and Castillo Pedroso sections of the Basque-Cantabrian Basin (Northern Spain); and Fuentelsaz, Sierra de los Cameros, Muro de Águas and Moyuela sections of the Iberian Cordillera (Central Sector of the Castilian Branch and Aragonaise Branch; NE Spain). In all the analyzed sections, including the Aalenian GSSP (Fuentelsaz section), the upper Toarcian corresponds to the Aalensis Biozone (Mactra and Aalensis Subzones at the Lusitanian Basin; Mactra, Aalensis and Buckmani Subzones at the remaining basins), and the lower Aalenian corresponds to the Opalinum Biozone (Opalinum and Comptum Subzones).

The detailed study of the benthic foraminiferal assemblages of the same sections has pointed out that they are constituted by typical taxa of the Jurassic platforms of the Boreal Realm.

From a biostratigraphic point of view two biozones have been established based on this microfossil group, well calibrated with the biostratigraphic units based on the ammonite record: the Astacolus dorbigny (Roemer) Biozone recognized in the upper Toarcian (Aalensis Biozone; Mactra and Aalensis Subzones) and in the lower Aalenian (Opalinum Biozone; Opalinum and Comptum Subzones), extending its record throughout the Aalenian in the Iberian basins; Lenticulina quenstedti (Gümbel) Biozone recognized in the Bradfordensis Biozone (Middle Aalenian) of the Lusitanian and Basque-Cantabrian basins. In the studied assemblages, both index species are abundant and easily identifiable.

The recognition of the same biostratigraphic units based in both fossil groups increases its correlation potential within Iberia and, concerning zonal scales based on benthic foraminifera, such scales represent a particularly useful tool for dating sample cores, namely under the frame of the oil industry activities.