Foraminifera from the Aalenian and the Bajocian GSSP (Middle Jurassic) of Murtinheira section (Cabo Mondego, West Portugal): Biostratigraphy and paleoenvironmental implications

Abstract

The detailed study of benthic foraminifera in 28 samples from Late Toarcian-Aalenian-Early Bajocian hemipelagic sediments of the Murtinheira section (Cabo Mondego, Lusitanian Basin, West Portugal), has permitted the recognition of the assemblage composition and the analysis of their evolution throughout this time interval. The representatives of the Suborder Lagenina dominate all the assemblages and, in the majority of them, the most abundant species is Lenticulina münsteri. The Late Toarcian and Early Aalenian assemblages are very similar. From the Comptum Subzone, a gradual replacement of the Early Jurassic forms by characteristic Middle Jurassic taxa takes place. During the Late Aalenian-Early Bajocian interval, a significant number of last occurrences (LOs) of taxa are recorded. The assemblages composition, the estimation of the relative taxa abundances and the data obtained from the calculation of several diversity indexes, together with the previous sedimentological and paleontological works in this area, indicate that these assemblages were developed in a shelfal basin environment. This platform showed normal marine salinity values, good oxygenation and was always located above calcite compensation depth (CCD). However, the environmental conditions were not always the best for the development of rich and diverse foraminiferal assemblages. These conditions were probably unstable during the Late Toarcian (Aalensis Subzone)-Early Aalenian (lower part of the Comptum Subzone). From the Comptum Subzone to the lower part of the Limitatum Subzone (Late Aalenian), conditions were more stable and more favourable for the development of foraminiferal communities. However, in the Aalenian-Bajocian transition the environmental conditions changed again and affected the development of this group. In the Limitatum Subzone a decrease in the diversity of the assemblages was recorded. This fact seems to be related to a global perturbation in the carbon-cycle that could have affected the foraminifera, together with other marine and continental organism groups.