Publication: Foraminifera from the Aalenian and the Bajocian GSSP (Middle Jurassic)
of Murtinheira section (Cabo Mondego, West Portugal):
Biostratigraphy and paleoenvironmental implications
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2008
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Elsevier Science B.V., Amsterdam.
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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.