Paleoenvironmental significance of the foraminiferal assemblages from the Bajocian GSSP (Cape Mondego, Portugal)
The taxonomical composition of the foraminiferal assemblages from the Bajocian GSSP (Cape Mondego, West Portugal) is typical of the Jurassic carbonate platforms of the Northern Hemisphere. They are dominated by the representatives of the Suborder Lagenina, and the genus Lenticulina is characterized by high abundance of specimens. In this work, the paleoecological significance of such assemblages are described and discussed for the first time, based on the results obtained through the application of several richness and diversity indexes, which reflect the response of the assemblages to variations in the paleoenvironmental factors. Throughout the lower part of the Limitatum Subzone (Upper Aalenian) until the lowermost part of the Discites Biozone (Lower Bajocian) it was detected a strong decrease in the obtained values, and a recovery of such values from the Discites Biozone upwards (Fig. 1). This bioevent can be recognized in other Iberian basins, therefore representing a biostratigraphic proxy of regional value.
Several hypotheses can be envisaged in order to explain the impoverishment of the foraminiferal assemblages. The taxonomical composition of the foraminiferal assemblages discards low oxygen conditions. As so, this biotic crisis is probably related to nutrient supply or it results from atypical physico-chemical features of the bottom water as a consequence of tectonical activity, as previously reported for the Lower Aalenian of the São Gião section (Lusitanian Basin).
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AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015