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1 2006

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Pliensbachian calcareous nannofossil zones vs ammonite zones along the western and northern margins of the Iberian Massif and biostratigraphical potential of nannobiohorizons

Perilli N., Veiga de Oliveira L., Comas-Rengifo M., Duarte L.

Volumina Jurassica

2006

Vol. 4, no. 1

202-203

Calcareous nannofossils, one of the main components of Lower Jurassic marly/limestone alternations, were studied along the western and northern margins of the Iberian Massif. Consequently, they were used to date the lithological successions as well abiotic signals (e.g. isotope or TOC profiles). Our work focuses on the main changes of calcareous nannofossil record and the biohorizons recognized in some reference Pliensbachian sections from Basque-Cantabrian area (Spain) and Lusitanian Basin (Portugal). The remarkable changes in composition are the appearances and abundance increases of the Biscutaceae (Similiscutum) and of Watznaueriaceae (Lotharingius). The appearances of large Biscutum (B. grande and B. finchii) and of medium-sized Lotharingius species (L. sigillatus) are also clearly detectable though their occurrence is discontinuous. The other events include the appearances of Biscutum dubium, Bussonius prinsii, Biscutum novum and Crepidolithus impontus and the disappearance of Parhabdolithus robustus. The reconstructed distribution pattern of the age-significant species supports the identification and description of the nannofossil zones and subzones proposed for NW Europe. The NJ3/NJ4, NJ4/NJ5 zone boundaries are easily identified by the FO of Similiscutum cruciulus (Lower Pliensbachian) and the FO of Lotharingius hauffii (Upper Pliensbachian), respectively. The subzone boundaries should be carefully checked because the zonal markers are rare and occur discontinuously. However, the other events are helpful to correlate the biostratigraphic frames outlined for the investigated areas and to calibrate the NJ4a/NJ4b, NJ5a/NJ5b zone boundaries with respect to the ammonite zones. Based on the achieved data, the main differences between the two schemes are related to the very low abundance and discontinuous occurrence of the some species in their initial (e.g., B. grande, B. finchii) or final (e.g., P. robustus) ranges. Since for the Basque-Cantabrian area ammonite zone and subzones are well constrained, some discrepancies should be related with a discontinuous or incomplete ammonite record from the Lusitanian Basin. Nevertheless, the biostratigraphic frames proposed for both areas could improve biochronocorrelation between the Pliensbachian successions cropping out along the western and northern margin of the Iberian Massif.