8th International Nannoplankton Association Conference


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Nicola Perilli, Maria J. Comas-Rengifo:
Pliensbachian calcareous nannofossil events of the Tudanca and Santotis sections (Basque-Cantabrian area, N Spain)

As part of a multidisciplinary study (palaeontology, sedimentology, geochemistry, etc.) focused on a Lower Jurassic 'marine succession' cropping out in N Spain, two expanded and well-dated sections have been sampled in the SW sector of the Basque-Cantabrian area, in order to study the calcareous nannofossil content. Spanning in age from latest Sinemurian (raricostatum Biozone) to latest Pliensbachian (spinatum Biozone), the investigated carbonate platform deposits consist of a regular alternation of marlstone and limestone, with two thick Late Carixian and Early Domerian organic-rich intervals and a Late Domerian bioclastic limestone member.

The aim of this paper is the detailed investigation of the calcareous nannofossil assemblages of the selected sections in order to calibrate against the ammonite zones (a) the main compositional changes, and (b) the succession of the nannofossil events. The achieved results are based on refined ammonite biostratigraphy, closely-spaced sampling, and a semiquantitative analysis of more than 200 samples. Smear-slides were prepared as homogeneously as possible, to get comparable concentrations on different slides; to retain the original ratio between calcareous nannofossils and other fragments, centrifugation and ultrasound were not applied.

Both the most-relevant Pliensbachian composition changes and the main events previously recognised in two Pliensbachian reference sections (Camino and San Andrés) of the Basque-Cantabrian areas have been also identified in the studied Tudanca and Santotis sections, although the nannofossil assemblages are neither abundant nor diverse. The first significant composition change is the appearance and the abundance increase of Similiscutum avitum and Similiscutum cruciulus in the Early Carixian. The other relevant composition change takes place in the Early Domerian, and includes the abundance increase of Mitrolithus jansae, the appearances of Lotharingius barozii and Lotharingius hauffii, and the abundance decrease of the genera Similiscutum, Crepidolithus and Parhabdolithus. In the Late Domerian, the assemblage composition further changes, particularly in the sharp abundance increase of Lotharingius hauffii. Quite easily recognisable, the most relevant nannofossil events include the first occurrence (FO) of Similiscutum cruciulus (jamesoni Biozone), the FOs of Lotharingius barozii (margaritatus Biozone) and Lotharingius hauffii (margaritatus Biozone), and the first common occurrence of Lotharingius hauffii (spinatum Biozone).


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 [Division of Micropalaeontology] [Department of Geosciences] [Bremen University]

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