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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

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

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tom Vol. 4, no. 1

202-203

EN

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.

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Lower Toarcian organic-rich sediments from the Tuscan Succession (Northern Apennines, Italy)

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Perilli N. , Veiga de Oliveira L. , Rodrigues R. , Catanzariti R.

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tom Vol. 4, no. 1

203-204

EN

This work is aimed to improve lithostratigraphy and biostratigraphy, based on calcareous nannofossils, of the deep-marine Lower Toarcian sediments belonging to Tuscan Nappe (Northern Apennines). The sampled lower part of the Calcari e marne a Posidonia (Posidonia Marls) correlates with the Marne del Monte Serrone of the Umbria-Marche Basin, which is characterized by the presence of the Lower Toarcian black shales. The Calcari e marne a Posidonia consists of grey to greenish hemipelagic to pelagic bivalves-bearing marlstones and limestones with interbedded grey to grey dark, sometimes reddish, clayey marlstones and marly claystones. In some localities, in the lower portion of this formation is recognized a thin to medium thick organic-rich interval of black marlstones and marly claystones. The recovered calcareous nannofossil assemblages allow to assigne the basal portion of the Marne a Posidonia to the Lower Toarcian with the thin organic-rich interval comprised between the appearances of the genera Carinolithus and Discorhabdus.

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The Pliensbachian/Toarcian boundary in the Almonacid de la Cuba section (Iberian Range, Spain)

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Goy A. , Comas-Rengifo J. , Gomez J. , Gonzales J. , Herrero C. , Palencia A. , Perilli N. , Rodrigo A.

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tom Vol. 4, no. 1

164-166

EN

The Almonacid de la Cuba section, representative of the Pliensbachian-Toarcian transition in the Iberian Range (Fig. 1), is reviewed. It is an expanded section where no important discontinuities have been detected. Four successive assemblages of ammonites, which are characterized by the presence of Pleuroceras (BH14-CU14), Canavaria (CU16-CU32), Dactylioceras (E.) (CU35.2-CU44) and Dactylioceras (O.) (CU44-CU87), are distinguished. The Pliensbachian/Toarcian boundary is located at the base of level CU35.2 with the first record of Dactylioceras (Fig. 2). These assemblages are mainly constituted by taxa typical of the NW European Province, such as Pleuroceras, Dactylioceras (O.) and P. paltum. However, frequent Mediterranean Province taxa such as Emaciaticeras, Canavaria, Lioceratoides, Neolioceratoides, Dactylioceras (E.) and P. madagascariense, are also recorded. In the Tenuicostatum Zone, dactylioceratidae are dominating with respect to harpoceratinae. In the Mirabile Subzone, species of Dactylioceras (E.) are coexisting with P. paltum. Brachiopods show two successive assemblages. The lower one is composed generally of the Pliensbachian taxa and the upper assemblage includes more endemic taxa. Coinciding with the Early Toarcian OAE, almost all these species disappeared at the end of the Tenuicostatum Chron. Foraminiferal assemblages are rich and diversified. Calcareous hyaline taxa are dominated by suborder Lagenina, agglutinated foraminifera are scarce, the suborders Spirillinina and Miliolina are represented by few specimens and taxa, and specimens of Robertinina have been recovered throughout the whole stratigraphic interval. The main biostratigraphical foraminiferal events can be recognized and compared with other sections of the Iberian Range and with another ones of selected NW European Basins. Ostracod assemblages of the Spinatum Zone are dominated by healdiids and cytheraceans, which decrease at the base of the Tenuicostatum Zone, where the cypridaceans are better represented. In the Semicelatum Subzone, coinciding with the disappearance of the healdiids, the cytheraceans become dominants.Calcareous nannofossils assemblages are rich and well preserved. This allowed locating precisely the biochronostratigraphical position of the main markers and events and comparing them with these recorded in other basins of Western Tethys. A magnetic polarity column for the Pliensbachian/Toarcian boundary has been constructed on the basis of the polarities of the 2C Component (Fig. 2). The lower boundary of the Toarcian is located within the R2 magnetozone. A relatively large magnetozone N3 of normal polarity is located within the Tenuicostatum Zone.