Abstract

A planktonic foraminiferal zonal scheme is proposed for the internal basinal successions of the Monferrato and Northern Apennines (late Bartonian-early Serravallian). This scheme is based on a synthesis of quantitative analyses performed on the foraminiferal assemblages from 30 stratigraphic sections, cropping out over a distance of 300 km, between the Piedmont and the Northern Apennines. The proposed zonal scheme represents the synthesis, with minor changes, of the biozonations proposed by Novaretti et al. (1995) and Mancin and Pirini (2001) with the addition of unpublished data from some new sections, spanning a broader temporal and geographical range. With respect to the previous regional and standard biozonations, this scheme shows a better biostratigraphical resolution resulting from the replacement of missing or unreliable standard bioevents with new “reproducible” and reliable regional biohorizons. These new events are: LO of muricate species, which identifies the top of the Acarinina spp., Globigerinatheka spp. and Turborotalia cerroazulensis Assemblage Zone because Truncorotaloides rohri is very rare to absent; LO of Turborotalia cerroazulensis lineage, which marks the top of the Turborotalia cerroazulensis Interval Zone because the Hantkenina group is absent; LO of psuedohastigerinids, which marks the upper boundary of the Pseudohastigerina spp. Interval Zone because Cassigerinella chipolensis and Pseudohastigerina micra do not co-occur; the former appears later, in the Globigerina ampliapertura Zone, whereas the latter often disappears near the Eocene/Oligocene boundary; FCO of Paragloborotalia opima opima, which identifies the Paragloborotalia opima opima Subzone boundary (IFP21a/IFP21b) because the LO of Chiloguembelina cubensis is a weakly reliable event; FO of Globoquadrina dehiscens, which marks the Paragloborotalia kugleri Subzone boundary (IFN1a/IFN1b) because the FCO of the Globigerinoides group occurs later; LO of Paragloborotalia kugleri, which identifies the top of the P. kugleri Total Range Zone because the FO of Globigerinoides altiaperturus occurs later and finally, the FO of Globigerinoides trilobus, used to subdivide the interval between the LO of P. kugleri and the LO of Catapsydrax dissimilis, because Globigerinatella insueta is absent.

Other homotaxial biohorizons have also been recognized and introduced among the primary bioevents. These “secondary events” can help in better placing zonal and subzonal boundaries when the primary events cannot be used and in the refinement of the correlations among the different sections.

The validity of this proposal, in our view, consists in the general correspondence between the adopted bioevents and the standard zonal boundaries and therefore in an easy correlation with the various standard schemes. Moreover, the area of applicability of this biozonation could be extended to the surrounding basins and also to coeval Mediterranean successions.

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