Premoli Silva, I.

Detailed investigations carried out in support of the new Italian geological map at the scale 1:50 000 (CARG Project) resulted in the definition of new formal lithostratigraphic units (Brenno Fm, Tabiago Fm, Cibrone Fm) within the previously undifferentiated Scaglia and in the first integrated studies on calcareous plankton, including calcareous nannofossils. The Tabiago Formation consists of marls and pelagic limestones with some intercalations of turbidites containing large shallow-water displaced foraminifera in the upper part. Its age ranges from the Early Paleocene (foraminiferal Zone P1b, nannofossil Zone NP2) to the Middle Eocene (foraminiferal Zone P12, nannofossil Zone NP15). The lowermost biozones of the Paleocene (P0, Pα=P. eugubina, and P1a) are missing. The Cibrone Formation is represented by scattered outcrops and consists of marls and silty marls with minor arenites. Its age is Middle Eocene (Zone P12, Zone NP15-NP16). The Paleocene portion of the well-exposed Tabiago section is characterized by alternating couplets of soft marls and hard limestones. A multidisciplinary investigation was undertaken in order to understand the environmental significance of these repetitive cycles. Abundance and composition of the planktonic foraminiferal populations, degree of bioturbation, and calcium carbonate content were measured. In the marly semicouplet the fauna is relatively poor; it is enriched in specimens of the genus Subbotina, an indicator of nutrient-rich, colder waters. In the calcareous semicouplet the fauna is enriched in taxa of the genus Morozovella, an indicator of warm, nutrient-poor tropical waters. A possible interpretation of the sedimentary rhythms is that the marly interbeds were deposited in periods of high seasonality, when enhanced circulation brings nutrient enrichment at the surface and good ventilation at the bottom. On the contrary, the calcareous interbeds were deposited at times of low seasonality with sluggish circulation and nutrient-poor surface waters. In order to evaluate the possible extra-terrestrial forcing of the observed cyclicity, a spectral analysis was carried out on hundreds of quantitative data concerning foraminiferal composition and carbonate content. The Milankovitch frequency for precession (21 ky), obliquity (41 ky) and short excentricity (100 ky) cycles were recorded with the strongest signal related to obliquity.

The Monte Orfano Conglomerate (MOC), exposed in the foothills of the Southern Alps (northern Italy), is one of the few outcrops of sediments documenting the Cenozoic tectonic evolution of the Alpine retrowedge. Calcareous nannofossil biostratigraphy allowed us to constrain the upper part of the MOC, formerly attributed to the Early-Middle Miocene in the type-locality, to the earliest Miocene (Neogene part of the NN1 nannofossil zone). A likely latest Oligocene age is therefore suggested for the bulk of the underlying conglomerates, whose base is not exposed. Deposition of the MOC can be placed within the post-collisional tectonic uplift of the Alps, documented in the Lake Como area by the Como Conglomerate (CC) at the base of the Gonfolite Lombarda Group, and supports the correlation with Upper Oligocene clastic sediments cropping out further to the East, in the Lake Garda and in the Veneto-Friuli areas (‘‘molassa’’). The remarkable difference in petrographic composition between the western (CC) and eastern (MOC) clastics deposited in the Alpine retro-foreland basin highlights the synchronous tectonic activity of two structural domains involving different crustal levels. Whilst the bulk of the CC, that straddles the Oligocene/Miocene boundary, records largely the tectonic exhumation of the Alpine axial chain crystalline complexes, the coeval MOC consists of detritus derived from the superficial crustal section (Triassic to Paleogene sedimentary rocks) of the Alpine retrowedge and constrains the onset of the post-collisional deformation phase of the Southern Alps as not younger than the Late Oligocene.

Calcareous nannofossil biostratigraphic investigations have been carried out on samples collected from the Tabiago section, Brianza, northern Italy. This section records early Maastrichtian (CC23b nannofossil zone) to early Eocene (NP10 nannofossil zone) calcareous nannofossil assemblages. The Cretaceous-Tertiary transition is characterized by major changes in sedimentation rates and two likely stratigraphic gaps. The Cretaceous Brenno Formation was deposited with a high sedimentation rate (~20 m/m.y.), whilst the Tabiago Formation of Paleogene age records a drastic decrease (~10 m/m.y.). The absence of the CC26 zonal marker Micula prinsii may indicate a stratigraphic hiatus or an extremely condensed level in the latest Maastrichtian. However, only two samples have been collected and analyzed in this interval due to low exposure of the outcrop, and the absence of M. prinsii could be the result of diagenetic overprint or insufficient sampling. A stratigraphic hiatus or a very condensed interval corresponds to the upper part of NP1 nannofossil zone and Pα foraminiferal Zone and P1a Subzone in the early Paleocene. The poor exposure of the outcrop prevents to precisely locate the zonal boundaries between NP4 and NP5 and between NP9 and NP10 and impedes the documentation of possible further stratigraphic gaps or condensed intervals.