Aggiornamenti biocronologici e nuove interpretazioni ambientali sul Paleocene-Eocene della Brianza (Lombardia)

Abstract

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.