Nuovi dati da sondaggi provenienti dai dintorni di San Colombano al Lambro e dalla Pianura Padana a Est di Milano: analisi litostratigrafica, composizionale e micropaleontologica

New data are presented about the subsoil of the San Colombano al Lambro and Lambro - Molgora (SW Pianura Padana, Northern Italy) areas, collected from core cuttings and geognostic surveys. In the area around the hill of San Colombano al Lambro, the succession, from the top to the bottom, consists of a first sedimentary sequence, followed by a predominantly metamorphic sequence. 14C analyses provided an age of approx. 19.909/20.785 cal BC for the former and 28.679/28.295 cal BC for the latter. The clasts source area is inferred in the Alps/Prealps domain. A detrital sequence follows, in which some layers are dominated by quartz and serpentine, and others are mainly made by chert and permian lava clasts. The debris originated in the Alps/Prealps domain, probably in the W/NW area, suggested by the occurrence of “Granito rosa di Baveno”. 14C dating provided an age >40.410 BP approx. At the bottom, a sharp erosive contact separates the previous unit from thinner sediments yielding marine macrofossils and vegetal debris. In the Genzone cores, foraminiferal assemblages indicative of a brackish paleoenvironment, with Ammonia tepida and Haynesina depressula occur from -156 m b.s.l. (-90m o.s.l.) to the core bottom. In the Casalpusterlengo (Zorlesco) core the silty clay at 102 m b.s.l. p.c. (-36m o.s.l.) contains marine and brackish foraminifers (Ammonia papillosa, A. tepida) and abundant megaspores of the freshwater ferns Azolla filiculoides and Salvinia natans, suggesting freshwater input to a shallow marine bottom during an interglacial interval of the Pleistocene. The occurrence of Bulimina marginata from Orio Litta samples indicates a time interval encompassing the Gelasian and the Holocene. 12 wells were studied in the area from the East of Milan, between the Lambro river and the Molgora stream. Compositional analyses on the gravels and geological sections evidenced 4 macrounits. The petrological composition shows that sediments originated from the north, coming from the Valtellina and the Prealps between Como and Lecco. Micropaleontologic analyses did not detected marine microfossils at depths comparable with those of the S. Colombano cores.