ASSETTO GEOLOGICO DELLA SUCCESSIONE QUATERNARIA NEL SOTTOSUOLO TRA MELEGNANO E PIACENZA

Abstract

Baio M. et al., Geology of the Quaternary succession in the subsurface between Melegnano and Piacenza. (IT ISNN
0394-3356, 2004).
The current work illustrates a stratigraphic cross-section which crosses the southern plain of Lodi, as far south as the Po river, 45 km
long. The work aims to present the newly available database consisting of the stratigraphic and petrographic data obtained by several
tens of boreholes, foundation piles and wells that were drilled for building the high speed railway Milano – Bologna; these data are
complemented by some new geochronometrical 14C ages obtained on samples of organic matter and wood fragments. A preliminary
2D stratigraphic interpretation of this database is provided, in the form of a working hypothesis. The geological cross-section illustrates subsurface stratigraphy, down to a maximum depth of 100 m below sea level. Three sectors have been separated on the basis of the geometrical and stratigraphic features, from North to South. In the Lodi plain sector (North) six stratigraphic units have been introduced, spanning the Middle Pleistocene (?) to Holocene. The Post Glacial fluviatile unit, confined within the Lambro valley, is entrenched into the glaciofluvial sediments which presumably correlate with the LGM alloformations of the Alpine area. These latter unconformably overlay two glacio-fluvial units which have been correlated with different alloformations of the Besnate Allogroup (Late – Middle Pleistocene), based on 14C age determinations, facies and gravel petrography. The lowermost two units are interpreted as fluviatile and glaciofluvial deposits which might be related to glaciations predating the age of the Besnate Allogroup. The glaciofluvial units terminate southwards, in the central San Colombano hill sector, onto the lowermost stratigraphic unit, whose marine and transitional depositional environments suggest a correlation with the outcropping San Colombano Fm. (Pliocene p.p. – Early Pleistocene). In the subsurface of this sector, the unit reaches shallow depths (20 m below topography) due to tectonic uplift of the San Colombano apenninic ramp anticline. In this same sector it is deeply dissected by an erosion surface which mimics the shape of entrenched valleys. It is covered by the uppermost Pleistocene sediments which correlate with the most recent alloformations of the Besnate Allogroup and with the LGM sediments. In the southern Po sector, the Holocene Po alluvial deposits are cut into glaciofluvial gravel and sands of Late Pleistocene age, whose petrography suggests a mixing of dominant apenninic gravel with subordinate alpine-derived clasts. The Middle – Upper Pleistocene sediments of the Po sector, progressively cover northwards the ancient marine and transitional fine-grained sediments of the San Colombano hill sector, which are progressively lowered southwards to depths higher than 100 m below sea level and are no longer reached by boreholes and wells.