Aquifer architecture of the Quaternary alluvial succession of the southern lambro basin (Lombardy-Italy)

Abstract

Aquifer stratigraphy of the sector of the Pleistocene - Holocene alluvial plain of Lombardy, run by the Lambro valley system south of Milan, has been reconstructed on the basis of geological mapping at 1:10.000 and subsurface interpretation and correlation of more than 150 among water wells, boreholes and deep excavations. The Middle (?) - Late Pleistocene sedimentary evolution of this sector includes four major progradation cycles of alluvial depositional systems that migrated from the alpine northern side towards the axial palaeo-Po depositional system. These cycles were governed by Pleistocene glacial cycles, in combination with ramp-folding of the San Colombano - Salerano apenninic anticlines and minor uplift of the alpine side. Every major cycle is soled by an erosion surface, and is shaped by minor fining upward sequences. Both major and minor sequences record at first the advance of coarse-grained units (distal braided alluvial fan or sandy braid plain) which fringe-out south-eastwards into meandering fluvial systems, and are replaced upwards by alluvial plain fines, which close the sequences. Physical stratigraphy and geomorphology, analysis of facies associations, characterisation of gravel composition, radiocarbon dating on 4 peat and plant relic samples and findings of transported artefacts allowed the correlation of the four cycles with the regional evolution. The Post Glacial meandering depositional systems of the deeply entrenched Lambro valley system (Unit 4, Holocene; Unit 5, historical), are cut into the braided stream to meandering depositional systems that developed during L.G.M. times, at present outside the Lambro valley (Unit 3, Late Pleistocene). These represent the uppermost aquifer unit, i.e. the phreatic - non saturated zone. The underlying Unit 2 (Late Pleistocene) can be correlated with the Besnate Allogroup, and therefore developed during the corresponding glaciations. It is formed by three stacked sequences, controlled by glacial cycles, which are deeply scoured into the lowermost succession that could be studied (Unit 1 Middle ? - Late Pleistocene ?). It represents the most important and permeable intermediate aquifer unit, that is only partly confined by the flood-plain fines of the uppermost sub-unit 2C