The last 40 ka evolution of the Central Po Plain between the Adda and Serio rivers

Abstract

We reconstructed the geological evolution and the history of the fluvial network in the central part of the Po Plain in Lombardy, northern Italy, since the Middle Würm. We focused on evidences of dissection and incision of former rivers into the large outwash fans originating from piedmont glaciers of the southern Alps in the Last Glacial Maximum. The studied area is located between the Po River and the Alps north of the Adda/Serio river confluence, around the town of Crema. Morphometric analysis and stratigraphic investigation were carried out, supported by palaeobotany, sand petrography and radiometric dating. The obtained stratigraphic framework is presented by cross sections and by a geological map. During the Middle Würm, an alluvial fan of the Adda River oriented NWN-SSE occupied the area, feeding south and west the Romanengo hill. A substantial reorganisation of the fluvial network occurred during the Last Glacial Maximum. Since 32-30 ka cal. BP, the area was fed by two outwash rivers, originated from the Adda and the Oglio piedmont glaciers, both characterised by a prevalent southward drainage, 160°-170° S in the LGM. After the LGM the fan-head was dissected by trunk channels and, at the downslope fan limit, the southward flow shifted to a southeastern direction. Thereafter, major dissection and downcutting occurred in the Lateglacial, confining the major rivers into large alluvial corridors, further shaped by lateral erosion during the Holocene. The Late Holocene history of the Serio River is marked by fluvial floods during the late Roman Age, and by a capture of the lower reach by the Adda River. The geomorphological evidence supports the historically inferred age assignment of the diversion to 12th-14th c. AD.