Multi-source detrital contributions in the Po alluvial basin (northern Italy) since the Middle Pleistocene. Insights into sediment accumulation in intermediate sinks

Integrated stratigraphic-compositional studies on alluvial successions provide a valuable tool to investigate the provenance of detritus in multi-source systems. The Po Plain is an intermediate sink of the Po-Adriatic source-to-sink system, fed by rivers draining two orogens. The Alps are characterized by extensive outcrops of plutonic-metamorphic and ultramafic rocks to the north-west and of Mesozoic carbonates to the east (Southern Alps). The Northern Apennines, to the south, are dominated by sedimentary successions. The Po River flows from the Western Alps to the Adriatic Sea, interacting with a dense network of transverse tributaries that drain the two orogens. Stratigraphic, sedimentological and compositional analyses of two 101 and 77.5 m-long cores, recovered from the Central Po Plain, reveal the stacking of three petrofacies, which reflects distinct provenance and configurations of the fluvial network. A South-Alpine sedimentary input between MIS 12 and MIS 10 is testified by petrofacies 1, characterized by carbonate- and volcanic-rich detritus from rocks exposed in the Southern Alps. A northward shift of the Po River of more than 30 km is marked by a quartz-feldspar and metamorphic-rich detritus (petrofacies 2), similar to modern Po River sands. This dramatic reorganization of the fluvial network likely occurred around MIS 9-MIS 8 and is possibly structurally controlled. A further northward shift of the Po River and the onset of Apennine sedimentation in the Late Holocene is revealed by petrofacies 3, rich in sedimentary lithics from the Apennine successions. The results of this study document how compositional analysis, if framed in a robust stratigraphic picture, may provide clues on the evolution of multi-source alluvial systems.Inferred palaeogeography of the central Po Plain, deduced from compositional variations of core sands, during: (a) MIS 12 and MIS 10 (petrofacies 1). (b) MIS 5-4 (petrofacies 2). (c) MIS 2 (petrofacies 2). (d) The early Holocene (petrofacies 2). (e) The late Holocene (petrofacies 3).image