The modern Po Delta system: Lobe switching and asymmetric prodelta growth

Abstract

The modern Po Delta system, comprising five main delta lobes, has been investigated by integrating VHR seismic surveys, recorded offshore from water depths as shallow as 5 m to the toe of the prodelta in about 30 m, with accurate historical cartography extending back several centuries. Previous studies give sedimentological and geochronological information from precisely positioned sediment cores. This combined historical and stratigraphic reconstruction of the modern Po prodelta allows volumetric reconstructions indicating an average sediment load of 9.4 10 6 t yr − 1 for Po di Pila and Po di Goro-Gnocca lobes. This estimate is remarkably consistent with the total sediment load of 11.5 10 6 t yr − 1 available for parts of the last century from a gauge station at the apex of the delta plain (in Pontelagoscuro). These integrated stratigraphic studies allow to explain the key characters of the Po delta system: a) the marked asymmetry of the whole delta-prodelta system reflecting prevailing sediment dispersal to the south of each individual delta outlet; b) the shore-parallel overlapping of successive prodelta lobes fed by distinct river outlets of ever changing relative importance; c) the delta outlets being artificially forced in a fixed position so that natural avulsion is prevented and delta lobes undergo headland retreat leaving a marked erosion on the prodelta; d) the presence of prodelta lobes showing widespread “cut-and-fill” features (ranging from 100 to 300 m and depths up to 4–5 m filled with massive silt to very fine sand) offshore of short-lived very active distributary channels (e.g.: Po di Tolle lobe) and suggesting that, in some particular interval, short-lived episodes of submarine erosion are induced by catastrophic increases in river discharge (of natural origin or induced by human maintenance). The seismic stratigraphy of the modern Po Delta documents that markedly distinct prodelta architectures form when a newly activated lobe is located updrift (north, in this case) or downdrift (south) of the one that is retreating: in the first case the abandoned lobe becomes sheltered by the new, rapidly advancing, one; in the opposite case the retreating lobe is updrift and a substantial portion of the sediment is cannibalized and transported to the new lobe, downdrift.