Stratigraphy and geomorphology of an abandoned northern branch of the Po river, the Po di Adria paleochannel (Italy)

Some authors (Veggiani, 1972; Castiglioni, 1978, Peretto, 1986) have evidenced the existence of palaochannel of the Po river delta which passed through the stretch of the present-day alluvial plain between Po and Adige rivers. A branch of this palaeochannel, called “ramo settentrionale del Po di Adria” extends from Rovigo to the lagoon of Venice near Chioggia. This multidisciplinary research provides sedimentological data and geomorphological evidence for the reconstruction of the stratigraphical architecture of the alluvial ridge, evolution of the palaeohydrography and the age of deactivation of this Po branch which is debated (De Bon, 1939, Bosio, 1967, Veggiani, 1972) because archeological and large scale palaeoclimatic considerations alone are not conclusive. Remote sensing from Landsat TM and Aster images allowed to analyze major landforms while the interpretation of oblique and vertical panchromatic aerial photos gave a finer detection of the landforms. Microrelief based on a manual interpolation of contour lines with spacing of 0.5 m from the “Carta Tecnica Regionale del Veneto” at scale 1:5000 has been used to develop a digital elevation model. Cross sections down to the depth of 5-8.5 m have been carried out with the Edelman hand auger in order to define the stratigraphy of the palaeochannel deposits. Characterization of the sediments sampled in the boreholes used several methods. Loss on ignition (LOI) gave organic versus carbonate mineral content ratio, palaeobotanical analyses established the vegetation assemblage of the swamp environment, radiocarbon dating dated the peat layers and petrographical analysis determined the composition of the sands in order to relate with the appropriate fluvial basin, either Adige or Po. The cross sections are very close to archaeological sites and locate in Sarzano (Rovigo), Saline and Chiaroni (San Martino di Venezze, Rovigo), Ca’ Motte (Villadose, Rovigo) and Conetta (Cona, Venezia). Exemplary is the case of Saline and Chiaroni: the first site is directly related to the palaeochannel and cross-sections show that the archaeological site lies on the western side of the main sand body of the alluvial ridge, contrary to the remote sensing indications. The transect in the second site area suggests the presence of a bifurcation of the palaeochannel, just upstream it is covered by the modern Adige alluvial ridge, in agreement with the digital elevation model. Furthermore the transect across major crevasse splay deposit, as recognized by remote sensing on the right side of the palaeochannel and located between Rovigo and modern Adige River, allowed to reconstruct the geometry of the deposits and to clarify the relationship between its sands and the northernmost Po distributary channel activity.