Stratigraphy and palynology of late Holocene Adige-Po alluvial plain near Adria

The distal portion of the Adige sedimentary system confines to the north with the Brenta River megafan and to the south with the Po sedimentary system. The boundary between Adige and Po sedimentary systems is not well defined, as branches of the Po River occasionally interested the Adige alluvial plain and, vice versa, Adige channels crossed the Po system, building several alluvial ridges. The study area is located between the cities of Rovigo and Adria, about 25 km from the Adriatic coast. It is crossed by the Adige and Adigetto alluvial ridges and the so-called Po di Adria ridge. The reconstruction of the Late Holocene palaeohydrography of this area has been mainly based on remote sensing interpretation and archaeological data. Palaeoenvironmental reconstructions, based on palynological analysis, are available only for the Grandi Valli Veronesi, about 60 km west of the study area and short time intervals (e.g. Bronze Age). In this work, results from previous stratigraphical (Piovan, 2008) and palynological studies (Marchesini et al., in prep.) are presented together with data from new corings. This, in order to start a multidisciplinary discussion about Holocene palaeoenvironmental conditions in the study area and the correlation between climate variations and changes in the alluvial sedimentary architecture. The geomorphological study has been carried out by means of digital terrain modeling and remote sensing. About 100 boreholes organized in 8 cross sections down to the depth of 5-9 m have been carried out with a cilidrical hand auger in order to define the stratigraphy of the major palaeochannel deposits and their relationship with the floodplain. In this work, we present new stratigraphic data from Ramalto cross-section and Montefalche area. From north to south, the Ramalto cross-section cuts Fasanara, Adigetto and Ca’ Matte alluvial ridges. The cross-section presents three main sand bodies and a very small one in correspondence of Adigetto ridge. The northermost sand body is related to the so-called Fasanara palaeochannel that is probably belonging to the sedimentary activity of the Adige River. The southernmost one is the result of a late activity of Po di Adria; its geomorphological evidence is the Po di Ramalto alluvial ridge. These two sand bodies, both cropping out, formed in a phase of aggradation which took place since 4-3 ka BP as also recognized in whole Venetian-Friulian Plain (Fontana et al., 2008). The third and largest sand body is buried under 3-5 m of fine-grained sediments and is probably related to a branch of the Po di Adria. Radiocarbon dating of a peat layer, at the depth of 260-265 cm, provides an age of 3143-4408 BP cal., indicative of the deactivation of this channel. An open section (4 m from ground level) has been uncovered during the Cavarzere-Minerbio methane pipeline excavation in Montefalche, about 200 m on left side of Ca’ Matte ridge. The stratigraphy consists of fine grained sediments (silt and clay) and is characterized by the presence of two organic-rich layers. Two 14C dates have been obtained from these layers: the more superficial, at the depth of 95-145 cm gave an age of 1416-1627 BP cal. and the deepest one, at 350-400 cm, 3377-3557 BP cal. This latter is comparable to the 14C dating of Ramalto cross-section. In the Montefalche section, about 40 samples have been analysed for palynology. The pollen diagram indicates a change to colder and wetter climate in correspondence to the deepest dated peat layer (Marchesini et al., in prep.), possibly related to the Löbben phase. Further studies are needed to obtain a more complete scenario on the response of the Adige-Po alluvial plain to late Holocene climate changes. For this purpose, new cores are going to be studied.