Holocene palaeohydrography and stratigraphy of the south Venetian Plain

The study area is located in the distal portion of the Adige alluvial system between the cities of Rovigo and Adria and is characterized by a complex network of alluvial ridges formed by the aggradation of sandy deposits along the river branch during the Holocene avulsions by Adige and Po rivers. The reconstruction of the palaeohydrographical conditions during the Late Holocene of this area has been mainly based on remote sensing interpretation and archaeological data (Veggiani, 1972; Peretto, 1986; Marcolongo, 1987; Cremonini, 2007). In this research, new chronostratigraphical and petrographical data are presented, in order to provide better insights on the Holocene palaeohydrography and stratigraphy of this area. Geomorphological, stratigraphical and petrographical results confirm the presence of a Po branch from Fratta Polesine through Sarzano to Cona which activity began after 4237-4979 cal. BP in Saline 1 and after 4141-4712 cal. BP in Cona, and produced the Villadose crevasse splay. The palaeochannel deactivation dates back in the Late Bronze Age as confirmed by archaeological settlement in Saline that lies on the natural levee deposits. In Cona 1, a calcic horizon has been reached at the mean depth of -7.5 m a.s.l. It can be correlated with the “caranto” palaeosoil of the Lagoon of Venice (Mozzi et al. 2003), produced during a sub-aerial exposure of the sediment after the last glacial maximum lowstand phase. In this interpretation, this buried soil marks the top of the Late Pleistocene sequence. As evidenced in the whole Venetian-Friulian Plain, the alluvial sequence is characterized by a sedimentary hiatus since 14500 to 8000 BP (Fontana et al., in press). After this hiatus, the transgressive phase began and the sea level rise caused an increase of the underground water table with the setting of swamp conditions, which characterized the floodplain during the maximum flooding (6000-5500 BP in the Po delta region (Stefani & Vincenzi, 2005). The peat deposits found in Cona, Saline and Chiaroni at the mean depth of -4 m a.s.l. may be correlated with those formed, in the alluvial plain behind the coast line, during the transgressive and maximum flooding phase (Bondesan et al., 1995). In the hypothesis that in Cona the sedimentation possibly began again around 8000 BP, the radiocarbon dating of the peat layer at 4435-5306 cal. BP, just ca. 1 m above the caranto palaeosoil, supports the presence of an Early Holocene sedimentary hiatus in the alluvial series. The overlying radiocarbon dating (4141-4712 cal. BP), at -4.5 m a.s.l., shows that the aggradation of the floodplain continues during the 3rd millennium B.C. with a deposition of about 3 m in 3500 years. After this date, a sandy channel body, which can be interpreted as related to a Po River palaeochannel, cut the floodplain and the deposition seems to increase. In the following 1500 years, calculated from the dated peat layer and the topographic surface that corresponds to the Late Bronze Age, 4.5 to 7.5 m of sediment were deposited, in the floodplain and in the channel-levee system, respectively. The chronostratigraphy of Cona section shows that in the study area, the highstand phase and the aggradation of the alluvial ridge of Po northern branch starts since about 4000 cal. BP. In Pettorazza, there is evidence of floodplain aggradation during the 2nd millennium B.C. until the 1st B.C.-4th century A.D. In this time interval a first sand body, belong to the Adige River, was deposited and the peat on top of these sands was formed in the 11th-13th century A.D. Thus, the main sand body, which corresponds to the modern alluvial ridge of the Adige River, formed in the Middle Ages. Some authors (Peretto, 1986; Balista, 2004) have suggested the presence of a southern branch of Adige River active during the Roman Age, which produced the alluvial ridge cut, in more recent times, by the Adigetto channel. Stratigraphical results show, in Ramalto, no important sand body in correspondence of the Adigetto alluvial ridge. On the other hand, the “Fasanara” alluvial ridge, about 1 km to the north, corresponds to an important sand body. In the Ramalto cross-section the stratigraphy of a branch of the Po di Adria is also presented.