Holocene stratigraphy of the south Venetian Plain: new insight data and correlation

The late Holocene distal tract of the Adige sedimentary system is bounded to the north by the Brenta River megafan and to the south by the Po sedimentary system (MURST, 1997; FONTANA, et alii, 2008). This latter boundary is not well defined, as branches of the Po River have occasionally intersected the Adige alluvial plain and, vice versa, Adige channels have crossed the Po Plain. The study area is located within the belt where the Adige and Po rivers have interfingered in the past. This alluvial plain is characterized by a complex network of alluvial ridges formed by the aggradation of sandy and silty channel deposits, natural levees and minor proximal crevasse splays (VEGGIANI, 1972; PERETTO, 1986; MARCOLONGO, 1987; CASTIGLIONI, 2001; PIOVAN et alii 2010). The interdistributary depressions are generally silty-clay, but frequently show evidence of accumulation of organic deposits in extensive swamps or are occupied by dense networks of distal crevasse channels and splays. High-resolution sedimentological and geochronological data, here presented, allow the description of major alluvial ridges. As well, the correlation among stratigraphical elements is shown in a number of cross-sections. A stratigraphic marker is provided by a calcic horizon which has been reached at average depth of 7 m in Cona and Conselve cross-sections. It can be correlated with the “caranto” palaeosoil of the Lagoon of Venice (GATTO & PREVIATELLO, 1974; MOZZI et alii 2003), which formed during the sedimentary hiatus and sub-aerial exposure of the alluvial plain between the end of the Last Glacial Maximum and the post-glacial transgression. This buried soil marks the top of the Late Pleistocene sequence in the whole Venetian-Friulian Plain (FONTANA et alii, 2008). Another important element for stratigraphic reconstruction is a 1 m thick peat layer recognized in the whole study area at the average depth of 5 m. Chronostratigraphic evidence indicates that this major organic-rich sedimentary event in the fluvial series may correlate with the marine maximum flooding surface. The radiocarbon dating of the bottom of this peat layer is 4435‒5306 cal BP at Cona and 3557‒3699 cal BP at Conselve, just ca. 1 m above the calcic palaeosoil; this points to condensed alluvial sedimentation during the early Holocene. Other ages from the bottom of this layer are 4570‒5330 cal BP in Saline and 5595‒5754 cal BP in Santa Margherita. The radiocarbon dating at the top of the peat provided the ages of 3455‒4094 cal BP in Cona, 3201‒3354 cal BP in Conselve, 4237‒4979 cal BP in Saline and 3158‒3383 cal BP in Santa Margherita cross sections. Radiocarbon datings evidence that the base of the peat layer is progressively younger in more landward locations. Peat started to form around 5700 cal BP in Santa Margherita, the place nearest to the coastline, at 5300‒4400 cal BP in Saline-Cona and 3600 cal BP in Conselve. After peat deposition, a phase of major aggradation occurred in the area, which resulted in the formation of alluvial ridges. Radiocarbon datings on organic clays in natural levee deposits in Conselve and Santa Margherita cross sections give the ages of 1950‒2119 cal BP at 3.53 m depth and 1995‒2157 cal BP at 2.5 m depth respectively. This evidences continuous aggradation until Roman times, mainly related to the Adige alluvial system as around 3000 BP the Po di Saline-Cona palaeochannel was not anymore active (PIOVAN et alii, 2010).