CONTRIBUTION OF SEISMIC PROFILES, HISTORICAL MAPS, AND DIGITAL ELEVATION MODEL TO DEFINE BURIED GEOMORPHOLOGICAL FEATURES IN THE VENICE LAGOON SUBSOIL (ITALY)

Abstract

Recently, the integrated analysis of Very High Resolution Seismic (VHRS) profiles, satellite images, aerial photographs, maps, and topographic/bathymetric data has given an important contribution to the identification of buried geomorphological features in the Venice lagoon subsoil down to about 30 m b.s.l.. Investigations allow to attribute these features to the Late Pleistocene and the Holocene and to point out their relation with the evolution of the lagoon basin. Results of this study are also assuming great importance in relation to coastal environmental problems. Relict sandy geomorphological features, characterized by high permeability, act as preferred pathways for groundwater flow and solute transport, enhancing saltwater intrusion in the watershed (Carbognin & Tosi, 2003; Carbognin et al., 2005; Pousa et al., 2007). Furthermore, salinization process can also trigger land subsidence induced by clayey particles rearrangement (Meade, 1964). In addition, the different kinds of deposits that characterize geomorphological features are responsible for a differential lowering of the territory (Teatini et al., 2005). Aerial photograph and satellite image interpretations, analysis of historical and recent maps, field surveys, and topographic/bathymetric investigations were first used to identify the main buried and surface geomorphological features. Afterwards, an important contribution to the present study was given by a single channel VHR seismic system, optimized for surveys in shallow water less than 1 m depth (Brancolini et al., 2006; Brancolini et al., 2007). Seismic profiles were calibrated and validated using geological information obtained from existing cores. The detailed reconstruction of the seismic-morpho-stratigraphic units present in the subsoil of the Venice Lagoon is still in progress. It is obtained integrating results of the investigations previously described with sedimentological, stratigraphic, geotechnical, mineralogical, textural, and paleoenvironmental data, and 14C dating (Serandrei Barbero et al., 2006; Tosi et al., 2007a; Tosi et al., 2007b). The combined interpretation of results obtained from remote sensing investigations, topographic/bathymetric measurements, VHRS surveys, and analysis of multidisciplinary geological data allowed the discovery and characterization of buried paleoriver beds, ancient tidal channels, and paleobeach ridges and pointed out the relation among geomorphological features occurring in the lagoon basin and in the watershed. In fact, most of the features recognized in the mainland, which apparently come to an end in correspondence to the lagoon margin, continue into the lagoon basin, where their identification is made difficult by the presence of water and by depositional/erosive processes active in this kind of environment. Data show that relict geomorphological features composed of high permeability deposits provide the hydraulic connection between freshwater aquifers and the sea. In particular, results of the present study point out that well developed paleoriver systems, intersecting the southern lagoon margin and the nearby coastline and characterized by permeable sediments, represent preferential way of communication among waters having different salinity. By contrast thick silty-clayey layers preclude the salty pollution in the aquifers from the lagoon and the sea. As pointed out close to the lagoon margin (Rizzetto et al., 2003), the different kinds of deposits, related to the presence of distinct geomorphological features, contribute to the differential lowering of the lagoon basin (Teatini et al., 2005). In particular, organic soils correspond to highly sinking areas, whereas sandy-silty sediments, which constitute fluvial and beach ridges, are more stable. Future investigations have to be addressed to the quantitative geomorphological analysis aimed to know the past hydrologic conditions of the drainage systems, and to analyze the formative processes that control the morphological setting and evolution of lowland fluvial river and tidal creek systems.