Alluvial plain formation during the Late Quaternary between the Southern Alpine margin and the lagoon of Venice (Northern Italy)

Abstract

The geomorphology of the central Veneto plain has been analysed using a high-resolution DEM, remote sensing and field survey. The study area consists of three alluvial megafans: the Montebelluna megafan (Piave River, pre-LGM); the Nervesa megafan (Piave River, LGM - Upper Holocene); the Bassano megafan (Brenta River, LGM). The gravelly, cone-shaped, steep (0.8-0.4 %) Montebelluna megafan outcrops just in the piedmont sector, as its distal reaches are buried by the Bassano and Nervesa megafans. These latter extend from the Southern Alps piedmont to the lagoon of Venice. Within a distance of 10-15 km from the Prealpine foothills, their apical parts are cone-shaped and rather steep (0.6-0.1 %), consisting mainly of gravels. Through a gradual transition, the distal parts become markedly less steep (reaching values ≤ 0.1%) and articulated in systems of fluvial ridges and elongated depressions. The ridges are generally ≤ 1 km wide and several km long. They are mainly composed of sandy deposits, while the depressions are silty-clay. The most significant erosive landforms are related to: i) the head trenching of the Bassano megafan, which probably took place at the end of the LGM because of a disequilibrium between solid and liquid discharges of the Brenta river, related to the deglaciation of the mountain catchment; ii) the downcutting and lateral erosion of the eastern lobe of the Montebelluna megafan during the Upper Pleistocene and the Holocene, due to the tectonic uplift of the piedmont sector between the Aviano and Sacile faults. Active tectonics at the buried Southern Alpine thrust front led to the faulting of the apical portions of the Montebelluna, Nervesa and Bassano megafans, with the formation of tectonic scarps. Geomorphological evidence of a south-west tectonic tilting of a relict alluvial surface is also detectable in the distal fringes of the Bassano megafan.