The Sandy Hills Of Belvedere – San Marco of Aquileia: The Largest System of LGM Continental Dunes In Northern Italy

Continental aeolian dunes are a major feature in many alluvial plains of Central and Western Europe, whereas they are completely unknown in the alluvial environments of northern Italy. Thanks to the analysis of satellite images, aerial photos, Lidar-derived DEMs, Ground Probing Radar (GPR) profiles and field survey we identified a large system of dunes formed in the LGM in NE Italy. The study area is located in the distal portion of the Friulian Plain, between the city of Aquileia and the Grado Lagoon, where a series of small elongated sandy reliefs is present and locally called “dunes of Belvedere – San Marco”. The zone is part of the distal sector of the megafan of Isonzo River and is surrounded by lagoon areas reclaimed since the 19^th century. This peculiar geographical position led many scholars to consider them as Holocene coastal dunes.

The investigated landforms cover an area of about 25 km² and they are mainly elongated in the ENE-WSW direction. The dunes have been largely quarried along history, but the highest crests reach up to 10 m above sea level (asl), in contrast with the surrounding reclaimed coastal plain (-1 m asl). The deposits consist of sands ranging from fine to medium-coarse. Near the dune’s ridges, concretions of cemented sand are abundant and largely exposed by ploughing practices. In outcrops and GPR radargrams the cross sections document an internal structure made of 20-30° inclined foresets, a few centimeters thick. The investigated dunes lay over the LGM alluvial plain forming the distal sector of Isonzo River and their base is radiocarbon dated to about 21 ka cal BP. Considering that vegetation should be scarce during the formation of the dunes, it is likely that they were built between the end of the LGM and the first stadial phases of Late Glacial.

The new data allow to interpret the Belvedere – San Marco reliefs as a system of parabolic dunes consisting of parallel and linear ridges, merging on the lee side in frontal lobes. They currently represent the largest and most complex continental aeolian dunes system in Italian Peninsula. Moreover, the orientation of the dunes is concordant with that of Bora, a katabatic wind which blows in Northern Adriatic from ENE and even today can reach peak velocity of 40 m/s. Similar wind, supported by the North European and Alpine ice caps could blow stronger and more frequently during the onset of deglaciation, when sandy sediment was largely available, supplied by braided channels of Isonzo River.

This discovery underlines the importance of wind-driven processes in the evolution of alluvial plain in northern Italy and suggests the presence of other continental dune systems, also on the seafloor of the Northern Adriatic, which was part of the continental plain until Late Glacial and was then submerged by sea-level rise.