GNGTS 2017 - 36° Convegno Nazionale

656 GNGTS 2017 S essione 3.2 uniform sedimentation rates, ranging between 0.3 and 0.9 mm/yr. Determining nature and thickness of lake sediments is therefore particularly relevant for an accurate reconstruction of the geological setting, the paleo-climatic record and the eventual colmatation risk. With this aim, given the high costs and technical challenges of direct sediment coring, geophysical surveys can be seen as a valuable tool either to locate significant cores or to extend existing core information. Within the frame of a long-lasting research concerning waterborne geophysical surveys over inland waters, several lakes of glacial origin, located at the foot of Northwestern Italian Alps, have been investigated. Here glacier’s retreat (about 15 kyr BP, according to Vescovi et al. , 2007) left well recognizable frontal and lateral morainic structures, basal erosion surfaces and depressions in the bedrock. The topographic constraints and permeability contrasts generated by both these positive and negative glacial forms promoted localized water accumulation and the consequent origin of several glacial lakes. One of the largest and best known Alpine complex of glacial forms, which is still hosting many glacial lakes, is the Ivrea Morainic Amphitheatre (IMA, Gianotti et al. , 2015). Two of the IMA lakes have been extensively studied: the Candia Lake (45°19’27.90”N, 7°54’40.85”E) and the Sirio Lake (45°29’12.08”N, 7°53’1.56”E). The Candia Lake originated in a proglacial position, located in the IMA frontal sector, at the southwestern internal edge of one of its outer and more extended morainic arcs. It currently lies on a suspended fluvial terrace, approximately 4-8 m above the present Dora Baltea River alluvial plain. The Sirio Lake is located northern the Candia Lake, in a more internal IMA position, and entirely lies on the granulitic rocks of the Ivrea-Verbano zone, which were intensely eroded by the glacier mass. Despite the different origin, the two lakes show some similar characteristics (Tab. 2) and both have no, or very scarce, inflows and outflows. Paleoclimatic studies, involving continuous drilling of the lakebed sediments, have been performed on both lakes. Facchinelli et al. (2005) estimated a mean sedimentation rate of 0.5 mm/yr for the last 2 kyr BP on a 1.7 m long sediment core sampled in the Sirio Lake. According to Lami et al. (2000), the bottom of a 1.1 m long core of the Candia Lake dates back to nearly 2 kyr BP, with a resulting similar sedimentation rate. An extensive waterborne Continuous Vertical Electric Sounding (CVES) investigation, aimed at the reconstruction of the Candia lakebed sediment nature and geometry, found a 10-to-15 m thick pack of sediments showing an average electrical resistivity of 30 Ωm, nearly throughout the whole lake bottom, and thus interpreted as fine-grained saturated lacustrine sediments (Colombero et al. , 2014). In this work our attention is focused on the Sub- bottom and GPR surveys recently acquired on the two lakes. Results of the new surveys arise a Tab. 1 - Sedimentation velocity in some lakes as referred to in literature. Paper Site Elevation Average sed Core Inflow [m a.s.l.] vel [mm/yr] length [m] Simonneau et al. , 2014 Lake Blanc Huez, W French Alps 2550 0.3 3.4 No [Quat. Sc. Rev.] Menounos, 1997 [The Holocene] Sky Pond, Colorado, USA 3000 0.3 3.8 No Lami et al. , 2000 [J. of Limnol.] Candia Lake, NW Italy 226 0.3 – 2 1.1 No Punning et al. , 2003 [J. Paleolimn.] Viitna Linajarv, N Estonia 75 0.9 9.5 No Punning et al. , 2003 [J. Paleolimn.] Viitna Pikkjarv, N Estonia 75 0.35 4 No Facchinelli et al. , 2005 Sirio Lake, NW Italy 266 0.5 (1) 1.7 No [RMZ Mat. and Geoenv.] Finsinger et al. , 2006 Avigliana Lago Piccolo, NW Italy 353 0.5 (2) 14.92 No [Quat. Sc. Rev.] Finsinger et al. , 2014 Avigliana Lago Grande, NW Italy 353 0.6 3 No [Jour. of Limnol.] (1) in the last 2000 years - (2) down to 8.3 m