Abstract
Abstract. During the Last Glacial Maximum (LGM), most of the major glaciated basins of the European Southern Alps had piedmont lobes with large outwash plains; only a few glaciers remained within the valley. Piedmont glaciers have left well-preserved terminal moraines, which allow for investigations to be carried out and inferences to be made regarding their evolution and chronology. Valley glaciers' remnants, on the contrary, are often scantly preserved, and changes can only be detected through correlations with glaciofluvial deposits in downstream alluvial basins. The Brenta glacial system's dynamics in the glacier's terminal tract have been inferred through a wide range of sediment analysis techniques on an alluvial stratigraphic record of the Brenta megafan (northeast Italy), and via the mapping of in-valley glacial/glaciofluvial remnants. Glaciers flowing across narrow gorges could possibly be slowed/blocked by such morphology, and glacial/sediment fluxes may then be diverted to lateral valleys. Moreover, narrow valleys may induce glaciers to bulge and form icefalls at their front, preventing the formation of terminal moraines. The Brenta Glacier was probably slowed/blocked by the narrow Valsugana Gorge downstream of Primolano and was effectively diverted eastwards across a wind gap (Canal La Menor Valley), joining the Cismon/Piave glaciers near Rocca and ending ∼2 km downstream. The Cismon and Piave catchments started to contribute to the Brenta system just after 27 ka cal BP until at least ∼19.5 ka cal BP. After the glaciers collapsed, the Piave River once again flowed into its main valley, whilst the Cismon continued to merge with the Brenta. This investigation shows that glacial catchments may vary significantly over time during a single glaciation in rugged Alpine terrains. Sand petrography and the chemical/mineralogical composition of sediments are powerful proxies for tracing such variations, as they propagate through the glacial and glaciofluvial systems and can be recognized in the alluvial stratigraphic record far downstream from the glacier front.
Highlights
Mountain glaciers are complex systems, and the evolution of these systems affects both their mountain basins and the alluvial plains that receive the glaciofluvial water and sediment flux (e.g. Russell et al, 2006)
In this paper we investigate the interaction between the Last Glacial Maximum (LGM) glaciers in the middle Brenta Valley and the related glaciofluvial system in the piedmont plain (Fig. 1)
Data gathered in the mountainous areas of this study and in the piedmont megafan are discussed here in order to reconstruct the evolution of the LGM Brenta Glacier and the tributary glacial systems
Summary
Mountain glaciers are complex systems, and the evolution of these systems affects both their mountain basins and the alluvial plains that receive the glaciofluvial water and sediment flux (e.g. Russell et al, 2006). Few minor valley glaciers are currently present in the highest areas of the European Alps (Evans, 2006), whereas during Pleistocene glaciations large ice streams flowed along most of the Alpine valleys, leading to deep landscape modifications Most of the major glaciated basins had piedmont lobes with large outwash plains, where stratigraphic reconstruction and available chronology has allowed researchers to infer the evolution of the glacial system (Monegato et al, 2007; Preusser et al, 2011; Ravazzi et al, 2012; Fontana et al, 2014). S. Rossato et al.: Glacial dynamics in pre-Alpine narrow valleys vial deposition in the piedmont area (van Husen and Reitner, 2011; Rossato et al, 2013)
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