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 few glaciers remained within the valley. The formers left well-preserved terminal moraines, whose investigation allowed to infer their evolution and chronology. Valley glaciers remnants, on the contrary, are often scantly preserved and changes can be detected only through the correlation with the glaciofluvial deposits in downstream alluvial basins. The Brenta glacial systems dynamics in its terminal tract was inferred through a wide range of sediment analysis techniques on an alluvial stratigraphic record of the Brenta megafan (NE Italy) and the mapping of in-valley glacial/glaciofluvial remnants. Glaciers flowing across narrow gorges turned out to be possibly slowed/blocked by such morphology and, if a lateral valley exists, glacial/sediment fluxes can be diverted. 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 windgap (Canal La Menor valley), joining the Cismon/Piave glaciers near Rocca and ending ~ 2 km downstream. The Cismon River started to flow along its present path just before 27 ka cal BP, while the Piave catchment contributed to the Brenta system at the acme of LGM, from ~ 27 to, at least, ~ 19.5 ka cal BP. Our investigation shows that glacial catchments may significantly vary over time during a single glaciation in rugged Alpine terrains. Sand petrography and chemical/mineralogical composition of sediments are good tracer of such variations, that reflects in the glacial and glaciofluvial systems and can be recognized in the alluvial stratigraphic record far downstream from the glacier front.
Lowlands fluvial sedimentation enlightens glacial dynamics in narrow valleys during the Last Glacial Maximum (Venetian Forealps, Italy)
