Shift and adjustment of fluvial distributary systems in the foreland of tectonically-active and cyclically-glaciated mountain belts: insights from the Piave River megafan in the southern Eastern Alps (Italy)

Abstract:

The development of large fluvial distributary systems at the foot of mountain belts depends from the efficiency of sediment production and routing from the uphill catchments. Valley glaciers reaching the foreland during Pleistocene glaciations were very efficient conveyors of sediments to alluvial basins, as demonstrated by megafan progradation in the Last Glacial Maximum (LGM) along the southern margin of the European Alps. In our study, stratigraphy of alluvial deposits, coupled by sand petrography and OSL and radiocarbon dating, allow to reconstruct the early aggradation of the Piave River megafan between 40 and 30 kyr BP, in response to the starting of ice accumulation in the higher Alpine catchments at the end of MIS 3. A lateral shift of 12 km of the megafan apex across the uplifting thrust-ramp anticline of the Montello hill has been framed at around 28 kyr BP. The river switched from a water gap to the neighboring one several times since the Lower Pleistocene, leading to the formation of 7 strath terraces at the western end of the Montello Hill. Our research indicates that the last river diversion took place at the onset of full glacial environmental conditions of the LGM. River shift had basin-scale impact on the overall distribution of both gravels and finer-grained sediments within 60 km from the mountain front. This suggests that abrupt changes of facies distribution and geometry of sedimentary bodies in alluvial basins, at the front of tectonically-active and cyclically-glaciated mountain chains, may primarily reflect climatically–driven fluctuations of sediment delivery from the highland glaciers, rather than variations in tectonic uplift, even if the river cuts through active and seismogenic (as for the Montello thrust) structures.