Alluvial megafans in the foreland of Southern Alps

1. Introduction Along the Southern Alps several large cone-shaped landforms are present. These relate to the alluvial systems fed by the main valleys and presently display an extent of 300-3000 km2, with a length of 30-70 km. We reviewed the available information related to the geomorphological, stratigraphical and geochronological aspects of these alluvial systems with the aim to identify the global forcing factors and the local constraints which influenced their late Quaternary evolution. The study area extends in the Alpine foreland for about 25,000 km2 and corresponds to the northern sector of the Po Plain from Ticino River to Garda Lake (i.e. the Lombard Plain: LP), the Venetian-Friulian Plain (VFP) and part of the Adriatic shelf. The related Alpine rivers drain a total mountain catchment of about 50,000 km2, with maximum elevations ranging between 2800-4810 m. 2. Results The largest cone-shaped depositional systems of the VFP have been described as alluvial megafans for their evident longitudinal differentiation (Fontana, et al., 2008; 2010). This characteristic is displayed also by several systems of the LP, but these were before considered only in their gravelly sector (Guzzetti et al., 1997; Marchetti, 2001). The steep piedmont sector consists of amalgamated gravels, while the distal portion has a gradient <2‰ and is dominated by fine sediments. In all the megafans the major depositional phase occurred between 26-19 ka cal BP, during the LGM marine lowstand, when the Alpine glaciers reached the plain and fed the related glaciofluvial systems. The easternmost megafans partly extended also on the Adriatic shelf whereas, West from Garda Lake, their downstream development was limited by the existence of the Po river plain. The thickness of LGM alluvial sedimentation ranges between 30-15 m and pinches out about 25 km from the present coast. Soon after the ice decay, since 19-17 ka cal BP, the sedimentary delivery from Alpine catchments to the plain dramatically decreased and in the central Alps large intramontane lakes formed, trapping the bedload almost completely. Thus, an erosive phase affected the whole pede-Alpine sector, leading the rivers to entrench for tens of meters in their LGM megafans. The funneling effect created by the incisions allowed the gravels to arrive tens of kilometers downstream than in LGM. In the Venetian-Friulian megafans a single valley formed in the piedmont sector, while 2-5 fluvial incisions developed in the distal sector. But these incised landforms have been almost completely filled by a depositional lobe formed in the last 8 ka, partly triggered by the marine highstand. The Alpine tributaries of Po haven’t yet been affected by sea-level influence and they still flow along a single incision from their megafan apex to the junction with Po. 3. Conclusions Alluvial megafans along the Southern Alps were mainly built by the activity of LGM Alpine glaciers and could be explained as a product of their erosive power in the mountain valleys and their efficiency in evacuate the sediment to the foreland. Since the glacial withdrawal occurred at the end of LGM the Alpine rivers incised in their megafans and were more sensitive to the local characteristics of their catchment. In particular the presence of large intramontane lakes, that trapped most part of the sediment. The main aggrading phase occurred in the megafans during the marine lowstand, while the post-LGM transgression had no important effects until 8 ka cal BP and the marine highstand influenced only the distal sector of the VFP megafans, but not the LP. Thus, in the last glacial cycle the alluvial evolution along Southern Alps was in antiphase in respect to the typical model proposed by sequence stratigraphy. The coupling between glacial aggradation and interglacial incision occurred also during MIS 6 and MIS 5 and it is likely also for the previous glacial cycles. Thus, along the Alps and in other similar temperate regions, the occurrence of megafans in the Quaternary stratigraphy could be a testimony of the colder phases. References Fontana, A., Mozzi, P. and Bondesan, A. (2008) Alluvial megafans in the Venetian–Friulian Plain (north-eastern Italy): Evidence of sedimentary and erosive phases during Late Pleistocene and Holocene. Quaternary International, 189, 71–90. Fontana, A., Mozzi, P. and Bondesan, A. (2010) Late Pleistocene evolution of the Venetian-Friulian Plain. Rendiconti Lincei, 21 (Suppl.1), 181-196. Guzzetti, F., Marchetti, M. and Reichenbach, P. (1997) Large alluvial fans in the north-central Po Plain (Northern Italy). Geomorphology 18, 119-136. Marchetti, M. (2001) Fluvial, fluvioglacial and lacustrine forms and deposits. In: Castiglioni G.B., Pellegrini, G.B. (Eds.), 2001. Illustrative Notes of the Geomorphological Map of the Po Plain. Geografia Fisica Dinamica Quaternaria (Suppl. 7), 73-104.