Late-Quaternary Alluvial Megafans of Europe

Megafans are huge alluvial landforms with a convex geometry, generated by the activity of an instable fluvial systems radiating from an apex. They have an extent that generally exceed 1000 km2 and are formed by a large and fine-dominated distal portion. According with reference literature, megafans can develop where large active mountain catchments can deliver their sedimentary supply to a wide facing plain. Considering these characteristics and the tectonic setting of continental Europe, megafans could be currently described in some of the alluvial areas existing along the Alps and the Carpathian chain. The adoption of the megafan concept has been rather delayed in Europe because of the strong and traditional use of the term “fan” also for describing the largest divergent alluvial landforms. Even in very recent literature, some huge alluvial systems with an extent of some thousands of km2, are described as “fan”, despite the gravelly portion stops in the first 20 km from the valley mouth. This research was limited to the late-Quaternary megafans that are still recognizable in the topography, while the previous similar landforms that now are no more connected to the present landscape or are part of the stratigraphic record, have not been discussed. The work was carried out starting from the analysis of the specific literature and of the DTMs obtained by SRTM data or, when available, by other more detailed information. The morphometric results were compared with remote-sensed images and with the specific literature describing geomorphological, sedimentological, stratigraphic and geochronological data. Along the southern Alps megafans are present from Milan (central Po Plain) to the whole Venetian-Friulian Plain. The major landforms are the ones formed by Tagliamento, Piave, Brenta, Adige, Oglio, Olona and Adda rivers. All these systems experienced a strong depositional phase in the LGM (29-17.5 ka BP), when the Alpine glaciers stationed at the mouth of their valleys and the rivers were their outwashes. In that period the megafans reached their maximum extent (e.g. Brenta 2600 km2, Adige 1600 km2, Tagliamento 1200 km2), coupled by a vertical aggradation of 15-30 m. During LGM the channels of Alpine rivers where unconfined in the apical portion of their megafans and gravels could be transported only for the first 15-25 km, while the distal sector was mainly formed by silt and clay. On the contrary, sedimentary starvation characterized all the megafans of northern Italy since the glaciers started to withdrawn. This setting led the rivers to down cut in their LGM deposits and to form incised valleys, even in the distal sector. Thus, during Late Glacial and early Holocene, gravels were conveyed along the incised valleys up to the present coast. A different context settled since about 8 ka BP, when the Adriatic Sea arrived at -10 -5 m below the present level and a new depositional phase started. This situation induced the progressive infilling of the incisions in the distal sector, that now are almost completely buried. According with their evolution history, the southern Alpine megafans can be mainly considered as relict products of the last glaciation, mainly controlled by the climate forcings. In the Little Hungarian Plain (LHP), the Danube River formed different generations of Quaternary megafans. The present one has its apex near Bratislava, extends for at least 4000 km2 and mainly consists of sandy and fine deposits. This megafan experienced a major forming phase during LGM and Late Galcial, but the Danube supported some aggradation even in the late Holocene. Up to its entrance in the LHP, the Danube is mainly supplied by Alpine catchments, as the Rába River (Raab in German), that flows from the Austrian Eastern Prealps (Burgenland) and created a megafan of ca. 1600 km2 in the SW portion of the LHP. Rába megafan was mainly generated during the last glaciation but, between 25-15 ka BP, the river abandoned large sectors of the megafan surface. In the Great Hungarian Plain (GHP), many fan-shaped alluvial systems are present, but only few of them can be currently described as megafans (e.g. Maros, Szamos and Timis rivers). The largest megafan in the Carpathian Basin is the one formed by the Maros River (Mures in Romanian), which is fed by the SE Carpathian and by the Apuseni Mountains. This megafan consists of a northern and a southern lobe that cover an area of 5000 and 2000 km2, respectively. Gravels can be found in the shallow subsoil up to 25 km from the apex. In the northern lobe, traces of braided and meandering channel belts have been dated through OSL and testify a continuative activity during Late Glacial and Holocene. The southern lobe was activated since about 2 ka BP. Compared with the southern Alpine ones, the megafans of the Carpathian Basin are fed by larger catchments, but these were not severely glaciated during LGM and sustained also important depositional phases in the Late Glacial and Holocene. In the GHP the planform and the development of the megafans was strongly influenced by differential subsidence.