Development of a Holocene glacier-fed composite alluvial fan based on surface exposure-age dating techniques: The Illåe fan, Jotunheimen, Norway

Holocene development of a composite alpine alluvial fan is reconstructed and dated based on its geomorphology and Schmidt-hammer exposure-age dating (SHD), supplemented by terrestrial cosmogenic-nuclide dating (TCND), lichenometry, radiocarbon dating, soil development, and palaeohydrological analysis. SHD ages of 7080 ± 450 to 8220 ± 440 years and a TCND age of 6075 ± 1220 years from the fan surface indicate that fan aggradation occurred in the early Holocene through episodic debris floods (phase 1) in response to paraglacial processes following deglaciation of the catchment (~9.7 ka). Stabilisation during the Holocene Thermal Maximum (phase 2) was followed by re-activation of the fan, involving entrenchment and terrace formation (phase 3), which occurred in response to neoglaciation after ~5.5 ka and culminated in the Little Ice Age. The main control on fan development was the effect of the changing extent of glaciers in the catchment on sediment supply, initially evinced in high-magnitude debris floods (hyperconcentrated flows) and later in water floods (typical fluvial processes). Aggradation switched to stabilisation as paraglacial activity waned, while later entrenchment was associated with relatively low bedload. Results provide the basis of a conceptual model of alluvial fan evolution for glacierized catchments related to Holocene environmental change