Late Holocene development of a Norwegian alpine alluvial fan affected by proximal glacier variations, episodic distal undercutting, and colluvial activity

Abstract

The late-Holocene, especially post-Little Ice Age, evolution of Nystølen alluvial fan, Langedalen, Jostedalsbreen region, southern Norway, is investigated using a combination of evidence from surface morphology, recent aerial photography, lichenometric dating, hydrological reconstruction, stratigraphy, and sedimentology. The fan is of predominantly fluvial origin, but it was effectively influenced by variations in the extent of the Nystølsbreen glacier, which advanced onto the fan apex at its Little Ice Age maximum about A.D. 1750, by colluvial activity (primarily snow avalanche) and by the Langedøla River undercutting the fan toe. The fluvial signature is evident in the pattern of distributary channels, lobate boulder berms on the proximal fan surface, three characteristic sedimentary facies associations, and a general down-fan fining identified within stratigraphical sections. The glacier influenced fan development through inputs of sediment (till and glaciofluvial material) and by modification of the flood regime. Snow avalanches added volumetrically small quantities of characteristically coarse and angular colluvial sedimentary material, but debris flows were unimportant. Episodic undercutting of the fan toe initiated a sequence of events consequent on the fall in the local base level, which rendered the distal fan susceptible to headward erosion of box-sided gullies during flood events and their subsequent infilling by the coarse-grained sediment. The evidence is used to develop a new dynamic evolutionary model that reflects these spatial and temporal complexities of glacier-fed, alpine alluvial fan development and incorporates submodels of process interaction, sedimentary architecture, and gully erosion.