Holocene alluvial fan evolution, Schmidt‐hammer exposure‐age dating and paraglacial debris floods in the SE Jostedalsbreen region, southern Norway

Abstract

The evolution of several sub‐alpine alluvial fans SE of the Jostedalsbreen ice cap was investigated based on their geomorphology and Schmidt‐hammer exposure‐age dating (SHD) applied to 47 boulder deposits on the fan surfaces. A debris‐flood rather than debrisflow or water‐flow origin for the deposits was inferred from their morphology, consisting of low ridges with terminal splays up to 100 m wide without lateral levees. This was supported by fan, catchment, and boulder characteristics. SHD ages ranged from 9480±765 to 1955±810 years. The greatest number of boulder deposits, peak debris‐flood activity and maximum fan aggradation occurred between ̃9.0 and 8.0 ka, following regional deglaciation at ̃9.7 ka. The high debris concentrations necessary for debris floods were attributed to paraglacial processes enhanced by unstable till deposits on steep slopes within the catchments. From ̃8.0 ka, fan aggradation became progressively less as the catchment sediment sources tended towards exhaustion, precipitation decreased during the Holocene Thermal Maximum, and tree cover increased. After ̃4.0 ka, some areas of fan surfaces stabilized, while Late‐Holocene climatic deterioration led to renewed fan aggradation in response to the neoglacial growth of glaciers, culminating in the Little Ice Age. These changes are generalized within a conceptual model of alluvial fan evolution in this recently‐deglaciated mountain region and in glacierized catchments. This study highlights the potential importance of debris floods, of which relatively little is known, especially in the context of alluvial fan evolution.