Diagnostic characteristics and paleodynamics of supraglacial rock avalanches, Innerdalen, Western Norway

Abstract

Misinterpretation of rock-avalanche deposits in mountain environments is not uncommon because of the complex interactions between rock avalanches and glaciers. This paper for the first time presents a detailed description of suspicious rock-boulder deposits, which are remnants of a rock avalanche onto a Late Pleistocene glacier. The boulder deposits cover around 1.1km2 and are distributed over an area of around 7.5km2 and a variety of landforms: three closely nested distinct terminal moraines (0.45km2) and dislocated terminal moraines (0.34km2) entirely built up of large boulders, lateral moraines that decrease in elevation over a short distance and connect to the terminal moraines, small patches (0.02km2) of boulders that occur in islands on a peat-dominated valley floor, and large patches of boulder fields (0.25km2 in size) hanging 350m above the valley floor. The latter are disconnected from any potential source area. 10Be cosmogenic nuclide ages of the three terminal moraines give an indistinguishable Late Pleistocene age of 13.6±1.4ka, whereas the largest boulder field that is located on the opposite side of the valley from the failed mountain yields a 10Be age of 14.1±0.4ka, which is slightly older but indistinguishable from the moraine deposits within uncertainty margins. All boulder deposits add up to a volume of 31×106m3. The interpretation of a single rock-avalanche source for those landforms is further confirmed by numerical runout modeling using DAN3D. A failure of the entire volume from Skarfjellet Mountain would result in a runout pattern that is identical to that of the distributed boulder fields when considering that the valley was filled with a glacier around 350m thick. The rock avalanche would not reach the position of the terminal moraines; however, a glacial re-advance could have transported the boulders to that position. The Late Pleistocene rock avalanche was followed by a Holocene rock avalanche with a volume of 23×106m3 and a typical lobate deposit damming the Innerdalen Valley and creating a lake. 10Be ages indicate that this event occurred 7.97±0.94ka ago. Structural measurements performed on high-resolution LiDAR scans show a strongly jointed source rock with three joint sets with dip/dip direction 81/034, 86/331, and 77/354 and a foliation with 22/003. The kinematic of the failures can thus be a combination of small-scale wedge failures and planar sliding on shallow dipping foliation. Our detailed description can enable a better identification and interpretation of similar deposits in other mountain areas.