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

• First description of deposits by rock avalanche onto a glacier not present any more
• 10Be dating and dynamic runout modeling (DAN3D) confirm the landform interpretation.
• Two large rock avalanches occurred from the same rock slope since Late Pleistocene.
• One of 31 × 106 m3 occurred ~ 14.1 ka ago onto a glacial ice body not present any more.
• One of 23 × 106 m3 occurred ~ 7.97 ka ago into a narrow ice-free valley.

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 1.1 km2 and are distributed over an area of around 7.5 km2 and a variety of landforms: three closely nested distinct terminal moraines (0.45 km2) and dislocated terminal moraines (0.34 km2) entirely built up of large boulders, lateral moraines that decrease in elevation over a short distance and connect to the terminal moraines, small patches (0.02 km2) of boulders that occur in islands on a peat-dominated valley floor, and large patches of boulder fields (0.25 km2 in size) hanging 350 m 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 ± 1.4 ka, 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 ± 0.4 ka, which is slightly older but indistinguishable from the moraine deposits within uncertainty margins. All boulder deposits add up to a volume of 31 × 106 m3. 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 350 m 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 × 106 m3 and a typical lobate deposit damming the Innerdalen Valley and creating a lake. 10Be ages indicate that this event occurred 7.97 ± 0.94 ka 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.