The first submarine sackungen: a new paper

A new paper published Geo-Marine Letters (Conway and Barrie 2018) describes the first known submarine sackungen. The Colorado Geological Survey has a nice page about the sackungen phenomenon, which includes this definition:

A sackung structure can be a trench (small-scale graben) or an upslope-facing scarp. They are found most commonly in Alpine glaciated regions near the crest of a range. Their origin is interpreted to be a result of post-glacial, gravitational spreading of the ridge crest and over-steepened ridge flanks. Plural of this German word is sackungen.

And they have this example on their website:-

The Crested Butte Sackungen in Colorado via Colorado Geological Survey

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These features are formed by slow, very deep-seated creep in a mountain flank. They are common across high mountain areas.  However, submarine sackungen had not been observed to date, although technically there is no reason why they should not exist.  The paper by Conway and Barrie (2018) describes deformation in the wall of central Douglas Channel in British Columbia.  Two large, creeping blocks are evident, as the image below (from the Researchgate version of the article) shows:

The submarine sackungen feature observed in the walls of Douglas Channel in British Columbia. Image from Conway and Barrie (2018) via Researchgate

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These are landslides on a giant scale – the blocks have moved over 350 m in each case and the blocks are 62 million and 70 million cubic metres in volume respectively.  Because this is an area that was heavily glaciated, the movements must have occurred since retreat of the ice.  The authors suggest that movement probably happened after retreat of the glaciers – i.e. between 15,800 and 13,400 calendar years ago – when the slopes would have been debuttressed. Evidence from the younger sediments that drape the submarine environment suggests that they are not currently moving in any substantial manner, although smaller-scale slope failures may still be happening.

It is not every day that a new type of landslide feature is identified.  The discovery of these submarine sackungen feels like an important advance in our understanding of slope processes.

Reference

Conway, K.W. and Barrie, J. 2018. Large bedrock slope failures in a British Columbia, Canada fjord: first documented submarine sackungen. Geo-Marine Letters. https://doi.org/10.1007/s00367-018-0533-y