The Helmet – a tale of a salt pillar | Norges Geologiske Undersøkelse

Lilja Rún Bjarnadóttir

Seksjonsleder

Phone: 73904288

Mail: lilja.bjarnadottir@ngu.no

Henning Jensen

Forsker

Phone: 73904305

Mail: henning.jensen@ngu.no

Published: June 23rd 2015 | Last change: June 23rd 2015

Overview map showing the location of the study area. The black arrow points to The Helmet, and the red polygon indicates MAREANOs 2015 sampling area in the Barents Sea. The peninsula Varangerhalvøya in northern Norway is marked in. TB: Tiddlybanken; TIB:

MAREANOs first video station of 2015 documented a conspicuous seabed structure, about 300 km north-northeast of mainland Norway, known as The Helmet. The circular structure which looks like the top of a helmet sticking out of the seabed is 30 m high and has a diameter of 800 m.

How does a giant bump on the seabed like this form? The explanation lies in the deeper layers of the continental shelf. Long ago, during Carboniferous-Permian times (350-250 mill. Years BP), piles of salt were deposited in a long-since evaporated sea. Over time the salt deposits rose slowly but steadily to the surface, as tenacious, pillar-shaped bodies known as salt diapirs. This upwards movement occured because salt has a lower density than the overlying sedimentary rocks, which makes it float slowly but steadily upwards through denser rocks. As it rose the salt lifted and deformed the rocks and sediment layers in its path, sometimes ripping out blocks from older deep layers and bringing them up to the surface of the seabed. In the surrounding area there are many salt diapirs that come close to the surface but none which actually protrude as clearly as The Helmet. (See also: Salt diapir and gas leakage on Tiddlybanken)

Shaded relief image of the seabed, based on a terrain model with 5 m resolution. The white arrow points to The Helmet, where the water depth is 165-210 metres. The furrows on the seabed are iceberg ploughmarks. The white circles marked a-l are located along the video transect and indicate the locations of pictures a-l below.

Filming the Helmet from east to west
We crossed The Helmet with the video rig, acquiring video data from a 1.4 km stretch of seabed over which large variations in bottom type were observed. At first, on the eastern side of The Helmet, there was not so much life to be seen but as we moved westwards and ascended the eastern slope of The Helmet, both the geology and biology got more exciting. The bottom type which started as mud became increasingly mixed with fine gravel then coarser gravel as we moved upwards, becoming dominated by cobbles and boulders by the time we reached the top of The Helmet. As we descended the western side of The Helmet the bottom type gradually became finer ending in mud again as we reached the flat seabed.