Giant sand waves in the Hola glacial trench, Vesterålen, north Norway

Recently, a combined sampling and video survey was conducted in the Vesterålen region offshore north Norway as part of the ongoing MAREANO programme (www.mareano.no). MAREANO is a multidisciplinary seabed mapping programme conducting physical, biological and environmental mapping in the Lofoten - Southern Barents Sea area. Data and analysis arising from this project will provide the foundation for an ecosystem based management of Norway’s coastal and offshore regions, and will be made available via the internet (www.mareano.no). The main idea of the sampling and video survey was to map biology and habitats, groundtruth the multibeam backscatter data, and to study other interesting features observed in the bathymetry. Multibeam batymetry acquired from the Hola trench off Vesterålen by the Norwegian Defence Research Institute reveal the morphology of giant sand waves in spectacular detail, and show that much of the area is influenced by strong bottom currents.

The continental shelf off Vesterålen is 30-40 km wide, and the water depth ranges from a few hundred metres on the shelf to more than 1000 metres on the continental slope. The region is divided into shallow banks (50-200 m deep) and deeper trenches (100-400 m deep) formed during the last ice age. Multibeam backscatter, shallow seismic, video analyses and seabed samples show a sedimentation pattern strongly influenced by pre-existing glacial features and sediments, bathymetry and ocean currents. Coarse lag deposits occur both on banks and in glacial trenches, testifying to the influence of strong bottom currents. Deposition of fine-grained sediments occurs in trenches and sheltered depressions.

Sand waves occur in three clearly defined wave fields in the Hola trench. Wave field 1 occurs in the northern part of the trench. Sand waves here exhibit a vortex-like distribution, they are up to 6 m high, 1-2 km long, and typically have wavelengths of 100-200 m (up to 500 m). Wave field 2 comprises around 50 sandwaves of approximately the same size as in wave field 1. Wave field 3 is complex, with different types of sand waves in different areas. Individual waves are up to 5 m high, 3 km long, and wave lengths are 50-150 m. In the west, sand waves are straight although curved over distances of 2-3 km. In the east, lunate and catenary (both in phase and out of phase) sandwaves occur.

Sand waves are interpreted to have formed by reworking of glacial deposits by geostrophic and tidal currents. The formation of sand waves probably started immediately after the area was deglaciated ca. 15 000 14C years BP. Video observations document that sand wave migration is still an active process. The general ocean currents in the Hola trench, interpreted from multibeam backscatter, multibeam bathymetry, sediment distribution, comet marks and sand wave orientation and asymmetry, define an anticlockwise circualation pattern. However, tidal currents probably have a strong influence on the sediment transport.