North Sea Fan

The North Sea Fan with an area of approximately 142 000 km2 (King et al., 1996), is one of the largest trough-mouth fans on the Norwegian Margin. The fan is located in front of a large cross-shelf trough, the Norwegian Channel, and is cut by the Storegga Slide in the east and flanked by the Miller Slide in the west. The prograding sediment wedge associated with the fan stretches from 400 m to 2000 m water depth into the Norwegian Basin, deflecting seaward the bathymetric contours by up to 30 km, with a gradient of about 0.5°.

The stratigraphic succession in the fan is characterized by an alternation between 1) intense deposition of glacial debris flow (GDFs), 2) periods when hemipelagic sedimentation prevailed and by 3) disturbed slide deposits (Nygård et al., 2005), with the GDF deposits constituting the majority of the material.

The sediments that build up the North Sea Fan were deposited after the lowermost glacial erosion surface at the base of the Quaternary beds and when the ice margin was located at the mouth of the Norwegian Channel (King et al., 1996), which has been a gateway for repeated fast-flowing ice streams (King et al., 1996). The large input of sediment to the North Sea Fan has subdued the erosional effect on the morphology due to slides in favour of a constructive margin with positive relief (Haflidason et al.2002). Glacigenic debris flows are unique to mid- to high-latitude type of debris flows, with their extremely long run-out distances (over 100 km) on very low slope gradients (<1°) and their roughly uniform thickness along their length (Elverhøi et al., 1997; Taylor et al., 2002) due to the input of clay-rich cohesive sediments (Dowdeswell et al., 1998). These debris flow deposits are assumed to be derived from discontinuous failure of glacier-derived diamictic sediments, brought out to the shelf break in a basal deformation till layer and deposited relatively rapidly on the upper slope (Dowdeswell et al., 1996; Nygård et al., 2002; Fig. 3). The material flowed down-slope from the pull of gravity, and was organized into lensoid debris-flow lobes.

Individual flows, which are acoustically homogeneous, may be separated from each other by low to medium amplitude reflectors. Laberg and Vorren (1995) interpreted the acoustic package units observed on seismic lines from the Bear Island Fan, made up of a group of stacked debris flow deposits and separated by major reflectors similar to the acoustic packages found at North Sea Fan, as comprising debris flow from a single glacial-interglacial cycle. Nygård (2002) has studied the depositional pattern on the North Sea Fan and has used the number of glacigenic debris flows packages as evidence of the presence of an ice-stream in the Norwegian Channel.

High-resolution seismic lines as well as several cores have revealed that the thickness of the post-glacial hemipelagic sequence of sediments overlying the glacial debris-flows on the fan is usually less than 3 metres and that Holocene sediments are commonly less than 30 centimetres thick (King et al., 1996).