Trough mouth fans as high-resolution source-to-sink archives

Trough mouth fans comprise the largest sediment deposits along glaciated margins, and record Pleistocene climate changes on a multi-decadal time scale. Sedimentation related to climate warming in polar regions and new challenges associated with the energy transition highlight the urge for better knowledge of these depocenters. Here, we present sedimentation models for the two largest of these depocenters – the Bear Island Fan on the western Barents Sea margin and the North Sea Fan on the northern North Sea margin – which are analogues for large glacial fans along the Antarctic and Greenland margins. We use extensive high-quality 3D reflection seismic cubes (37,200 km²) as well as conventional 2D reflection seismic lines, and combine these datasets with lithological and geophysical borehole logs.

The stratigraphy of trough mouth fans is dominated by contourites, glacigenic debris flows, meltwater turbidites, and megaslides, which together result in a thickness exceeding 2 km. Neogene to early Quaternary-age contourites are characterized by continuous and high-amplitude reflections in the seismic data. The contourites of the late Quaternary, in contrast, have a more transparent seismic facies, and onlap the escarpments shaped by the megaslides. The lithology of the contourites varies from fine clays to coarse sands. Meltwater turbidites are identified as high-amplitude reflections characterized by 4-100 m deep channels and sourced from multiple regions along the paleo-shelf break. The well-connected turbidite channels are 90-2100 m wide, and can be traced for distances of >100 km. These channels are both deeper and wider in the North Sea Fan compared to the Bear Island Fan. The lithology of these deposits has yet to be cored. Glacigenic debris flows are transparent packages of sediments, with a lens-shaped expression in the seismic profiles and lobe-shaped geomorphology in planar view. The grain size of glacigenic debris flows is typically more mud-dominated than for contourites, but glacigenic debris flows do include sandy beds at selected intervals. Glacigenic debris flows are more dominant in the high-latitude Bear Island Fan compared to the mid-latitude North Sea Fan. Megaslides consist of high-amplitude, deformed sediment that is constrained by steep headwalls and sidewalls. The megaslides within these two trough mouth fans have mainly occurred since the Late Pleistocene and fail along contouritic basal layers.

The contourites were fed by fluvial systems on the East Shetland Platform and the Norwegian mainland, and are, in turn, often trapped by megaslide escarpments or deeper structural elements. The rapid glacial deposition of debris flows and turbidites delivered large sediment volumes to gently-dipping slopes, which then failed as megaslides. Trough mouth fans are excellent depocenters to study the interaction between along-slope and down-slope processes in high-resolution, both in space and time. We suggest that contourites are most active during interglacial periods, while turbidites and debris flows are more common during glacial periods. The timing of the megaslides, however, still includes large uncertainties. Meltwater contribution seems to be a more dominant factor for sedimentary processes in mid-latitude glacial fans than in their high-latitude counterparts.