Marine mine tailings disposal at Lillebukt, Stjernsundet, North Norway: distribution, sedimentary processes and depositional impacts

Sibelco Nordic's mine on Stjernoya, North Norway, disposes mine tailings into the fjord Stjernsundet. The tailings, discharged at the shoreline in the bay of Lillebukt, comprise c. 85% fine silt to medium-grained sand (0.01-0.5 mm). Upon discharge of the mine tailings into the fjord, they are redistributed by slides and density currents along major channels with pronounced levees. Multibeam echosounder data show sand waves in the channels, while seabed samples and cores document sand ripples and layers of mud between the sand layers. Mud accumulates outside of the channels on the seabed in Lillebukt and as a thin veneer along the shores to the east and west. The bathymetry data show partly buried slide escarpments and slide deposits, while smaller slide scars are evident in the levees. Three slide events in the tailings are documented of which the most recent occurred 9th October 2017. Comparison of bathymetry data collected in 2016 and 2018 show changes in bathymetry in the channels of +/- 3 m. Slides are partly initiated along fine-grained layers and caused by hypersedimentation. From 60 m depth, one single channel continues down to 100 m, where the sediment transport is along a gully in the steep (45 degrees) bedrock slope down to c. 400 m depth. A sand-dominated submarine fan has accumulated at the foot of the slope, extending to c. 463 m depth. Bathymetric data, seismic data and core analysis show that the fan has a radius of up to 1500 m and covers an area of c. 1.5 km(2). Comparison of bathymetry data collected in 1998 and 2016 shows that a large part of the tailings disposed of in that 18-year period (4 million tons) have accumulated on the submarine fan.