Iceberg deformation and erosion in soft sediments, southeast Norway

Abstract

Trenches in soft glaciomarine sediments have been excavated through two relict iceberg scours — one upslope, and the other horizontal — and through one iceberg gravity crater. The scours are 70 and 30 m wide respectively, and are ∼1 m deep. The grounding features were formed by current-driven icebergs during flooding of previously emergent sea bottom ∼9200 yrs B.P. The clay beneath the scours and craters is deformed, and small and larger scale folding, small faults and thrusts, joints, fissures and collapsed lamination are found. Different scouring mechanisms apply for initial phases of upslope scouring and horizontal scouring. At the start of upslope scouring the sediment mass is removed by thrusting, and the depth of deformation within the subsoil is shallow. In horizontal scouring, two-thirds of the scour depression is due to erosion and one-third to deformation of the subsoil. The deformation can, in this case, be detected down to four times the scour depth. The current velocities are critical for survival of the plough ridges. Along the horizontal scour, the plough ridges are missing because of current erosion during and after scouring.