Contourites and their relevance for mass wasting along the Mid-Norwegian Margin

Between about 12 and 4-3 Ma, slope-parallel bottom currents dominated the Neogene sedimentation on the slope of the Mid-Norwegian margin, and large-scale sediment bodies accumulated to thicknesses up to 1000 m. These bedforms are morphologically classified as sheeted and elongated mounded contourite drifts. An increase in sediment supply from about 4 to 3 Ma, due to onshore uplift and glaciation, resulted in a massive influx of downslope-derived material onto the continental margin. Only on the Vøring Plateau was contourite sedimentation little affected by this change in sedimentary style. During the last 0.5 My contouritic deposits occurred also in the Storegga area as 100-150 m thick infill drifts inside the scars of two palaeo-slides (R and S). Holocene post-slide deposits in the scar of the Storegga Slide (8.2 ka) have the typical morphological features and distribution of contourite drifts with the upslope limit at the lower base of the warm northward flowing North Atlantic Current at 500-700 m depth and the downslope limit at depths of approximately 1200 m. Detailed chronostratigraphy of the post-slide sediments show relatively constant high sedimentation rates in the order of 1-2 m/ky during the last 8000 years. This is equivalent to the estimated rates for the older infill drifts. The infill drifts consist mainly of clays with silty and sandy lamina and represents marine and glaciomarine conditions. The main influence of contourite drifts on slope stability relates to their physical properties forming a more sensitive and brittle deposit than the glacial deposits and their exposure to downslope sedimentation. In addition, the potential for build-up of excess pore pressure due to rapid loading from subsequent glacial depositions, their seabed smoothing effect, and great lateral extent, make the contourite drift deposits particularly important for slope instability in this region.