The Storegga slide

Abstract One of the World’s largest known submarine slides is found in the Storegga area off the coast of Mid-Norway. The slide area has been investigated by seismic profiling, seabed sampling and long-range (Gloria) and medium-range side-scan sonars. The 290 km long headwall is located at the shelf edge 100 km off the coast. The slide extends down the continental slope and into the abyssal plain to a distance of more than 800 km. The maximum thickness is 450 m and a total of about 5600 km3 of sediment was involved in the sliding. Three main slide events are distinguished. The First Storegga Slide was the largest (about 3880 km3) and probably was formed 30000—50000 years BP. The two other events seem to have occurred in near succession about 6000—8000 years BP. The Second Slide, which consisted of more consolidated sediments than the First Slide, cut back 6-8 km headwards beyond the First Slide and removed some 450 km2 of the continental shelf edge. It involved large blocks (olistoliths) of sediments that can be recognized in hummocky slide deposits both within the slide scar and on the abyssal plain. Two huge sediment slabs, 150—200 m thick and up to 10 x 30 km wide, were transported about 200 km down an average slope 0.3°. The Third Storegga Slide was limited to the upper part of the Second Slide scar, and probably occurred as a final, somewhat delayed stage of the Second Slide. In the deepest part of the Norway Basin, more than 750 km from the headwall, a thick (more than 6 m) fine-grained turbidite is related to the Second Storegga Slide. Several other turbidites are found in cores from within the slide scar and on the inner part of the abyssal plain. We believe that earthquake loading and decomposition of gas hydrates caused liquefaction of the sediments and thus triggered the slides.