The Pleistocene to recent geological development of the Ormen Lange area

A large volume of geological, geophysical and geotechnical data have been acquired over the last decade in order to perform a safe development of the Ormen Lange gas field, which is located in the scar of the Holocene Storegga Slide, the last in a series of large slides in this region over the last 500 ky. Based on high resolution seismic data and a series of geotechnical borings, a unified Pleistocene stratigraphy has been established for the area. Two main modes of deposition prevailed in the Pleistocene. During periods of peak glaciation, when glaciers extended to the shelf break, basal tills were deposited on the shelf, and glacial debris flows on the continental slope. During the much longer periods of reduced ice cover, including interglacial periods, normal marine and distal glacial marine deposition prevailed, partly developed as contourites on the slope. The resulting two sediment types are unsorted glacial diamictons, and fine grained, partly laminated sediments, respectively. These display distinctly different sedimentological and geotechnical properties, and the failure planes for all the major slides are found in the fine grained marine deposits.