3D seismic data indicate potential glide planes for submarine sliding : the mid-Norwegian margin Nyegga area.

Abstract

The Nyegga study area of the thesis is located at the north-eastern escarpment of the Holocene giant Storegga Slide. It lies on the southern part of the Vøring plateau on the mid-Norwegian continental margin at approximately 700-1000m water depth. The area has been known to be prone for submarine sliding and is therefore an excellent location for studying morphological features to infer slide mechanisms and development. A major effect of submarine slides is seen on the seafloor of the Nyegga area, as the Storegga Slide sidewall marks a clear transition from undisturbed marine sediments to the disrupted, chaotic morphology in the slide scar. Two other slides termed the T and U slides have been discovered in the study area. All three slides lie within the Plio-Pleistocene Naust Formation; both the T and U slides are likely of middle Pleistocene age, at 200 and 400 Ka respectively. The three slides have been investigated using 3D seismic data provided by Statoil (ST0408 cube). It allowed mapping the top and bottoming horizons of the slides and utilizing a volumetric approach for visualization and interpretation of the sliding processes and directions. The identified kinematic indicators, which include slide blocks, ridges and head-/sidewalls, suggest a similar north-south direction of material transport for all three slides. For the Storegga Slide it refers to the post-sliding after the major east-west directed slide event at 8180 cal years BP. 27 depression, fault-like features interpreted to be crown cracks distributed amongst the slides were identified and their extension quantified. The presence of crown cracks and other morphological features suggest retrogressive slide developments though alternatives are possible. Failure within the study area was initiated because of a retrogression of slide material that occurred down-slope where excess pore pressure may have decreased along layers that provided zones of weakness. A number of glide planes have been observed for each slide, all of them occurring in marine deposits representing zones of weakness. Today’s areas of spreading and crown cracks at Nyegga may be a geohazard and a risk for nearby pipelines and installations on the seabed.