High-resolution seismic stratigraphy, sedimentary processes and the origin of seabed cracks and pockmarks at Nyegga, mid-Norwegian margin

Abstract

Densely spaced high-resolution TOPAS seismic profiles and EM1002 bathymetric data reveal the presence of numerous pockmarks, mound-like structures and elongated seabed cracks at Nyegga, offshore mid-Norway. The seabed cracks are located adjacent to the northern escarpment of the Storegga Slide, appearing as graben-like structures in the TOPAS data. Unlike the cracks, pockmarks and mound-structures are largely associated with vertical zones of acoustic blanking at depth, interpreted as pathways for vertically migrating gaseous fluids. Based on the TOPAS data, a new seismostratigraphic framework has been established and correlated to previously published age models of IMAGES cores MD99-2291 and MD99-2289. Seismic facies interpretation suggests repeated and rapid deposition of up to 40 m thick glacigenic wedges in the eastern part of the study area around 18.2 14C ka BP (21.8 cal. ka), 17.5 14C ka BP (20.8 cal. ka) and 16.9 14C ka BP (20 cal. ka). Towards the west, glacimarine deposition has prevailed, characterized by progressively increasing sedimentation rates with peak values of 30 m/ka during the period from 15.0 14C ka BP (18.2 cal. ka) to 15.8 14C ka BP (19 cal. ka). As the distribution of the Nyegga pockmarks closely coincides with the main Late Weichselian sediment depocenters, we suggest a relation between rapid and repeated sedimentation and periodic overpressure generation at depth, ultimately leading to fluid expulsion at the seabed and the formation of the Nyegga pockmark field. In contrast, seabed cracks at Nyegga appear to have formed due to local extension which we relate to horizontal stress reduction as a consequence of the Storegga Slide event. Potentially, this event has been accompanied by renewed vertical fluid migration and the most recent stage of pockmark development.