Mineralogical and microstructural development of the sediments on the Mid-Norwegian margin

As a contribution to the understanding of the mechanisms and causes for Storegga Slide, the mineralogy and microstructure of samples from a series of boreholes from the slide neighbourhood have been analysed. The results show a change from kaolinite-rich oozes through smectite-rich fossilferous clays/clayey oozes to illite dominated hemipelagic and glacial sediments from the Brygge (Eocene to earliest Miocene) to Kai (earliest Miocene to late Pliocene) to Naust Formations (late Pliocene to present), respectively. The deposits are interpreted as the effect of denudation of weathered regolith and an increase in physical erosion in conjunction with the deterioration of the climate and the initiation of glacial cycles during the late Cenozoic. During the Quaternary, climate has controlled the mixing of current transported, fine grained sediments that are probably derived from the region around the Faeroe Islands and coarser grained glacial sediments containing a broad spectrum of minerals eroded from the North Sea and Scandinavian mainland. Since currents have been most important during warm periods, sediments from these periods are finer grained and contain more smectite than the glacial sediments. However, it is concluded that the mineralogical differences between finer grained sediments from warm periods and the coarser grained glacial sediments are not great enough to explain the ubiquitous utilisation of glacial marine/hemipelagic sediments as slip planes for the slides in the area, but that this is probably a consequence of the differences in the grain size distribution of the sediments. Voids that may be casts left behind by gas hydrates have been observed. (c) 2004 Elsevier Ltd. All rights reserved.