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Keywords:

  • Event bed;
  • fjord;
  • landslide;
  • Norway;
  • quick clay;
  • turbidite

Abstract

Distinct, clay-rich beds are common in fjord-marine deposits in Trondheimsfjorden near the outlet of the Nidelva River. Their characteristic light-grey colour makes the beds easily distinguishable from the surrounding brownish, bioturbated, muddy fjord sediments. The clay-rich beds commonly display a clear stratification in clay, silt and very fine sand. The beds are interpreted as originating primarily from large quick-clay landslides upstream along the Nidelva River. Such events resulted in a sudden increase in the supply of fines to the fjord from disintegrating landslide debris and heavily loaded effluent plumes, possibly favouring hyperpycnal flow. Typical beds can be divided into a clay-rich lower section, reflecting an initial surge with high concentrations of suspended mud, and a sandier upper section reflecting pulses of higher energy. This development can be explained, for example, by a lowering in the supply of mud, an increasing activity of deltaic sediment gravity flows due to a higher availability of sandy sediments in the landslide-affected river, and by flooding and/or breaching of landslide dams. The typical, stratified beds are interpreted as the result of one quick-clay landslide, whereas exceptionally thick, less organized, stratified beds are possibly the result of several large and/or complex landslides. Radiocarbon dating of mollusc shells has helped to establish a chronology for major terrestrial landslides in the area. The frequency of landslides increases towards the end of the Holocene. This is explained by a progressively deeper incision of rivers during glacioisostatic rebound, possibly combined with a change to a wetter climate. The marine core record displays deformation structures and hiati representing submarine mass-wasting events, and supports the evidence that the clay-rich beds are weak layers in the fjord-marine stratigraphy. The inherent weakness of these layers may be explained by their composition, immature texture, loose fabric and contrasting permeabilities in the deposits. Slide-prone layers similar to the clay-rich beds described here may be found in other comparable fjord-marginal settings and are considered to be of importance for geohazard assessments.