Turbiditic, clay‐rich event beds in fjord‐marine deposits caused by landslides in emerging clay deposits – palaeoenvironmental interpretation and role for submarine mass‐wasting

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 landslideaffected 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 masswasting 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.

Submarine Mass Movements and Their Consequences

“…A prerequisite for the occurrence of the weak event beds, such as those found in this study, is the presence of prehistoric clay-slide activity in the catchment and a low to moderately dipping fjord margin on which the event beds are able to accumulate (Hansen et al 2011). As sensitive, glacio-marine sediments and clay-slides are in Alaska, beds with a similar origin as to those found in this study could play an et al 199 ).…”

Multidisciplinary investigation of a shallow near‐shore landslide, Finneidfjord, Norway

“…Similar beds observed in other fjords around Norway (L’Heureux et al , ; Hansen et al ) and Canada (St‐Onge et al ) were identified as being the result of terrestrial quick‐clay landslides in the catchment of the fjord. Such events can produce a turbitdity current that propagates downstream into the fjord, rapidly draping the fjord bed.…”

“…The morphology and internal architecture of this event bed as imaged in the 3D seismic volume is consistent with this hypothesis. In particular, the manner in which the lower clay‐rich layer demonstrates limited run‐up on the foreshore slope and thickens south‐west, toward the Røssåga River, agrees with the fine‐grained, clay‐rich gravity flow that is expected to form the first deposit in such an event (Hansen et al ). The sandy interval may be associated with failure of the delta slope and/or flooding of the river (i.e., dam breach) following clay slide activity in the uplifted valley (L’Heureux et al ).…”

Comparison of earthquake-triggered turbidites from the Saguenay (Eastern Canada) and Reloncavi (Chilean margin) Fjords: Implications for paleoseismicity and sedimentology

“…Fjords are unique archives of climatic and environmental changes, but also of natural hazards (e.g., [Syvitski et al, 1987] and [Syvitski and Shaw, 1995]; see also Howe et al, 2010 for a recent compilation of papers). They can preserve thin sediment sequences deposited under very high sediment accumulation rates, making them ideally suited to record historical and pre-historical sedimentological events such as major landslides (e.g., [L'Heureux et al, 2009] and [Hansen et al, 2011]), floods (e.g., [Bornhold et al, 1994], [Urgeles et al, 2002] and [Mulder and Chapron, 2010]) or earthquakes (e.g., [Blais-Stevens and Clague, 2001], [St-Onge et al, 2004], [Chapron et al, 2006], [Dallimore et al, 2008] and [Sepulveda et al, 2010]). These types of events may trigger the deposition of rapidly deposited layers (RDL) 1 with specific characteristics and geometries.…”