GeologyRocks

About the author

Joana Gafeira
University of Edinburgh & BGS

Introduction

Submarine mass movement processes, driven by gravitational forces, represent an important mechanism whereby vast amounts of sediment are rapidly transported downslope and redistributed into deep-water from an originally shallow-water setting, and their deposits are widely recognised in continental margins. Although there is a tendency to consider these processes independently, there is an increasing awareness that in many cases these processes need to be integrated to produce a model of margin development. The mass movement processes display large temporal and spatial variations and have different importance, but their interrelationship needs to be understood and background sedimentation processes must be considered. Increasing knowledge of mass movement processes on the continental slope also has interest for natural hazard assessment.

The scale and diversity of mass movement processes that shaped the glaciated NW European margin, combined with the extensive geophysical and geological datasets from the area, many acquired by the exploration industry, makes it an excellent area to study the interrelationship of mass movements. This margin has been affected by recurrent climate-related episodes of growth and retreat of ice sheet, which covered entirely Scandinavia and part of the UK during the Quaternary (Sejrup et al., 2000).In each glacial episode, ice advanced onto the continental shelf and on some occasions reached the shelf break delivering directly sediments to the upper continental slope. Although today there are no areas where glacier ice is grounded at the shelf edge, the evidence of the past ice sheet activity is still visible in the seabed morphology, both on the shelf and the slope, as the topography has remained nearly unchanged since the last ice sheet retreat approximately 15 ka BP (Dahlgren et al., 2002). The glaciated European margin has also experienced widespread submarine landslides in the past 1 Ma, and is the location of one of the world largest landslides, the Storegga Slide - with a total of to 3200 km³ of sediments displaced (Haflidason, Haflidi et al., 2004)

The Glaciated European margin is characterized by trough-mouth fans (TMFs) and large-scale submarine landslides.The TMFs are composed largely of stacked glacigenic debris flow deposits that were laid down in front of cross-shelf troughs cut by former fast flowing ice streams. There have been several studies looking at the morphology and sedimentary architecture of this glacier-influenced continental margin at a regional scale (Dowdeswell et al., 1998).However, there is still a need to understand better the link, revealed by King et al. (1996) between large-scale mass transport and glacially delivered sediments.

Submarine mass movement

The biggest landslides occur on the seafloor however, until recently it was thought that these types of events were rare and limited to areas of high slope, deltas or areas of high seismic activity. The increasing knowledge of seafloor topography correlated with exponential use of acoustic remote systems of observation (e.g. multibeam bathymetry, sidescan sonar, seismic profiles), resulting not just from the technical advances but also to the growing development of offshore activities, have uncovered that landslides have been far more frequent than previously believed. This represents a substantial risk to the coastline, as the collapse of large amounts of sediment could induce the formation of tsunamic waves with catastrophic consequences, especially to offshore structures such as pipelines, as the cable breaks on the Grand Banks slide are an example Piper et al., 1988).

Slope instability can occur in many environments: fjords, active river deltas, submarine canyons, oceanic volcanic islands or ridges and open continental margin slopes (Lee et al., 1991), and result from an increase in the driving stresses, a decrease in strength, or a combination of the two. In a situation of slope instability, mass movement can be induced by several trigger mechanisms, including:

• earthquakes (e.g. Grand Banks Slide, Piper et al., 1988);
• sedimentary accumulation (e.g. Mississippi Fan, Coleman et al., 1993),
• oversteepening (e.g. Canary Islands, Gee et al., 2001);
• loading by human activity (e.g. Nice, Assier-Rzadkieaicz et al., 2000);
• diapirism (e.g. Gulf of Mexico, Prior & Hooper, 1999);
• storms (e. g. Mississippi Hurricane Camille, Bea et al., 1983);
• sea-level changes (e.g. Madeira Abyssal Plain, Weaver & Kuijpers, 1983)

Regardless of the mass transport process acting, all continental margins shaped by these processes make slope instability an important factor in continental margin evolution. However it can be consider that there is a mixture of "local" and "universal" mass movement processes, since glacigenic debris flows and volcanic island collapse only occur in specific environments, whereas large-scale sediment slab failure is a universal processes.

The classification of submarine mass movements has been the object of continuing debate, in part due to the fact that during and following slope failure, sediments can be transported by a broad variety of processes, from rigid block motion to turbulent flow generation.

Storegga Slide

The Holocene Storegga Slide, adjacent to the North Sea Fan, is the most studied of all the Norwegian slides and the subject of ongoing studies (Bugge et al., 1988; Evans et al., 1996; Bryn, P. et al., 2003; Haflidason et al. 2004; Solheim et al., 2005). This was a huge slope failure of Pleistocene sediment, which took place about 8200 years ago (Bryn et al. 2005). Haflidason et al. (2004) estimated that the minimum volume of sediments displaced was 2400 km3 and the maximum was 3200 km3.

The Storegga Slide depression separates the North Sea Fan in the south from the Vøring Plateau to the north. The erosive scar measures some 290 kilometres north to south across the headwall, commonly with 30-50 m height but that can reach 120 m along limited parts of the northern escarpment (Haflidason et al.,2004), and extends at least 225 kilometres north-west from the headwall into the ocean basin. However, the development of the depression and the detailed architecture of the immediately surrounding are results from a large number of palaeoslides that have occurred intermittently since perhaps late Pliocene times (Evans et al., 2005; Nygård et al., 2005). Due to extensive and repeated sediment removal, the volumes of the older slides are difficult to establish, but it seems likely that some were at least as large as the Holocene event (Evans et al., 2005).

Numerous regional models relating to trigger mechanisms and scenarios have been proposed for the Holocene Storegga Slide, many of which would also be applicable to older slides in the area. While some authors associate the failure of the Storegga Slide with excess pore pressures caused by gas-hydrate dissociation due to sea-level/water-temperature change (e.g. Mienert et al., 2005), other authors consider that the Storegga Slide may have been triggered by offshore earthquakes (e.g. Atakan & Ojeda, 2005).

Although the mechanism that initiate the Storegga Slide is still not well understood, the first event of the Holocene Storegga Slide, according to Bryn et al.(2002) was followed by successively smaller slides. In some areas sediments have moved as flows, whereas in others they remain as more-or-less coherent blocks. While the slides are generally in tectonic terms, extensional features, numerous zones of compression can be seen where individual flows and slide lobes terminate.

Studies of the sub-surface reveal a large number of palaeoslides with movements that have occurred on a number of different failure surfaces. The present-day absence of non-slide material within the Storegga area indicates that a large-scale present day regional failure in this area is highly unlikely.

Glaciated European margin

Ice sheets have advanced and retreated across the glaciated margins in a series of climate-related cycles. Research focus on regional morphology and sedimentary architecture of continental slopes (e.g. Laberg & Vorren, 1995; e.g. Dowdeswell et al., 1996) has shown that during glacial maxima, the ice sheets expanded across continental shelves to the shelf edge. However, the delivery of ice and sediments to the continental shelf edge is heterogeneous; a pattern of flow-partition into low and fast-flowing ice (ice streams) having been described (Sejrup et al., 2000). Ice streams are responsible for discharging the majority of the ice and sediment within ice sheets. Thousands of cubic kilometres of sediment were transported to the Norwegian shelf edge (e.g. Hjelstuen et al., 2004b) during the mid and late Pleistocene by ice streams. Ice streams also play a critical role in driving abrupt changes in high-latitude climate and oceanography.

This high sediment flux supplied enough material for the development and the build-up of the large glacier-fed fan systems on the continental slope of offshore of these fast-flowing ice streams. These fans have been termed Trough-Mouth Fans (TMFs) by Vorren and Laberg (1997), as they are located in front of depressions on the shelf, and were the main depocentres of glacigenic sediments on the Norwegian Margin during the Quaternary (King et al., 1996; Dowdeswell et al., 1998; Taylor et al., 2002). It has been estimated that 15-20% of the Late Weichselian sediment input to the deep-basins was by the North Sea Fan and the Bear Island Fan, contributing approximately 1000 km3 each (Taylor et al., 2002). However, the flux of sediment delivered to the deep-basins through the fan is only high during glacial maxima when ice sheets are located at or near the shelf break.

Last Glacial Maximum (28-22 ka) extent of the Fennoscandian, UK, and western Barents ice sheets, with distribution of glacial fed fans on the margins and possible location of former ice streams indicated.

The topography/morphology of the glaciated margins is only slightly modified during the deglaciation of the shelf. However, even during the interglacials the glaciated margins retain a glacimarine overprint since ice sheets terminating in marine waters still deliver icebergs, meltwater and debris. Apart from a few slides, the sea-floor remained largely unchanged during the Holocene, only modified by current activity and deposition of a very thin drape of sediments (<10 centimetres on average), iceberg reworking of the shallow areas, contourite drifts, and the formation of pockmarks and other gas-escape structures.

Although shaped by the same main processes, glaciated margins can also be quite distinct. That can be observed by comparing the Eastern North Atlantic, characterised by few large slope failures and large fans, with the Western North Atlantic, which accommodates abundant small failures (Huhnerbach & Masson, 2004).

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