The role of sedimentation rate and permeability in the slope stability of the formerly glaciated Norwegian continental margin: the Storegga slide model

Despite the gently dipping slopes (ca 1°), large-scale submarine slope failures have occurred on the mid-Norwegian continental margin (Storegga, Sklinnadjupet, Traenadjupet), suggesting the presence of special conditions predisposing to failure in this formerly glaciated margin. With a volume estimated between 2,400 and 3,200 km³ and an affected area of approximately 95,000 km², the Storegga slide represents one of the largest and best-studied submarine slides of Holocene age known worldwide. Finite element modeling of slope failure indicates that a large (6.5 < Ms < 7.0) seismic triggering mechanism would not be sufficient to cause failure at more than 110 m below the seabed as observed for the slip planes at Storegga (northern sidewall). This implies that other factors (e.g., liquefaction, strain softening, gas charging, rapid burial) are needed to explain the occurrence of the Storegga slide with a deep surface of failure. In this paper, we discuss the importance of the compaction effect of rapidly accumulated sediments in the slide area. During compaction, sediment grains reorganize themselves, thereby, expelling pore water. Consequently, depending on sedimentation rate and permeability, excess pore pressures might result beneath less permeable sediments. Our modeling and cross-checking illustrate how excess pore pressure generation due to high sedimentation rate could explain the development of layers of weakness, and thus, how such a large slide might have been initiated in deep sediments. Using the highest sedimentation rate estimated in the area (36 and 27 m/kyr between 16.2 and 15 kyr BP), 1D modeling shows excess pore pressure values of around 200 kPa at a depth of 100 m below the seafloor 15 kyr BP and 60 kPa at a depth of 100 m at the time of the slide (8 kyr BP). Excess pore pressure apparently drastically reduced the resistance of the sediment (incomplete consolidation). In addition, 2D modeling shows that permeability anisotropies can significantly affect the lateral extent of excess pore pressure dissipation, affecting, that way, normally consolidated sediments far from the excess pore pressure initiation area.     

High-resolution seismic studies of gas hydrates west of Svalbard

 A strong bottom-simulating reflection (BSR) with high-amplitude variations is detectable in high- resolution reflection seismic profiles west of Svalbard. Above the BSR, anomalously high velocities up to 1840 m/s, calculated from high-frequency ocean-bottom hydrophone (HF-OBH) data, indicate the existence of gas-hydrated sediments. Below the BSR, a low-velocity layer, interpreted as gas-bearing sediments, shows thickness variations from 12 to 25 m. In addition, two other low-velocity layers clearly containing free gas are detected within the classic hydrate stability zone (HSZ) where, a theoretical viewpoint, free gas cannot exist. Received: 6 August 1997 / Revision received: 26 January 1998     

New insights into slide processes and seafloor geology revealed by side-scan imagery of the massive Hinlopen Slide, Arctic Ocean margin

The submarine Hinlopen Slide, located along the Arctic Ocean margin, is one of the largest known mass movements on Earth. The slide scar has several unusual morphometric characteristics, including headwalls up to 1,500 m high and spectacularly large, steep-sided rafted megablocks. The slide processes and continental margin properties that produced these features are not well known. A new high-resolution TOBI (towed ocean bottom instrument) side-scan sonar dataset reveals information about the detailed seafloor morphology and, therefore, slide dynamics during the final stages of sliding. First, the headwall area was efficiently and almost completely evacuated of slide debris, which is unusual for large submarine slides. Second, features relating to the propagation of extension to the shelf behind the headwall are absent, suggesting “strong” cohesive shelf material here or that a very stable shelf configuration was reached, possibly defined by NE-SW-trending faults. Third, there is little evidence for the translation of shelf material, again uncommon for submarine slides. Taken together with the occurrence of massive megablocks in the slide debris, Hinlopen Slide is distinct because of the juxtaposition of apparently “stronger” shelf material that has remained intact (headwalls, megablocks), and “weaker” shelf material that disaggregated fully during slope failure. Nevertheless, there is sonograph evidence of variable post-slide disintegration of the megablocks. Contrary to previous interpretations, this suggests that the blocks comprise sedimentary lithologies that are prone to failure, a key aspect awaiting confirmation.     

The enigma of double BSRs: indicators for changes in the hydrate stability field?

 A classical bottom simulating reflector (BSR) and a presently unknown double BSR pattern are detectable in reflection seismic profiles from the Storegga Slide area west of Norway. Pressure and temperature modeling schemes lead to the assumption that the strong BSR marks the base of a hydrate stability zone with a typical methane gas composition of 99%. The upper double BSR may mark the top of gas hydrates and the lower double BSR may represent a relict of former changes of the hydrate stability field from glacial to interglacial times or the base of gas hydrates with a gas composition including heavier hydrocarbons.     

The Trænadjupet Slide: a large slope failure affecting the continental margin of Norway 4,000 years ago

¹⁴C AMS-dated gravity cores reveal that the Trænadjupet Slide offshore Norway occurred about 4,000 ¹⁴C years B.P. (ca. 4,000 cal. years ago). From 4,000 to 3,000 years B.P., minor areas of the newly formed slide scar were probably eroded, the result of smaller episodes of mass wasting caused by delayed collapse of part of the western, upper sidewall or by bottom currents. From about 3,000 years B.P. to the present, sediments were derived from alongslope-flowing, north-eastward-oriented ocean currents carrying sediments in suspension. These results demonstrate that large-scale mass wasting during sea-level highstand is rather common on passive continental margins.     

Submarine mass movements on glaciated and non-glaciated European continental margins: A review of triggering mechanisms and preconditions to failure

In this paper we present an overview of the major triggering mechanisms and preconditions for slope failure on the European continental margins, a vast area in which the dominant factors on sedimentation and erosional processes vary both spatially and temporally. Therefore, we have collated and integrated new as well as published data for both the formerly glaciated and non-glaciated areas of this highly dynamic margin for a time period mainly from the Last Glacial Maximum (LGM) to the present. Mass transport type is predominantly translational sliding on the high-latitude continental margins (north of 52°N), whereas turbidites dominate on lower latitudes. This is partly related to the average slope of the respective continental margin segments and differences in both sediment types and soil properties. Additionally, on low latitudes, submarine slope failures mainly occurred during glacial conditions with low sea level, whereas on high latitudes, they occur during the relatively fast transition from glacial to interglacial conditions (i.e. during periods of sea level rise). The largest submarine slides (e.g. Storegga, Trænadjupet, Andøya) on the glaciated Norwegian margin occurred during the Holocene, a time of rapid ice sheet decay, continental uplift and increased seismic activity, one of the most important triggering mechanisms for large failures during deglaciation processes. Preconditioning factors such as weak layers related to contourite drifts and rapid loading by glacial sediments may enhance strain localization and creep processes on the slope.     

Bottom-simulating reflections and geothermal gradients across the western Svalbard margin

Seismic reflection data reveal prominent bottom-simulating reflections (BSRs) within the relatively young (<0.78 Ma) sediments along the West Svalbard continental margin. The potential hydrate occurrence zone covers an area of c. 1600 km². The hydrate accumulation zone is bound by structural/tectonic features (Knipovich Ridge, Molloy Transform Fault, Vestnesa Ridge) and the presence of glacigenic debris lobes inhibiting hydrate formation upslope. The thickness of the gas-zone underneath the BSR varies laterally, and reaches a maximum of c. 150 ms. Using the BSR as an in-situ temperature proxy, geothermal gradients increase gradually from 70 to 115 °C km⁻¹ towards the Molloy Transform Fault. Anomalies only occur in the immediate vicinity of normal faults, where the BSR shoals, indicating near-vertical heat/fluid flow within the fault zones. Amplitude analyses suggest that sub-horizontal fluid migration also takes place along the stratigraphy. As the faults are related to the northwards propagation of the Knipovich Ridge, long-term disturbance of hydrate stability appears related to incipient rifting processes.     

Fluid flow impact on slope failure from 3D seismic data: a case study in the Storegga Slide

Analysis of three‐dimensional (3D) seismic data from the headwall area of the Storegga Slide on the mid‐Norwegian margin provides new insights into buried mass movements and their failure mechanisms. These mass movements are located above the Ormen Lange dome, a Tertiary dome structure, which hosts a large gas reservoir. Slope instabilities occurred as early as the start of the Plio‐Pleistocene glacial–interglacial cycles. The 3D seismic data provide geophysical evidence for gas that leaks from the reservoir and migrates upward into the shallow geosphere. Sediments with increased gas content might have liquefied during mobilization of the sliding and show different flow mechanisms than sediments containing less gas. In areas where there is no evidence for gas, the sediments remained intact. This stability is inherited by overlying strata. The distribution of gas in the shallow subsurface (<600 m) may explain the shape of the lower Storegga headwall in the Ormen Lange area.     

Evidence of shallow- and deep-water gas hydrate destabilizations in North Atlantic polar continental margin sediments

 Destabilization of gas hydrates from the North Atlantic polar continental margins is geophysically detectable within hydrate stability zones (HSZ). High-frequency seismic surveys of structures and propagation velocities of compressional waves have changed the classic conception of a consistently stable hydrate zone. The results are important in two respects: (1) unstable shallow-water gas hydrates can substantially contribute to the transfer of methane into the atmosphere, and (2) deep-water gas hydrates also indicate destabilization, which results in slope instability with probably only a secondary role in the transfer of methane to the atmosphere and thus in the greenhouse effect. Received: 6 August 1997 / Revision received: 26 January 1998     

Gas hydrate stability zone modelling in areas of salt tectonics and pockmarks of the Barents Sea suggests an active hydrocarbon venting system

The Barents Sea seabed exhibits an area of major glacial erosion exposing parts of the old hydrocarbon basins. In this region, we modelled the gas hydrate stability field in a 3D perspective, including the effects of higher order hydrocarbon gases. We used 3D seismic data to analyse the linkage between fluid-flow expressions and hydrate occurrences above old sedimentary basin systems and vertical faults. Pockmarks showed a relation to fault systems where some of them are directly connected to hydrocarbon bearing sedimentary formations. The influence of bottom water temperature, pore water salinity and geothermal gradient variation on gas hydrate stability zone (GHSZ) thickness is critically analysed in relation to both geological formations and salt tectonics. Our analysis suggests a highly variable GHSZ in the Barents Sea region controlled by local variations in the parameters of stability conditions. Recovery of gas-hydrate sample from the region and presence of gas-enhanced reflections below estimated BSR depths may indicate a prevalent gas-hydrate stable condition.