Russell B. Wynn

Submarine landslides: processes, triggers and hazard prediction

Huge landslides, mobilizing hundreds to thousands of km3 of sediment and rock are ubiquitous in submarine settings ranging from the steepest volcanic island slopes to the gentlest muddy slopes of submarine deltas. Here, we summarize current knowledge of such landslides and the problems of assessing their hazard potential. The major hazards related to submarine landslides include destruction of seabed infrastructure, collapse of coastal areas into the sea and landslide-generated tsunamis. Most submarine slopes are inherently stable. Elevated pore pressures (leading to decreased frictional resistance to sliding) and specific weak layers within stratified sequences appear to be the key factors influencing landslide occurrence. Elevated pore pressures can result from normal depositional processes or from transient processes such as earthquake shaking; historical evidence suggests that the majority of large submarine landslides are triggered by earthquakes. Because of their tsunamigenic potential, ocean-island flank collapses and rockslides in fjords have been identified as the most dangerous of all landslide related hazards. Published models of ocean-island landslides mainly examine ‘worst-case scenarios’ that have a low probability of occurrence. Areas prone to submarine landsliding are relatively easy to identify, but we are still some way from being able to forecast individual events with precision. Monitoring of critical areas where landslides might be imminent and modelling landslide consequences so that appropriate mitigation strategies can be developed would appear to be areas where advances on current practice are possible.

Classification and characterisation of deep-water sediment waves

Deep-water sediment waves can be classified using a combination of grain size and wave-forming process, although in some cases one or other of these criteria may be indeterminable. Sediment waves are generated beneath currents flowing across the seabed, in the form of either downslope-flowing turbidity currents or alongslope-flowing bottom currents. Waves formed by either process show varying characteristics, depending on whether they are constructed of coarse- or fine-grained sediments. Sediment wave studies over the last five decades are reviewed, and clear trends can be discerned. Early descriptive studies in the 1950s and 1960s relied almost exclusively on seismic reflection profiles, and the wave-forming process was often a subject of much debate. In the 1970s and 1980s the quality of sediment wave datasets increased, with sidescan sonar, deep-sea drilling and numerical modelling all applied to sediment wave studies. Consequently, the wave-forming process became more easily identifiable, and models for the growth of bottom current and turbidity current sediment waves were introduced. Most studies from the 1990s onwards have focussed on turbidity current sediment waves, in response to the increasing demand for data from turbidite systems from the hydrocarbon exploration and production industry. Studies of bottom current sediment waves during this period have focussed on the applications to palaeoceanography, in response to the recent boom in climate change studies. The main focus of this paper is the characterisation of both fine- and coarse-grained, turbidity and bottom current sediment waves, including the depositional environment, wave morphology, wave sediments and migration, and the wave-forming process. In addition, criteria for distinguishing between fine-grained bottom current and turbidity current waves are discussed, and also for identifying other wave-like features formed by different processes, such as creep folds. Although in many sediment wave studies the dominant wave-forming process is easy to determine, in others it is likely that a more complex combination of processes has occurred. Further studies should concentrate on methods for identifying these processes and how they interact, and also investigate the exact mechanisms for the initiation and evolution of sediment wave fields.

Onset of submarine debris flow deposition far from original giant landslide

Submarine landslides can generate sediment-laden flows whose scale is impressive. Individual flow deposits have been mapped that extend for 1,500 km offshore from northwest Africa. These are the longest run-out sediment density flow deposits yet documented on Earth. This contribution analyses one of these deposits, which contains ten times the mass of sediment transported annually by all of the world’s rivers. Understanding how this type of submarine flow evolves is a significant problem, because they are extremely difficult to monitor directly. Previous work has shown how progressive disintegration of landslide blocks can generate debris flow, the deposit of which extends downslope from the original landslide. We provide evidence that submarine flows can produce giant debris flow deposits that start several hundred kilometres from the original landslide, encased within deposits of a more dilute flow type called turbidity current. Very little sediment was deposited across the intervening large expanse of sea floor, where the flow was locally very erosive. Sediment deposition was finally triggered by a remarkably small but abrupt decrease in sea-floor gradient from 0.05° to 0.01°. This debris flow was probably generated by flow transformation from the decelerating turbidity current. The alternative is that non-channelized debris flow left almost no trace of its passage across one hundred kilometres of flat (0.2° to 0.05°) sea floor. Our work shows that initially well-mixed and highly erosive submarine flows can produce extensive debris flow deposits beyond subtle slope breaks located far out in the deep ocean.

Characterization and recognition of deep-water channel-lobe transition zones

The channel-lobe transition zone (CLTZ) is an important, but commonly overlooked, element of many deep-water turbidite systems. Recognizing this zone is difficult in both modern and ancient environments and depends largely on the quality and resolution of the data obtained. In this article, three case studies of modern CLTZs are presented, largely based on high-resolution side-scan sonar imagery. These data are then compared to other well-defined CLTZs, both modern and ancient, and the common characteristics identified. CLTZs occur at canyon/channel mouths and are commonly associated with a break of slope. Most sediment bypasses this zone, and consequently only coarse sands and gravels are deposited, although these are commonly patchily distributed and extensively reworked. The CLTZ is characterized by abundant erosional features, including isolated spoon- and chevron-shaped scours up to 20 m deep, 2 km wide, and 2.5 km long. In areas of more widespread erosion, these merge to form amalgamated scours several kilometers across. Depositional bed forms include sediment waves with wavelengths of 1-2 km and wave heights up to 4 m. The presence or absence of a CLTZ has important implications for hydrocarbon exploration and development, especially in terms of the connectivity between sandy channel-fill and lobe facies.

The origin of deep-water, coral-topped mounds in the northern Rockall Trough, northeast Atlantic

Mounds associated with the cold water coral Lophelia pertusa are widespread in the North Atlantic, although the factors controlling their distribution are not well understood. Here we examine a group of small, coral-topped mounds (the Darwin mounds) which occur at 1000 m water depth in the northern Rockall Trough, northwest of the UK. Individual mounds are up to 75 m in diameter and 5 m high, although some ‘mound-like’ targets seen on sidescan sonar have little or even negative relief. Some mounds are associated with ‘tail-like’ features, imaged as elongate patches of moderate backscatter up to 500 m long, elongated parallel to prevailing bottom currents. High-resolution sidescan images and seabed photographs show hundreds of coral colonies, each a metre or so across, on each individual mound. Many other organisms, mainly suspension feeders, occur in association with the coral. Piston cores from the mounds contain predominantly quartz sand with only scattered coral fragments, showing that bioclastic material is not a major contributor to mound building. A field of seabed pockmarks occurs immediately south of the Darwin mounds. On sidescan sonar data, pockmarks are low relief, circular depressions, typically around 50 m in diameter. The seafloor around the pockmarks consists of uniform, heavily-burrowed, muddy sediments and no specific biological communities, nor any sedimentological or photographic evidence for active seepage, were observed. The distribution of mounds and pockmarks suggests a gradual transition from mounds in the north to pockmarks in the south. This, combined with the lack of bioclastic material in the mound sediments, suggests that both mounds and pockmarks are created by fluid escape from below the seafloor. Mounds occur where fluids carry subsurface sand to the surface, where it forms mounds because bottom currents are not strong enough to disperse it. Pockmarks form where muddy material is eroded by fluid escape but dispersed by bottom currents. Despite the origin of mounds through fluid escape, we suggest that it is the elevated mound topography, rather than any fluid escape, that is advantageous to the corals. This is supported: (1) by the wide variety of suspension-feeding organisms that occur on the mounds, since all of these are unlikely to have a specialised seepage-related lifestyle, and (2) because corals and their associated community do not occur around pockmarks, where seepage has also occurred but elevated topography is absent.

The northwest African slope apron: a modern analogue for deep-water systems with complex seafloor topography

The Northwest African slope apron is an interesting modern analogue for deep-water systems with complex seafloor topography. A sediment process map of the Northwest African continental margin illustrates the relative roles of different sedimentary processes acting across the entire margin. Fine-grained pelagic and hemipelagic sedimentation is dominant across a large area of the margin, and is considered to result from background sedimentary processes. Alongslope bottom currents smooth and mould the seafloor sediments, and produce bedforms such as erosional furrows, sediment waves and contourite drifts. Downslope gravity flows (debris avalanches, debris flows and turbidity currents) are infrequent but important events on the margin, and are the dominant processes shaping the morphology of the slope and rise. The overall distribution of sedimentary facies and morphological elements on the Northwest African margin is characteristic of a fine-grained clastic slope apron. However, the presence of numerous volcanic islands and seamounts along the margin leads to a more complex distribution of sedimentary facies than is accounted for by slope apron models. In particular, the distribution and thickness of turbidite sands are controlled by the location of the break-of-slope, which is itself controlled by the pre-existing submarine topography.

Turbidity current sediment waves on the submarine slopes of the western Canary Islands

Two sediment wave fields have been identified on the flanks of the western Canary Islands of La Palma and El Hierro, using a high-quality 2-D and 3-D dataset that includes GEOSEA and TOBI imagery, 3.5-kHz profiles, and short sediment cores. The La Palma sediment wave field covers some 20,000 km2 of the continental slope and rise, and consists of sediment waves with wave heights of up to 70 m and wavelengths of up to 2.4 km. The wave crestlines have a complex morphology, with common bifurcation and a clear sinuosity. Waves have migrated upslope through time. Cores recovered from the wave field contain volcaniclastic turbidites interbedded with pelagic/hemipelagic layers. The wave field is interpreted as having formed beneath unconfined turbidity currents. A simple, previously published, two-layer model is applied to the waves, revealing that they formed beneath turbidity currents flowing at 10–100 cm/s−1, with a flow thickness of 60–400 m and a sediment concentration of 26–427 mg/l. The El Julan sediment wave field lies within a turbidity current channel on the southwest flank of El Hierro. The sediment waves display wave heights of about 6 m and wavelengths of up to 1.2 km. The waves are migrating upslope, and migration is most rapid in the centre of the channel where the flow velocity is highest. This wave field has been formed by channelised turbidity currents originating on the flanks of El Hierro.

Generation and migration of coarse-grained sediment waves in turbidity current channels and channel–lobe transition zones

Large-scale sediment waves, composed of gravels and sands, have been studied using deep-water sidescan systems. New data are presented from submarine channels off the Canary Islands and from canyon mouths off Portugal. Data from other areas are briefly reviewed, including a re-interpretation of data from Laurentian Fan, in order to summarise the varied morphology and setting of these bedforms. Coarse-grained sediment waves are found in the proximal, dominantly bypassing areas of deep-water turbidite systems, within canyons, channels and channel-lobe transition zones. Wave heights are in the region of 1-10 m, and wavelengths are up to several hundred metres. The distribution of waves, and sparse sedimentological evidence from modern and ancient sediment wave fields, suggests that initial transport and deposition of coarse sediment occurs within a high-density turbidity current, and not as a non-Newtonian debris flow. In some cases the development of pronounced wave asymmetry, and evidence of wave disruption and reworking, suggests that the wave morphology is at least partially controlled by a later phase of low-density turbidity flow. Grain size also appears to exert some control on wave morphology, for example, gravel-rich waves have a greater height for the same wavelength than sand-rich waves. Coarse-grained sediment waves are often difficult to recognise on the seafloor because of reworking or burial by younger turbidity currents, and are equally difficult to recognise in outcrop because of their large size

New insights into the morphology, fill, and remarkable longevity (>0.2 m.y.) of modern deep-water erosional scours along the northeast Atlantic margin

A series of large-scale erosional scours are described from four modern deep-water canyon and/or channel systems along the northeast Atlantic continental margin. Regional-scale geophysical data indicate that most scours occur in zones of rapid flow expansion, such as canyon and/or channel termini and margins. High-resolution images of the scours cover ∼25 km² at 2 × 2 m pixel size, and were obtained at depths of 4200–4900 m using Autosub6000, an autonomous underwater vehicle equipped with an EM2000 multibeam bathymetry system. Sedimentological and microfossil-based chronological data of scour fills and interscour areas were obtained via accurately located piston cores that targeted specific sites within imaged areas. These core data reveal a number of key findings. (1) Deep-water scours can be very long lived (>0.2 m.y. ) and may undergo discrete phases of isolation, amalgamation, and infilling. (2) Deep-water scours can develop via a composite of cutting and filling events with periodicities of between tens of thousands and hundreds of thousands of years. (3) Immediately adjacent scours may have strikingly different sedimentological histories and do not necessarily evolve contemporaneously. (4) Scour infills are typically out of phase with sedimentation in intrascour areas, having thin sands internally and thick sands externally, or thick muds internally and thin muds externally. (5) Erosional hiatuses within scour fills may represent hundreds of thousands of years of time, and yet leave little visible record. Four distinct morphologies of scour are identified that range from 40 to 3170 m wide and 8 to 48 m deep: spoon shaped, heel shaped, crescent shaped, and oval shaped. Isolated scours are shown to coalesce laterally into broad regions of amalgamated scour that may be several kilometers across. The combined morphosedimentological data set is used to examine some of the putative formative mechanisms for scour genesis.

Near‐synchronous and delayed initiation of long run‐out submarine sediment flows from a record‐breaking river flood, offshore Taiwan

Subsea fiber-optic telecommunication cables can break under fast sediment flows that travel 100s of kilometers through the deep ocean in response to earthquakes and submarine landslides. Similar flows are inferred to form from major river floods whose sediment-laden waters plunge and travel along the seabed. However, the complex initiation of flood-related flows and their hazard potential have not been observed until now. Here we use cable fault data from the Gaoping Canyon/Manila Trench off Taiwan to show that a major river flood, formed during Typhoon Morakot (2009), generated two, long run-out, destructive sediment flows; one during peak flood and the other 3 days later. The latter flow was more damaging with speeds and run-out similar to that of landslide-triggered turbidity currents formed in the same catchment. If the second flow was due to remobilized canyon sediment, it occurred during low earthquake (>Mw 2.0) activity, suggesting other triggering mechanisms.