A giant three-stage submarine slide off Norway

One of the largest submarine slides known, The Storegga Slide, is located on the Norwegian continental margin. The slide is up to 450 m thick and has a total volume of about 5,600 km3. The headwall of the slide scar is 290 km long and the total run-out distance is about 800 km. The slide involved sediments of Quaternary to Early Tertiary age and occurred in three stages. Earthquakes combined with decomposition of gas hydrates are believed to be the main triggering agents for the slides. The first slide event is tentatively dated to be about 30,000 to 50,000 years B.P. and the two last major events are dated to be at 6,000 to 8,000 years B.P.     

Scale invariant characteristics of the Storegga Slide and implications for large-scale submarine mass movements

This study documents the fractal characteristics of submarine mass movement statistics and morphology within the Storegga Slide. Geomorphometric mapping is used to identify one hundred and fifteen mass movements from within the Storegga Slide scar and to extract morphological information about their headwalls. Analyses of this morphological information reveal the occurrence of spatial scale invariance within the Storegga Slide. Non-cumulative frequency-area distribution of mass movements within the Storegga Slide satisfies an inverse power law with an exponent of 1.52. The headwalls exhibit geometric similarity at a wide range of scales and the lengths of headwalls scale with mass movement areas. Composite headwalls are self-similar.One of the explanations of the observed spatial scale invariance is that the Storegga Slide is a geomorphological system that may exhibit self-organized criticality. In such a system, the input of sediment is in the form of hemipelagic sedimentation and glacial sediment deposition, and the output is represented by mass movements that are spatially scale invariant. In comparison to subaerial mass movements, the aggregate behavior of the Storegga Slide mass movements is more comparable to that of the theoretical ‘sandpile’ model. The origin of spatial scale invariance may also be linked to the retrogressive nature of the Storegga Slide. The geometric similarity in headwall morphology implies that the slope failure processes are active on a range of scales, and that modeling of slope failures and geohazard assessment can extrapolate the properties of small landslides to those of larger landslides, within the limits of power law behavior. The results also have implications for the morphological classification of submarine mass movements, because headwall shape can be used as a proxy for the type of mass movement, which can otherwise only be detected with very high resolution acoustic data that are not commonly available.     

Linked turbidite–debrite resulting from recent Sahara Slide headwall reactivation

The northwest African margin has been affected by numerous large-scale landslides during the late Quaternary. This study focuses on a recent collapse of the Sahara Slide headwall and characterises the resulting flow deposit. Core and seismic data from the base of the upper headwall reveal the presence of blocky slide debris, comprising heavily deformed hemipelagic slope sediments. The blocky slide debris spilled over a lower headwall 60 km downslope and formed a thick transparent debris flow unit. Cores recovered 200–250 km farther downslope contain a surficial turbidite that is interpreted to be linked to the headwall collapse event based on timing and composition. One core located approximately 200 km from the headwall scar (C13) contains debrite encased in turbidite. The debrite comprises sheared and contorted hemipelagic mudstone clasts similar as those seen in the vicinity of the Sahara Slide headwall, and lacks matrix. This debrite pinches out laterally within 25 km of C13, whereas the accompanying turbidite can be correlated across 700 km of the northwest African margin. The linked turbidite–debrite bed is interpreted to have formed through recent failure of the steep Sahara Slide headwall that either 1) generated both a debris flow and a turbidity current almost simultaneously, or 2) generated a debris flow which with entrainment of water and progressive dilution led to formation of an accompanying turbidity current.     

Evidence for long-term instability in the Storegga Slide region off western Norway

Shallow-seismic surveys around the Storegga Slide off western Norway have allowed greater understanding of the development of this part of the European margin. The northern flank of the scarp is formed of seismically well-layered, hemipelagic and distal-glaciomarine deposits in which a variety of luid-escape structures, probably due to gas, are locally abundant. There is evidence of slides that substantially pre-date the earliest slide previously recognized. Surveying on the North Sea Fan to the southwest of the Storegga Slide shows the markedly different nature of the autochthonous sediments on the southern flank of the Storegga Slide; there is a predominance of glacigenic debris flows in the upper part of the sequence, lesser maximum slopes, and an apparent absence of interstitial gas and/or hydrates. This contrast has had considerable effect on slope stability and has influenced the position of the southwestern Storegga Slide boundary. The North Sea Fan succession includes at least three major buried slides, termed the Vigra, Møre and Tampen slides, all of which substantially pre-date the Storegga event and probably pre-date predominantly glacigenic margin sedimentation. Post-late Weichselian slope failure is locally significant. The region has a long, but as yet chronologically poorly defined history of instability in which seismic triggering is considered to have been important.     

Mass-transport complex evolution in a tectonically active margin (Gioia Basin,Southeastern Tyrrhenian Sea)

Along the southeastern Tyrrhenian Sea margin, the Gioia Basin formed as a result of extensional tectonics at the rear of the Maghrebian thrust belt. In the central part of the basin, mass-transport deposits represent up to 80% of its recent infill. The basin-wide Nicotera slump is the deepest mass-transport deposit present in the basin and was followed by sheet turbidite deposition. Above the turbidite package, a mass-transport complex (MTC) formed through the stacking of different mass-transport deposits due to repeated failures of the continental slope and of a base of slope channel levee wedge, which is still preserved in the western side of the basin. The Villafranca frontally-confined slide, a body mainly consisting of coherent blocks, represents the bulk of the MTC. The failure of the Villafranca slide was due to asymmetric loading of a permeable condensed horizon in the thinnest, distal lateral part of the channel levee wedge. The relatively large thickness of the Villafranca slide caused it to remain confined at its toe region. Smaller scale mass-transport deposits, a debris-flow sheet and a debris-flow lobe, followed the Villafranca slide and were sourced from the same headwall area. Their different run out and internal character are possibly a function of the lithology of the material involved in the collapse. A slab slide, characterized by little internal deformation and frontal contractional ridges, originated when seafloor instability propagated towards the north, causing clockwise rotation of a sediment wedge. Along the linear headwall of the slab slide, a localized upslope failure propagation is shown by a small scale re-entrant. The Sicilian margin, along which the Gioia Basin develops, is characterized by strong differential vertical movements due to ongoing extensional tectonics. The effects of both local and regional strong earthquakes are frequently felt in the area. Thus, slope oversteepening and earthquakes are suggested as the more likely causes for the observed repeated events of seafloor failure. In addition, an evolution of the MTC through larger slides controlled by the migration of uplift of the basin bounding submarine ridge, followed by smaller scale failures due to the consequent slope profile modification, is here advanced.     

Acoustic evidence of a submarine slide in the deepest part of the Arctic,the Molloy Hole

The western Svalbard continental margin contains thick sediment sequences with areas known to contain gas hydrates. Together with a dynamic tectonic environment, this makes the region prone to submarine slides. This paper presents results from geophysical mapping of the deepest part of the high Arctic environment, the Molloy Hole. The mapping includes multibeam bathymetry, acoustic backscatter and sub-bottom profiling. The geophysical data reveal seabed features indicative of sediment transport and larger-scale mass wasting. The large slide scar is here referred to as the Molloy Slide. It is located adjacent to the prominent Molloy Hole and Ridge system. The slide is estimated to have transported >65 km³ of sediments over the deep axial valley of the Molloy Ridge, and further into the Molloy Hole. A unique feature of this slide is that, although its run-out distance is relatively short (<5 km), it extends over an enormous vertical depth (>2,000 m) as a result of its position in a complex bathymetric setting. The slide was most likely triggered by seismic activity caused by seafloor spreading processes along the adjacent Molloy Ridge. However, gas-hydrate destabilization may also have played a role in the ensuing slide event.     

Sediment slides on the northwestern continental margin of Africa

A sediment slide complex has been mapped on the West African continental margin north of Dakar, Senegal. Four major slides covering approximately 44,300 km² were delineated by seismic reflection profiles, 3.5 and 12 kHz echograms and piston cores. Although the slide areas have been altered by later erosion and deposition by turbidity flows, the major components of the slides — slide scar, zones of hummocky and blocky slide material and zones of debris flow — are recognizable. Cores containing flow folds with horizontal axial surfaces substantiate the echogram interpretations of debris flow. Morphology and depositional areas of the slides indicate that several major slide movements have occurred in each of the various slide areas. The triggering mechanism for these slides is perhaps earthquakes associated with the Cape Verde Islands, Cape Verde Plateau, and adjacent fracture zones.     

Late Cenozoic geological development of the south Vøring margin,mid-Norway

Late Cenozoic seismic stratigraphy of the Vøring continental margin has been studied in detail, with emphasis on the geological development of the Naust Formation deposited during the last 3 million years. The Kai Formation (15–3 Ma) comprises mainly biogenic ooze deposited in the Møre and Vøring Basins. In Naust time, there was a marked increase in supply of sediments from the inner shelf areas and the western part of the Scandinavian mountain range, and glaciers expanded to the shelf and reached the shelf edge several times during the last 1.5–2 million years. During early to mid Naust time the shelf was widened by westerly prograding sediment units, but for a long period the shallowest part of the Helland-Hansen Arch (HHA) formed a barrier preventing glacially derived debris from being distributed farther west. West of the HHA, mainly stratified marine and glacimarine sediments were deposited. A substantial part of these sediments were transported by the north-flowing Norwegian Atlantic Current, which redistributed suspended particles from ice streams, rivers, coastal erosion and seabed winnowing. After burial of the crest of the HHA at c. 0.5 Ma, glacial debris and slide deposits were also deposited west of this high. In the north, massive units of glacial debris were distributed beyond the crest of the HHA, also in mid Naust time, thinning westwards and interfingering with fine-grained sediments on the lower slope. The Sklinnadjupet Slide, inferred to be c. 250,000 years old, corresponds in age with an earlier huge slide in the Storegga area. An elongated area of uneven seabed topography previously interpreted as diapirs (Vigrid diapirs) on the slope west of the HHA is shown to be formed by ooze eruption from the crest of the arch and submarine sliding.     

Return to ranger submarine slide,Baja California,Mexico

Ranger Slide is a modest (12 km³) slide deposit of Pliocene and younger sediment on the continental slope in northern Sebastian Vizcaino Bay, Mexico. A limited survey using a deeply-towed instrument shows that hummocky terrain immediately downslope from the slide scar consists of large blocks of semiconsolidated sediment, some exceeding a kilometer in length and 10⁷ m³ in volume. Most blocks have rotated, fallen apart, and/or deformed during movement. The form, structure, and processes related to emplacement of the blocks within the hummocky topographic zone of Ranger Slide may be common to many submarine slides on slopes involving semiconsolidated, terrigenous sediment.     

New evidence for widespread mass transport on the Northeast Newfoundland Shelf revealed by Olex single-beam echo sounding

Based on Olex single-beam sounder data, multibeam sonar surveys, and sparse seismic reflection profiles, we recognize a large area of anomalous bathymetry on the Northeast Newfoundland Shelf as having formed as a result of mass-transport processes. Transported masses include (1) an arcuate ridge of deformed material with an area of 430 km², which has moved distances of ~20 km; (2) a 70-km² mass of deformed material displaced 50 km along a nearly horizontal track flanked by 90-m-high berms. The movement of these and other sediment bodies has created a 150-m-high headwall escarpment extending 110 km along the north flank of the Notre Dame glacial trough. In addition, a 35-km² block of undeformed material has moved 5 km to the southeast, away from the headwall, creating a gap of the same dimensions, while a smaller block of material originating in this vicinity has been displaced 24 km in the opposite direction, creating a 20-m-deep groove on the seafloor. There is evidence for mass transport and headwall formation elsewhere on the Northeast Newfoundland Shelf. Analysis of seismic reflection data indicates that the transported material most likely consists of stacked Quaternary till sheets that overlie Cenozoic, Mesozoic and older sedimentary rocks. Given the very low gradients involved, glaciotectonism is the most likely process to account for transport and deformation of the large sediment masses. However, some mass transport may have resulted from submarine sliding away from the headwalls that were created by the glacial transport.     

Architecture and development of a multi-stage Baiyun submarine slide complex in the Pearl River Canyon,northern South China Sea

The Baiyun submarine slide complex (BSSC) along the Pearl River Canyon of the northern South China Sea has been imaged by multibeam bathymetry and 2D/3D seismic data. By means of maximum likelihood classification with slope aspect and gradient as inputs, the BSSC is subdivided into four domains, denoted as slide area I, II, III and IV. Slide area I is surrounded by cliffs on three sides and has been intensely reshaped by turbidity currents generated by other kinds of mass movement outside the area; slide area II incorporates a shield volcano with a diameter of approximately 10 km and unconfined slides possibly resulting from the toe collapse of inter-canyon ridges; slide area III is dominated by repeated slides that mainly originated from cliffs constituting the eastern boundary of the BSSC; slide area IV is distinguished by a conical seamount with a diameter of 6.5 km and a height of 375 m, and two slides probably having a common source that are separated from each other by a suite of residual strata. The BSSC is interpreted to be composed of numerous slide events, which occurred in the period from 10.5 to 5.5 Ma BP. Six specific factors may have contributed to the development of the BSSC, i.e., gas hydrate dissociation, gas-bearing sediments, submarine volcanic activity, seismicity, sedimentation rate and seafloor geomorphology. A 2D conceptual geological model combining these factors is proposed as a plausible mechanism explaining the formation of the BSSC. However, the BSSC may also have been affected by the Dongsha event (10 Ma BP) as an overriding factor.     

Submarine debris flow deposits detected by long-range side-scan sonar 1,000-kilometers from source

Debris flow deposits of large aerial extent have been detected on the lower continental rise off northwest Africa using GLORIA long-range dual sidescan sonar. A preliminary interpretation of the sonographs with high-resolution (3.5 kHz) seismic profiles and gravity cores illustrates the potential for spatial mapping of these deposits. The transport directions indicated on the sonographs show that these sediments, emplaced by mass transport, are the downslope continuation of the Saharan Sediment Slide. The distance from an observed “toe” of a debris flow lobe to the most southerly slide scar is of the order of 1,000 km.     

台湾海峡南部海底地形的新发现

我们于1975年对台湾海峡南部进行海底地形和底质调查研究时发现:1.台湾浅滩发育着水下沙丘群。2.陆架地形受现代环境的作用,正经历着冲刷、侵蚀和堆积的再改造过程。3.陆架边缘曾发生了巨大的滑坡。其原因是陆源物质供应严重缺乏,在风浪和各种流系作用下,使沉积物不断进行分异作用,再加上自1938年以来多次发生5—7级海底地震而导致滑坡,滑坡又促使沉积物向深水搬运,加速陆架地形的冲刷。     

Evolution and depositional structure of earthquake-induced mass movements and gravity flows: Southwest Orphan Basin, Labrador Sea

A series of submarine canyons on the southwest slope of Orphan Basin experienced complex failure at 7–8 cal ka that resulted in the formation of a large variety of mass-transport deposits (MTDs) and sediment gravity flows. Ultra-high-resolution seismic-reflection profiles and multiple sediment cores indicate that evacuation zones and sediment slides characterize the canyon walls, whereas the canyon floors and inner-banks are occupied by cohesive debris-flow deposits, which at the mouths of the canyons on the continental rise form large, coalescing lobes (up to 20 m thick and 50 km long). Erosional channels, extending throughout the length of the study area (<250 km), are observed on the top of the lobes. Piston cores show that the channels are partially filled by poorly sorted muddy sand and gravel, capped by inversely to normally graded gravel and sand. Such deposits are interpreted to originate from multi-phase gravity flows, consisting of a lower part behaving as a cohesionless debris flow and an upper part that was fully turbulent.The Holocene age and the widespread synchronous occurrence of these failures indicate a large magnitude earthquake as their possible triggering mechanism. The large debris-flow deposits on the continental rise originated from large failures on the upper continental slope, involving proglacial sediments. Retrogression of these failures led to the eventual failure of marginal sandy till deposits on the upper slope and outer shelf, which due to their low cohesion disintegrated into multi-phase gravity flows. The evacuation zones and slide deposits on the canyon walls were triggered either by the earthquake, or from erosion of the canyon walls by the debris flows. The slides, debris-flows, and multi-phase gravity flows observed in this study are petrographically different, indicating different sediment sources. This indicates that not all failures lead through flow transformation to the production of a multi-phase gravity flow, but only when the sediment source contains ample coarse-grained material. The spatial segregation of the slide, debris-flow, and multi-phase gravity-flow deposits is attributed to the different mobility of each transport process.     

Distribution and morphology of large submarine sediment slides and slumps on Atlantic continental margins

Abstract

Numerous large sediment slides and slumps have been discovered and surveyed on the continental margins of Northwest Africa, Southwest Africa, Brazil (Amazon Cone), the Mediterranean, the Gulf of Mexico, and North America over the past 10 years. The mass movements are of two primary types: (1) translational slides, and (2) rotational slumps. Translational slides are characterized by a slide scar and a downslope zone of debris flows, after traveling in some areas for several hundreds of kilometers on slopes of less than 0.5°. Rotational slumps are bounded by steep scarps, but they do not involve large‐scale translation of sediments, although seismic records indicate disturbance in the down‐dropped block. Many of the slides and slumps have occurred in water depths greater than 2000 m on initial slopes of less than 1.5°. The largest slide so far discovered is off Spanish Sahara; in this case, the slide scar is 18,000 km² in area, at least 600 km³ in volume of translated sediments. No apparent consistent relationship has yet been observed between the presence of the slides and the sedimentary environment in which they occurred. The slides off Southwest Africa and Spanish Sahara occurred in pelagic sediments rich in planktonic organic matter. In contrast, the slides off North America, Senegal‐Mauritania, and Brazil (Amazon Cone) occurred in sediments containing a high percentage of terrigenous material from nearby landmasses. Large sediment slides have also occurred in pelagic sediments on isolated oceanic rises such as the Madeira Rise (East‐Central Atlantic) and the Ontong‐Java Plateau (Pacific), where sedimentation rates are less than 2 cm/1000 years. The failure mechanism of the slides initiated near the shelf edge can probably be explained by sediment overloading during low glacio‐eustatic sea level, which allowed rivers to debouch sediments directly onto the outer shelf or upper slope. Possible mechanisms of failure of the deepwater slides and slumps include earthquakes, undercutting of the slope by bottom currents, and changes in porewater pressures induced as a direct or indirect result of glacio‐eustatic changes in sea level.     

The stratigraphic and geographic distribution of giant craters and remobilised sediment mounds on the mid Norway margin,and their relation to long term fluid flow

Large craters associated with mounds of remobilised sediment have been recently mapped on the mid Norway margin in the Møre Basin. These craters and mounds may be linked to the long term migration of fluids upwards from the lower levels of the Møre Basin which exploit hydrothermal vent complexes emplaced in the late Paleocene and early Eocene. All of the craters are located on a regionally correlative seismic surface that is correlated with the basal shear plane of Slide W, a slide located at the base of the Plio-Pleistocene Naust Formation. The Craters are positioned in the western area of the Møre Basin at the foot of the continental slope on the crests and flanks of Miocene domes, where Oligocene biosiliceous ooze subcrops on the basal shear surface of Slide W. Not all of the craters are filled by Slide W. Mounds are emplaced above those craters which are filled by Slide W on the top surface of Slide W. Stratal relationships show that the mounds were emplaced on the paleo-seabed. We present and discuss two models that illustrate processes that may have been involved in the formation of craters and remobilisation of sediments. In one model, an eruption of fluid from beneath remobilises ooze into ooze mounds in a single event triggering slope failure, whereas in the other model the emplacement of Slide W and later slides loads low density ooze causing it to undergo liquefaction, a process which may have been facilitated by the trapping of continuous long term fluid migrating from beneath, causing the ooze to remobilise into ooze mounds in two or more events.     

Bioerosion by chemosynthetic biological communities on Holocene submarine slide scars

Geomorphic, stratigraphic, and faunal observations of submarine slide scars that occur along the flanks of Monterey Canyon in 2.0–2.5 km water depths were made to identify the processes that continue to alter the surface of a submarine landslide scar after the initial slope failure. Deep-sea chemosynthetic biological communities and small caves are common on the sediment-free surfaces of the slide scars, especially along the headwall. The chemosynthetic organisms observed on slide scars in Monterey Canyon undergo a faunal succession based in part on their ability to maintain their access to the redox boundaries in the sediment on which they depend on as an energy source. By burrowing into the seafloor, these organisms are able to follow the retreating redox boundaries as geochemical re-equilibration occurs on the sole of the slide. As these organisms dig into the seafloor on the footwall, they often generate small caves and weaken the remaining seafloor. While chemosynthetic biological communities are typically used as indicators of fluid flow, these communities may be supported by methane and hydrogen sulfide that are diffusing out of the fresh seafloor exposed at the sole of the slide by the slope failure event. If so, these chemosynthetic biological communities may simply mark sites of recent seafloor exhumation, and are not reliable fluid seepage indicators.     

Morphosedimentary features and recent depositional architectural model of the Cantabrian continental margin

Multibeam bathymetry, high (sleeve airguns) and very high resolution (parametric system-TOPAS-) seismic records were used to define the morphosedimentary features and investigate the depositional architecture of the Cantabrian continental margin. The outer shelf (down to 180–245 m water depth) displays an intensively eroded seafloor surface that truncates consolidated ancient folded and fractured deposits. Recent deposits are only locally present as lowstand shelf-margin deposits and a transparent drape with bedforms. The continental slope is affected by sedimentary processes that have combined to create the morphosedimentary features seen today. The upper (down to 2000 m water depth) and lower (down to 3700–4600 m water depth) slopes are mostly subject to different types of slope failures, such as slides, mass-transport deposits (a mix of slumping and mass-flows), and turbidity currents. The upper slope is also subject to the action of bottom currents (the Mediterranean Water — MW) that interact with the Le Danois Bank favouring the reworking of the sediment and the sculpting of a contourite system. The continental rise is a bypass region of debris flows and turbidity currents where a complex channel-lobe transition zone (CLTZ) of the Cap Ferret Fan develops.The recent architecture depositional model is complex and results from the remaining structural template and the great variability of interconnected sedimentary systems and processes. This margin can be considered as starved due to the great sediment evacuation over a relatively steep entire depositional profile. Sediment is eroded mostly from the Cantabrian and also the Pyrenees mountains (source) and transported by small stream/river mountains to the sea. It bypasses the continental shelf and when sediment arrives at the slope it is transported through a major submarine drainage system (large submarine valleys and mass-movement processes) down to the continental rise and adjacent Biscay Abyssal Plain (sink). Factors controlling this architecture are tectonism and sediment source/dispersal, which are closely interrelated, whereas sea-level changes and oceanography have played a minor role (on a long-term scale).     

Origin of continental margin morphology: Submarine-slide or downslope current-controlled bedforms, a rock magnetic approach

Morphological features observed in both swath bathymetry and seismic reflection data are not unique, which introduces uncertainty as to their origin. The origin of features observed in the Humboldt Slide has generated much controversy because the same features have been interpreted as a submarine failure deposit versus current-controlled sediment waves. It is important to resolve this controversy because similar structures are observed on many continental margins and the origin of these features needs to be understood. Anisotropy of magnetic susceptibility (AMS) measurements on sediment samples acquired from the Humboldt Slide reveal that the top  8 m have not experienced post-depositional deformation. This suggests that these features are formed by primary deposition associated with downslope currents. Using the same AMS technique on a core acquired north of the Humboldt Slide in a region with no geophysical evidence for post-depositional deformation, we were able to identify a  1 m thick deposit that appears to be a small slump.