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.     

The structural styles and formation mechanism of salt structures in the Southern Precaspian Basin: Insights from seismic data and analog modeling

Using the seismic profiles and analog modeling, this paper addresses the salt structures in the M and B blocks in the Southern Precaspian Basin. The salt structural features, the formation mechanism and the controlling factors of structural deformation are investigated and discussed systematically. The interpretation of the seismic profiles shows that typical salt-related structures include salt wall, (flip-flop) salt diapir, salt roller, salt pillow (dome), salt weld, salt withdrawal minibasin and drag structure (or drape fold). In addition, model results demonstrate that the gravity spreading driven by progradation and aggradation is probably the primary factor in controlling the formation of the salt structures in the research area. Due to the differential loading driven by progradation, passive salt diapir developed near the progradational front followed by the formation of intrasalt withdrawal minibasin bounded by two salt diapirs, and secondary reactive triangle salt diapir or salt pillow might form within the intrasalt withdrawal minibasin. Model results also indicate that the pattern of the subsalt basement has important influence on the formation and evolution of salt structures. Salt diapirs primarily developed along the margin of the subsalt uplift basement, where high shear deformation was induced by differential sedimentary loading between the uplift area and the slope area.     

Large submarine slide (olistostrome) associated with Sunda Arc subduction zone,northeast Indian Ocean

Reflection profiling in a region of anomalous topography and structure in the Bay of Bengal off Burma has revealed the presence of a large submarine slide (olistostrome) at the base of the continental slope off the Bassein River. The slide overlies a thick section of Bengal Deep-Sea Fan turbidites and has a complex internal structure consisting of two primary elements. The lower element is pervasively disturbed and is interpreted as a mudflow generated at the time of the slide which spread over a large area to as much as 35 km beyond the topographic toe. This mudflow poured into a distributary channel on the Bengal Fan and virtually filled it for 145 km along its length. The upper element comprises a series of relatively coherent blocks of stratified sediments (olistoliths) bounded by curved fault planes. The blocks have been transported as much as 55 km from the original Sunda Trench wall. Their dimensions, up to 360 m thick and 2.8 km between faults, are similar to olistoliths of the slide terrain in the Apennines. The blocks are blanketed by younger slope strata. The total area covered by the slide, including the mudflow, is almost 4,000 km², and total volume of the slide is over 900 km³. Material of the slide consists of Bengal Fan turbidites offscraped above the Sunda Subduction zone and blanketed by rapidly deposited slope sediments from a western Irrawaddy River distributary (the Bassein River) during Late Quaternary glacial low sea level. This rapid loading, probably coupled with a large earthquake, triggered the slide.     

Extensional rise and fall of a salt diapir in the Sørvestsnaget Basin,SW Barents Sea

Regional extension which initiates and promotes the rise of salt diapirs can also make diapirs fall once the supply of salt from its source is restricted. New observations on the 3D seismic data from a salt diapir in the Sørvestsnaget Basin suggest that salt moves until the end of the Eocene and is subtle to minor readjustments afterwards, revealing a more complex kinematics that previously described. Observations such as salt horns and sags and an antithetic fault linked to the western flank of the diapir suggest that salt syn-kinematics during Middle-Late Eocene included passive rising of the salt, followed by a fall. The salt horns are remnants of a taller salt diapir that, together with the indentation of the Middle-Late Eocene syn-kinematic sediment overburden above the salt, indicate diapiric fall due to restriction of salt supply by extension. Post-kinematic readjustments did not include diapiric reactivation by tectonic compression as previously thought, but minor salt rise by shortening due to gravity gliding after the tilting of the margin during Plio-Pleistocene glacial sediment loading and differential compaction of surrounding sediments. The salt diapir appears to be presently inactive and salt supply may have been restricted from its source already since Late Eocene.     

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.     

Geologic evolution of the Canarian Islands of Lanzarote, Fuerteventura, Gran Canaria and La Gomera and comparison of landslides at these islands with those at Tenerife, La Palma and El Hierro

In this paper we discuss the results of a swath bathymetric investigation of the Canary archipelago offshore area. These new data indicate that volcanism is pervasive throughout the seafloor in the region, much more that would be suggested by the islands. We have mapped tens of volcanic edifices between Fuerteventura and Gran Canaria and offshore Tenerife, La Gomera, El Hierro and La Palma. Volcanic flows are present between Tenerife and La Gomera and salic necks dominate the eastern insular slope of La Gomera. This bathymetry also supports land geologic studies that indicate that the oceanic archipelago has acquired its present morphology in part by mass wasting, a consequence of the collapse of the volcanic edifices. In the younger islands, Tenerife, La Palma and El Hierro, the Quaternary (1.2 to 0.15 Ma) debris avalanches are readily recognizable and can be traced offshore for distances measured in tens of km. Off the older islands, Lanzarote, Fuerteventura, Gran Canaria and La Gomera (<20 to 3.5 Ma), the avalanches have been obscured by subsequent turbidity current deposition and erosion as well as hemipelagic processes. The failure offshore western Lanzarote is in the form of a ramp at the base of the insular slope bound on the seaward side by a scarp. Its size and the lack of evidence of rotation along its landwards side precludes the possibility that it is a slump. It probably represents a slide whose outer scarp is caused by break-up of the slide. Mounds on the ramp’s surface may represent post-displacement volcanic structures or exotic blocks transported to their present locations by the slide. The failures offshore Fuerteventura are so large that, although they occurred in the Miocene-Pliocene, exotic blocks displaced from upslope are still recognizable in the insular margin morphology. The Canary Island insular margin appears to be a creation of Miocene-Pliocene mass wasting and more recent turbidity current deposition and erosion, and hemilepagic deposition. Failures offshore La Gomera are due to debris flows and/or turbidity currents. These events have obscured earlier mass wasting events. An erratum to this article is available at .     

Superposed deformation straddling the continental-oceanic transition in deep-water Angola

The Angolan margin is the type area for raft tectonics. New seismic data reveal the contractional buffer for this thin-skinned extension. A 200-km-long composite section from the Lower Congo Basin and Kwanza Basin illustrates a complex history of superposed deformation caused by: (1) progradation of the margin; and (2) episodic Tertiary epeirogenic uplift. Late Cretaceous tectonics was driven by a gentle slope created by thermal subsidence; extensional rafting took place updip, contractional thrusting and buckling downdip; some distal folds were possibly unroofed to form massive salt walls. Oligocene deformation was triggered by gentle kinking of the Atlantic Hinge Zone as the shelf and coastal plain rose by 2 or 3 km; relative uplift stripped Paleogene cover off the shelf, provided space for Miocene progradation, and steepened the continental slope, triggering more extension and buckling. In the Neogene, a subsalt half graben was inverted or reactivated, creating keystone faults that may have controlled the Congo Canyon; a thrust duplex of seaward-displaced salt jacked up the former abyssal plain, creating a plateau of salt 3–4 km thick on the present lower slope. The Angola Escarpment may be the toe of the Angola thrust nappe, in which a largely Cretaceous roof of gently buckled strata, was transported seawards above the thickened salt by up to 20 km.     

Salt diapirs in the Dead Sea basin and their relationship to Quaternary extensional tectonics

Regional extension of a brittle overburden and underlying salt causes differential loading that is thought to initiate the rise of reactive diapirs below and through regions of thin overburden. We present a modern example of a large salt diapir in the Dead Sea pull-apart basin, the Lisan diapir, which we believe was formed during the Quaternary due to basin transtension and subsidence. Using newly released seismic data that are correlated to several deep wells, we determine the size of the diapir to be 13×10 km, its maximum depth 7.2 km, and its roof 125 m below the surface. From seismic stratigraphy, we infer that the diapir started rising during the early to middle Pleistocene as this section of the basin underwent rapid subsidence and significant extension of the overburden. During the middle to late Pleistocene, the diapir pierced through the extensionally thinned overburden, as indicated by rim synclines, which attest to rapid salt withdrawal from the surrounding regions. Slight positive topography above the diapir and shallow folded horizons indicate that it is still rising intermittently. The smaller Sedom diapir, exposed along the western bounding fault of the basin is presently rising and forms a 200 m-high ridge. Its initiation is explained by localized E–W extension due monoclinal draping over the edge of a rapidly subsiding basin during the early to middle Pleistocene, and its continued rise by lateral squeezing due to continued rotation of the Amazyahu diagonal fault.     

Comparison of evolutionary and static modeling of stresses around a salt diapir

We compare an evolutionary with a static approach for modeling stress and deformation around a salt diapir; we show that the two approaches predict different stress histories and very different strains within adjacent wall rocks. Near the base of a rising salt diapir, significantly higher shear stresses develop when the evolutionary analysis is used. In addition, the static approach is not able to capture the decrease in the hoop stress caused by the circumferential diapir expansion, nor the increase in the horizontal stress caused by the rise of the diapir. Hence, only the evolutionary approach is able to predict a sudden decrease in the fracture gradient and identify areas of borehole instability near salt. Furthermore, the evolutionary model predicts strains an order of magnitude higher than the strains within the static model. More importantly, the evolutionary model shows significant shearing in the horizontal plane as a result of radial shortening accompanied by an almost-equivalent hoop extension. The evolutionary analysis is performed with ELFEN, and the static analysis with ABAQUS. We model the sediments using a poro-elastoplastic model. Overall, our results highlight the ability of forward evolutionary modeling to capture the stress history of mudrocks close to salt diapirs, which is essential for estimating the present strength and anisotropic characteristics of these sediments.     

Salt tectonics in the Cap Boujdour Area,Aaiun Basin,NW Africa

Seismic reflection data indicate the Moroccan salt basin extends to the Cap Boujdour area in the Aaiun Basin. Two salt diapir structures have been identified and several areas of collapsed strata indicate probable salt removal at the shelf edge. The presence of salt in this area correlates to the conjugate southern George's Bank Basin and the Baltimore Canyon area, and it is suggested that the salt extends southward from the known salt diapir province in the George's Bank Basin southward to the Great Stone Dome. The paucity of salt diapirs is attributed to the thick carbonate and anhydrite sequence, which was deposited soon after salt deposition that inhibited halokinesis. The presence of salt along this large segment of the Atlantic margin should increase its hydrocarbon potential with traps created around salt diapirs and provision of migration pathways from deep potential source rocks in the early Cretaceous and Jurassic strata to shallower levels.     

Stress and pore pressure histories in complex tectonic settings predicted with coupled geomechanical-fluid flow models

Most of the methods currently used for pore pressure prediction in sedimentary basins assume one-dimensional compaction based on relationships between vertical effective stress and porosity. These methods may be inaccurate in complex tectonic regimes where stress tensors are variable. Modelling approaches for compaction adopted within the geotechnical field account for both the full three-dimensional stress tensor and the stress history. In this paper a coupled geomechanical-fluid flow model is used, along with an advanced version of the Cam-Clay constitutive model, to investigate stress, pore pressure and porosity in a Gulf of Mexico style mini-basin bounded by salt subjected to lateral deformation. The modelled structure consists of two depocentres separated by a salt diapir. 20% of horizontal shortening synchronous to basin sedimentation is imposed. An additional model accounting solely for the overpressure generated due to 1D disequilibrium compaction is also defined. The predicted deformation regime in the two depocentres of the mini-basin is one of tectonic lateral compression, in which the horizontal effective stress is higher than the vertical effective stress. In contrast, sediments above the central salt diapir show lateral extension and tectonic vertical compaction due to the rise of the diapir. Compared to the 1D model, the horizontal shortening in the mini-basin increases the predicted present-day overpressure by 50%, from 20 MPa to 30 MPa. The porosities predicted by the mini-basin models are used to perform 1D, porosity-based pore pressure predictions. The 1D method underestimated overpressure by up to 6 MPa at 3400 m depth (26% of the total overpressure) in the well located at the basin depocentre and up to 3 MPa at 1900 m depth (34% of the total overpressure) in the well located above the salt diapir. The results show how 2D/3D methods are required to accurately predict overpressure in regions in which tectonic stresses are important.     

Submarine canyon morphologies in the Gulf of Palermo (Southern Tyrrhenian Sea) and possible implications for geo-hazard

The continental shelf and the upper slope of the Gulf of Palermo (Southern Tyrrhenian Sea) in the depth interval ranging from 50 to 1,500 m were mapped for the first time with Multi Beam echosounder and high resolution seismic. Seven submarine canyons are confined to the upper slope or indent the shelf-edge and enter the Palermo intraslope basin at a depth of around 1,300 m. The canyons evolved through concurrent top-down turbiditic processes and bottom-up retrogressive mass failures. Most of the mass failure features of the area are related to canyon-shaping processes and only few of them are not confined to the upper slope. In general, these features probably do not represent a significant tsunami hazard along the coast. The geological element that controls the evolution of the canyons and induces sediment instability corresponds to the steep slope gradient, especially in the western sector of the Gulf, where the steepest canyons are located. The structural features mapped in the Palermo offshore contributed to the regulation of mass failure processes in the area, with direct faults and antiform structures coinciding with some of the canyon heads. Furthermore, the occurrence of pockmarks and highs that probably consist of authigenic carbonates above faulted and folded strata suggests a local relationship between structural control, fluid escape processes and mass failure. This paper presents a valuable high-resolution morphologic dataset of the Gulf of Palermo, which constitutes a reliable base for evaluating the geo-hazard potential related to slope failure in the area.     

Gas venting that bypasses the feather edge of marine hydrate,offshore Mauritania

Methane can be released from the vast marine hydrate reservoirs that surround continents into oceans and perhaps the atmosphere. But how these pathways work within the global carbon cycle now and during a warmer world is only partially understood. Here we use 3-D seismic data to identify what we interpret to be a gas venting system that bypasses the hydrate stability zone (HSZ) offshore of Mauritania. This venting is manifested by the presence of the acoustic wipe-out (AWO) across a densely faulted succession above a salt diapir and a set of morphological features including a substantial, ∼260 m wide and ∼32 m deep, pockmark at the seabed. The base of the HSZ is marked by a bottom simulating reflector (BSR) which deflects upwards above the diapir, rather than mimicking the seabed. We use a numerical modelling to show that this deflection is caused by the underlying salt diapir. It creates a trapping geometry for gas sealed by hydrate-clogged sediment. After entering the HSZ, some methane accumulated as hydrate in the levees of a buried canyon. Venting in this locality probably reduces the flux of gas to the landward limit of feather edge of hydrate, reducing the volume of gas that would be susceptible for release during a warmer world.     

The Rockall Bank Mass Flow: Collapse of a moated contourite drift onlapping the eastern flank of Rockall Bank,west of Ireland

The Rockall Bank Mass Flow (RBMF) is a large, multi-phase submarine slope failure and mass flow complex. It is located in an area where the Feni Drift impinges upon the eastern flank of the Rockall Bank in the NE Atlantic. A 6100 km² region of slope failure scarps, extending over a wide water depth range and with individual scarps reaching up to 22 km long and 150 m high, lies upslope of a series of mass flow lobes that cover at least 18,000 km² of the base of slope and floor of the Rockall Trough. The downslope lobe complex has a negative topographic relief along much of its northern boundary, being inset below the level of the undisplaced contourite drift at the base of slope. The southern margin is topographically more subtle but is marked by the sharp termination of sediment waves outside the lobe. Within the lobe complex the southern margin of the largest lobe shows a positive relief along its southern margin. The initial failure is suggested to have occurred along coherent layer-parallel detachment surfaces at depths of up to 100 m and this promoted initial downslope block sliding which in turn transformed into debris flows which moved out into the basin. The remains of a deep erosional moat linked to the onlapping contourite complex bisects the region of failed slope, and post-failure thermohaline currents have continued to modify the mass flow in this area. Differential sedimentation and erosion associated with the moat may have promoted slope instability. Following the major failure phase, continuous readjustments of the slope occurred and resulted in small-volume turbidites found in shallow gravity cores collected on the lobes. The short term trigger for the failure remains uncertain but earthquake events associated with a deep-seated tectonic lineament to the north of the mass flow may have been important. A Late Pleistocene age for the slope failure is likely. The RBMF is unusual in that it records large-scale collapse of a contourite body that impinged on a sediment-undersupplied slope system. Unlike many other large slope failure complexes along the NE Atlantic margin, the RBMF occurs in a region where there was little overloading by glacial sediment.     

New evidence of slope instability in the Outardes Bay delta area, Quebec, Canada

The Outardes Bay delta constitutes one of the best sites to study the formation of failure deposits in a modern lowstand environment. These deposits are located in a pseudo-shelf-edge position along the northern part of the Laurentian Channel in the St. Lawrence Estuary. The site has been investigated over the past 20 years with a Raytheon model RTT1000 boomer (3.5 kHz, 400 J) on the shelf, and most recently with a Simrad model EM 1000 multibeam sonar (95 kHz) on the slope to provide high-resolution seismic and bathymetric data. The seismic data show wavy, chaotic and contorted reflectors which are typical in marine environments characterised by instability features. The multibeam sonar data have revealed many slope instability features such as creep folds, channel incisions, debris flows, and rotational slide scars. Thus, these interpreted features are in direct relationship with the seismic interpretation of the data collected upslope. These geomorphological and geophysical signatures express both past and present sedimentological processes. Some of the mass movement signatures observed in the surveyed area are believed to be related with the great MS~7 Charlevoix earthquake in 1663.     

New evidence of slope instability in the Outardes Bay delta area, Quebec, Canada

The Outardes Bay delta constitutes one of the best sites to study the formation of failure deposits in a modern lowstand environment. These deposits are located in a pseudo-shelf-edge position along the northern part of the Laurentian Channel in the St. Lawrence Estuary. The site has been investigated over the past 20 years with a Raytheon model RTT1000 boomer (3.5 kHz, 400 J) on the shelf, and most recently with a Simrad model EM 1000 multibeam sonar (95 kHz) on the slope to provide high-resolution seismic and bathymetric data. The seismic data show wavy, chaotic and contorted reflectors which are typical in marine environments characterised by instability features. The multibeam sonar data have revealed many slope instability features such as creep folds, channel incisions, debris flows, and rotational slide scars. Thus, these interpreted features are in direct relationship with the seismic interpretation of the data collected upslope. These geomorphological and geophysical signatures express both past and present sedimentological processes. Some of the mass movement signatures observed in the surveyed area are believed to be related with the great MS~7 Charlevoix earthquake in 1663.     

Salt structures in the Laizhouwan depression, offshore Bohai Bay basin, eastern China: New insights from 3D seismic data

Using the new high-quality 3D seismic data, this paper addresses the salt structures in the KL11 area of the Laizhouwan depression in the southern offshore Bohai Bay basin. In the study area, the salt in the Sha-4 Member of the Paleogene Shahejie Formation thickened, and then formed an S–N trending salt wall, which changes shape regularly along its trend from salt diapir to salt pillow. The change in thickness of the suprasalt layers record five growth phases of the salt wall from the Eocene to the Quaternary: (1) early diapirism, (2) active diapirism, (3) passive diapirism, (4) relative structural quiescence, and (5) arching. The evolution of the salt structures was mostly governed by the multi-phase compression induced by the dextral strike-slip of the Tan–Lu fault, which formed a restraining bend in the study area. There was an original passive stock in the south, which was later tectonically squeezed by E–W compression and became a diapir. As the shortening propagated to the north from the original stock, the salt pillow was created in the north. Relative structural quiescence then followed until the next phase of compression, which arched the thick roof of the salt wall.     

Sediment transport on subaqueous fan delta slopes, Britannia Beach, British Columbia

A high-resolution acoustic survey over a fjord side fan delta revealed distinctive bottom features resulting from slope instability processes. Delta-front chutes occurring on slopes of l3° are partially filled with radiating splays of coarse-grained sediment, apparently transported downslope by coarse-grained debris flows that originated on the subaerial slopes above the fan. Arcuate scarp patterns represent shallow successive, rotational slides, with numcrous small displacements of individual blocks and slabs of sediment. Blocky, ridged depositional areas occur at the base of the fan delta, but there is no evidence of long-distance mass movement farther downfjord.     

Mechanisms of collapse of the cretaceous helvetiafjellet formation at Kvalvågen,eastern Spitsbergen

Coastal cliffs at Kvalvågen, eastern Spitsbergen, expose palaeolandslide blocks and related slope failure features that record local collapse of an otherwise undisturbed succession of paralic sediments. The collapse occurred along a shallow sloping shelf at the edge of an epicontinental sea in the Early Cretaceous. The event was coincident with a rise in relative sea level along a coast that had just previously experienced a major paralic regression across a muddy marine environment. The low relief environment in which the slope failure structures formed, as well as the timing of the collapse raise questions regarding the cause of the topographic instability and the possible influence of sea level changes or tectonic activity. These outcrops have been previously interpreted as the collapse of a deltaic system triggered by local seismic activity or collapse of a shelf break in the headwall regions of submarine canyons. This paper presents new structural and stratigraphic data that refine and adjust the previous interpretations through the evaluation of a variety of possible mechanisms for collapse based on the new data. Our data suggest that active delta deposition was not occurring at Kvalvågen at the time of collapse and that the collapse was likely due to allogenic forcing. Despite the possible influence of pore fluids and rheological controls on the collapse, we find that the geometry and kinematics of landslide slip planes, synsedimentary folds, and other slope failure related features require over-steepened topography and that these are most consistent with westward-directed collapse off of a north striking escarpment with elevated topography on the east side. Fault exposures, a large contrast in palaeo-elevation, and liquifaction features support previous interpretations of a tectonic cause for the collapse and suggest that this topographic feature may have been a fault or fold scarp. This study demonstrates the importance of combining stratigraphic and sedimentological data with structural data and kinematic analysis in the interpretation of sedimentary processes.     

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.