Input of Glaciomarine Sediments along the East Antarctic Continental Margin; Depositional Processes on the Cosmonaut Sea Continental Slope and Rise and a Regional Acoustic Stratigraphic Correlation from 40° W to 80° E

Multichannel seismic reflection data from the Cosmonaut Sea margin of East Antarctica have been interpreted in terms of depositional processes in the continental slope and rise area. A major sediment lens is present below the upper continental rise along the entire Cosmonaut Sea margin. The lens probably consists of sediments supplied from the shelf and slope, being constantly reworked by westward flowing bottom currents, which redeposited the sediments into a large scale drift deposit prior to the main glaciogenic input along the margin. High-relief semicircular or elongated depositional structures are also found on the upper continental rise stratigraphically above the regional sediment lens, and were deposited by the combined influence of downslope and alongslope sediment transport. On the lower continental rise, large-scale sediment bodies extend perpendicular to the continental margin and were deposited as a result of downslope turbidity transport and westward flowing bottom currents after initiation of glacigenic input to the slope and rise. We compare the seismostratigraphic signatures along the continental margin segments of the adjacent Riiser Larsen Sea, the Weddell Sea and the Prydz Bay/Cooperation Sea, focussing on indications that may be interpreted as a preglacial-glaciomarine transition in the depositional environment. We suggest that earliest glaciogenic input to the continental slope and rise occurred in the Prydz Bay and possibly in the Weddell Sea. At a later stage, an intensification of the oceanic circulation pattern occurred, resulting in the deposition of the regional plastered drift deposit along the Cosmonaut Sea margin, as well as the initiation of large drift deposits in the Cooperation Sea. At an even later stage, possibly in the middle Miocene, glacial advances across the continental shelf were initiated along the Cosmonaut Sea and the Riiser Larsen Sea continental margins.     

Recognition of contour-current influence in mixed contourite-turbidite sequences of the western Weddell Sea,Antarctica

Sedimentary processes and structures across the continental rise in the western Weddell Sea have been investigated using sediment acoustic and multichannel seismic data, integrated with multibeam depth sounding and core investigations. The results show that a network of channels with associated along-channel ridges covers the upper continental slope. The seismic profiles reveal that the channels initially developed as erosive turbidite channels with associated levees on their northern side due to Coriolis force. Later they were partly or fully infilled, probably as a result of decreasing turbidite activity. Now the larger ones exist as erosive turbidite channels of reduced size, whereas the smaller ones are non-erosive channels, their shape being maintained by contour current activity. Drift bodies only developed where slumps caused a distinctive break in slope inclination on the upper continental rise, which served to initiate the growth of a drift body fed by contour currents or by the combined action of turbidites and contourites. The history of sedimentation can be reconstructed tentatively by correlation of seismo-stratigraphic units with the stages of evolution of the drifts on the western side of the Antarctic Peninsula. Three stages can be distinguished in the western Weddell Sea after a pre-drift stage, which is delimited by an erosional unconformity at the top: (1) a growth stage, dominated by turbidites, with occasional occurrence of slumps during its initial phase; (2) during a maintenance stage turbiditiy-current intensity (and presumably sedimentation rate also) decreased, probably as a result of the ice masses retreating from the shelf edge, and sedimentation became increasingly dominated by contour current activity; and (3) a phase of sheeted-sequence formation. A southward decrease in sediment thickness shows that the Larsen Ice Shelf plays an important role in sediment delivery to the western Weddell Sea. This study shows that the western Weddell Sea has some characteristics in common with the southern as well as the northwestern Weddell Sea: contour currents off the Larsen Ice Shelf have been present for a long time, probably since the late Miocene, but during times of high sediment input from the shelves as a result of advancing ice masses a channel-levee system developed and dominated over the contour-current transport of sediment. At times of relatively low sediment input the contour-current transport dominated, leading to the formation of drift deposits on the upper continental rise. Seaward of areas without shelf ice masses the continental rise mainly shows a rough topography with small channels and underdeveloped levees. The results demonstrate that sediment supply is an important, maybe the controlling factor of drift development on the Antarctic continental rise.     

The Cosmonaut Sea Wedge

A set of multi-channel seismic profiles (∼15,000 km) is used to study the depositional evolution of the Cosmonaut Sea margin of East Antarctica. We recognize a regional sediment wedge, below the upper parts of the continental rise, herein termed the Cosmonaut Sea Wedge. The wedge is situated stratigraphically below the inferred glaciomarine section and extends for at least 1,200 km along the continental margin with a width that ranges from 80 to about 250 km. The morphology of the wedge and its associated depositional features indicate a complex depositional history, where the deep marine depositional sites were influenced by both downslope and alongslope processes. This interaction resulted in the formation of several proximal depocentres, which at their distal northern end are flanked by elongated mounded drifts and contourite sheets. The internal stratification of the mounded drift deposits indicates that westward flowing bottom currents reworked the marginal deposits. The action of these currents together with sea-level changes is considered to have controlled the growth of the wedge. We interpret the Cosmonaut Sea Wedge as a composite feature comprising several bottom current reworked fan systems. The wide spectrum of depositional geometries in the stratigraphic column reflects dramatic variations in sediment supply from the continental margin as well as varying interaction between downslope and alongslope processes.     

Echo character of the western equatorial Atlantic floor and its relationship to the dispersal and distribution of terrigenous sediments

The floor of the western equatorial Atlantic Ocean can be divided into several distinct provinces based on detailed characteristics of the bottom echos recorded with short-ping (< msec.) 3.5 and 12 kHz sound sources. Two major types of echos are recorded: (I) distinct echos; and (II) indistinct echos.Indistinct echos can be further sub-divided into (A) continuous prolonged echos; and (B) hyperbolic echos. Each class of echos contains two or more unique echo types. The regional distributions of the various echo types recorded from the continental rise, Amazon Cone, and abyssal plains reveal much information about sedimentary processes.In the western equatorial Atlantic, hyperbolic echos are recorded only from small, isolated portions of the continental rise. This contrasts with the continental rise of the western North Atlantic where previous investigators have shown that hyperbolic echos parallel bathymetric contours along the entire rise and thus reflect shaping of the rise by geostrophic contour currents (Heezen et al., 1966; Hollister, 1967). The fact that regions of hyperbolic echos show little or no relationship to bathymetric contours of the continental rise of the western equatorial Atlantic suggests that contour currents have been unimportant in shaping the rise in this region.The three most widespread echo types recorded from the continental rise, Amazon Cone, and abyssal plains reveal much information about terrigenous sediment dispersal and deposition in the western equatorial Atlantic. Comparison of the thicknesses and frequencies of coarse (silt- to gravel-size), bedded, terrigenous sediment in piston cores with the echo type recorded at each coring site shows a correlation between echo type and the relative amount of coarse, bedded sediment within the upper few meters of the sea floor. The regional distributions of these three echo types indicate that dispersal of coarse terrigenous sediment has been downslope across the continental rise and Amazon Cone to the abyssal plains via gravity-controlled sediment flows. The Amazon River is the major sediment source and most coarse sediment is deposited on the lower Amazon Cone and proximal portions of the Demerara abyssal plain.     

A Compilation of Geophysical Data from the Lazarev Sea and the Riiser-Larsen Sea, Antarctica

On the basis of new geophysical data acquired by the Federal Institute of Geosciences and Natural Resources (BGR) and the Polar Marine Geological Research Expedition (PMGRE) as well as existing data new geophysical maps were compiled for the Lazarev Sea and the Riiser-Larsen Sea between 10°W and 25°E. The new results are: – The drastic change in the strike direction of the volcanic Explora Wedge between longitudes 10°W and 5°W is accompanied with a gradual change from one major wedge, i.e. the Explora Wedge, into at least two wedge-shaped volcanic constructions, each manifested by a sequence of seaward-dipping reflectors in the seismic records. – The southern Lazarev Sea is best described as a continental margin affected by multiple rifting episodes accompanied with transient volcanism. – A distinct N80°E striking basement depression separates the volcanic-prone continental margin of the southern Lazarev Sea from oceanic crust upon which the Maud Rise rests. The southern scarp of the narrow depression was presumably aligned with the eastern scarp of the Mozambique Ridge during the Early Cretaceous. – The Astrid Ridge proper occupies the transition from the volcanic-prone continental margin of the Lazarev Sea to old oceanic crust of the Riiser -Larsen Sea, and it rests upon a large volcanic apron which covers the basement of the southwestern Riiser-Larsen Sea. – No evidence was found that prolific volcanism has affected the early opening of the Riiser-Larsen Sea. – The Lazarev Sea is a sediment-starved region.     

Physiography of the Exmouth and Scott Plateaus,Western Australia,and adjacent northeast Wharton Basin

The continental margin of Western Australia is a rifted or “Atlantic”-type margin, with a complex physiography. The margin comprises a shelf, an upper and lower continental slope, marginal plateaus, a continental rise, and rise or lower slope foothills. Notches or terraces on the shelf reflect pre-Holocene deposition of prograded sediment, whose seaward limit was determined by variations in relative sea level, wave energy, and sediment size and volume. The upper continental slope has four physiographic forms: convex, due to sediment outbuilding (progradation) over a subsiding marginal plateau; scarped, due to erosion of convex slopes; stepped, due to deposition at the base of a scarped slope; and smooth, due to progradation of an upper slope in the absence of a marginal plateau. Lying at the same level as the upper/lower slope boundary are two extensive marginal plateaus: Exmouth and Scott. They represent continental crust which subsided after continental rupture by sea-floor spreading. Differential subsidence, probably along faults, gave rise to the various physiographic features of the plateaus. The deep lower continental slope is broken into straight northeasterly-trending segments, that parallel the Upper Jurassic/Lower Cretaceous rift axis, and northwesterly-trending segments that parallel the transform direction. The trends of the slope foothills are subparallel to the rift direction. The four abyssal plains of the region (Perth, Cuvier, Gascoyne and Argo) indicate a long history of subsidence and sedimentation on Upper Jurassic/Lower Cretaceous oceanic crust.     

Crustal structure and tectonic provinces of the Riiser-Larsen Sea area (East Antarctica): results of geophysical studies

About 16,000 km of multichannel seismic (MCS), gravity and magnetic data and 28 sonobuoys were acquired in the Riiser-Larsen Sea Basin and across the Gunnerus and Astrid Ridges, to study their crustal structure. The study area has contrasting basement morphologies and crustal thicknesses. The crust ranges in thickness from about 35 km under the Riiser-Larsen Sea shelf, 26–28 km under the Gunnerus Ridge, 12–17 km under the Astrid Ridge, and 9.5–10 km under the deep-water basin. A 50-km-wide block with increased density and magnetization is modeled from potential field data in the upper crust of the inshore zone and is interpreted as associated with emplacement of mafic intrusions into the continental margin of the southern Riiser-Larsen Sea. In addition to previously mapped seafloor spreading magnetic anomalies in the western Riiser-Larsen Sea, a linear succession from M2 to M16 is identified in the eastern Riiser-Larsen Sea. In the southwestern Riiser-Larsen Sea, a symmetric succession from M24B to 24n with the central anomaly M23 is recognized. This succession is obliquely truncated by younger lineation M22–M22n. It is proposed that seafloor spreading stopped at about M23 time and reoriented to the M22 opening direction. The seismic stratigraphy model of the Riiser-Larsen Sea includes five reflecting horizons that bound six seismic units. Ages of seismic units are determined from onlap geometry to magnetically dated oceanic basement and from tracing horizons to other parts of the southern Indian Ocean. The seaward edge of stretched and attenuated continental crust in the southern Riiser-Larsen Sea and the landward edge of unequivocal oceanic crust are mapped based on structural and geophysical characteristics. In the eastern Riiser-Larsen Sea the boundary between oceanic and stretched continental crust is better defined and is interpreted as a strike-slip fault lying along a sheared margin.     

Morphology and Late Quaternary sedimentation in the Gulf of Oman Basin

The morphology of the Gulf of Oman Basin, a 3,400 m deep oceanic basin between Oman and southern Pakistan and southern Iran, ranges from a convergent margin (Makran margin) along the north side, a passive type (Oman margin) along the south side, translation types along the basin's west (Zendan Fault-Oman Line) and east (Murray Ridge) sides and a narrow continental rise and a wide abyssal plain in the centre of the basin. Sediment input into the basin during the Late Quaternary has been mainly from the north as a result of the uplift of the Coast Makran Mountains in the Late Miocene-Pliocene. Today most of this detritrus is deposited on the shelf and upper continental slope and perched basins behind the fold/fault ridges on the lower slope. The presence of fans and channels on the continental rise on the north side of the basin indicate, however, that continental derived debris was, and possibly is, being transported to the deep-sea by turbidity currents via gaps in the ridges on the lower slope. In addition to land derived terrigenous sediments, the basin deposits also contain biogenic (organic matter and calcium carbonate), eolian detritus and hydrates and authigenic carbonates from the tectonic dewatering of the Makran accretionary wedge. The eolian sediment is carried into the Gulf of Oman Basin from Arabia and the Mesopotamia Valley by the northwesterly Shamal winds. This type of detritus was particularly abundant during the glacial arid periods 21,000–20,000 and 11,000 (Younger Dryas) years ago when exposure of the Persian (Arabian) Gulf increased the area of dust entrainment and shifted the position of the source of the eolian sediments closer to the basin.     

Structural lineations in the Canary basin,eastern central North Atlantic

A corridor 315 km wide centered along the southeast projection of the Atlantis fracture zone between 21°W and 29°W was investigated with seismic reflection, bathymetric, gravity, and magnetic profiles. Six sub-parallel, sediment-filled troughs in acoustic basement trend about 106° across the abyssal hills and lower continental rise off northwest Africa. Where the southernmost structural lineations cross the abyssal plain, they are interrupted by a ridge trending 080° surmounted by volcanic peaks.The structural lineations become less distinct landward of the western margin of the abyssal plain coincident with a decrease in topographic relief on acoustic basement and increasing sediment thickness. This transition is coincident with a reduction in the amplitude of gravity and magnetic anomalies.     

Structure and morphology of the west Reykjanes basin and the southeast Greenland continental margin

The continental-shelf morphology is dominated by glacial erosion and deposition. Erosion is prominent on the near-shore shelf and deposition along the outer shelf edge. The continental slope is characterized by delta-shaped progradations (glaciomarine-sediment fans) seaward of the shelf channels. Canyons cross the continental slope only in the region southeast of Cape Farewell. The continental rise is incised by a number of submarine canyons. Broad sediment ridges on the upper continental rise are probably canyon-eroded remains of extensive Plio-Pleistocene fans. A mid-ocean channel which crosses the continental rise is possibly related to the axis of maximum depth of Denmark Strait. Despite the presence of strong bottom currents, there is no indication of depositional sediment drifts along the continental margin of Greenland between Cape Farewell and Denmark Strait. This may be a function of high current velocity or low sediment load.Sea floor older than 60 m.y. B.P. is present just seaward of the Greenland continental margin implying either downwarped continental material or an early rift formed prior to the separation of Greenland from the European plate. A left lateral offset of anomalies 20 and 21 at 65°N indicates a major fracture zone related to the Greenland continental margin offset nearby.     

Determination of sediment volumes, accumulation rates and turbidite emplacement frequencies on the Madeira Abyssal Plain (NE Atlantic): a correlation between seismic and borehole data

The sedimentary infill history of the Madeira Abyssal Plain (MAP) is established from correlation of ODP Leg 157 drillsites (Sites 950–952) with an almost regular grid of 7000 km of intermediate-resolution seismic reflection profiles covering the central part of the abyssal plain. The most conspicuous seismic reflectors bounding the seismostratigraphic units have been identified and mapped. Correlation between seismic and borehole data using synthetic seismograms allows the lithological attribution and dating of the reflectors and seismostratigraphic units. Lateral mapping and correlation of seismic units also allows both the volumes and rates of accumulation of sediments within each seismostratigraphic unit and equivalent time periods of deposition to be determined. These calculations have been corrected for the effect of compaction, calculated at around 40% at the base of the drillholes. Three main turbidite types have been identified at the drillsites and their emplacement frequency has been calculated for each site and time period. Our results show that Cretaceous oceanic crust was draped with red pelagic clays, and the fracture-zone valleys were completely infilled and levelled in a geologically rather short time, probably during the latest Oligocene and Early Miocene, by organic-rich turbidites derived from the NW African continental margin. At 16 Ma, the topography was levelled enough to allow large turbidity current flows to cover the entire plain. During the Middle and Late Miocene (16–5.9 Ma), organic-rich turbidites were emplaced on the abyssal plain at a low rate of accumulation (12 m/my). In the uppermost Miocene–Early Pliocene (5.9–3.6 Ma), turbidite emplacement increased markedly in both frequency and accumulation rate (e.g., 26 m/my for organic-rich turbidites). During this time, period emplacement of volcanic-rich turbidites also increased in volume and frequency, a trend that continued into the Pliocene. Increased volcanic-rich turbidite emplacement correlates well with increased volcanic activity on the Canary Islands, and increased organic-rich turbidite emplacement may correlate with periods of erosion on the NW African continental margin. These erosional periods may be related to global cooling and falling sea level, intensification of bottom-water currents, and enhanced upwelling on the margin.     

The abyssal bounty fan and lower Bounty Channel: evolution of a rifted-margin sedimentary system

The Bounty Channel and Fan system provides the basis for a model for deep-sea channel and fan development in a rifted continental margin setting. The sedimentary system results from an interplay between tectonics (fan location; sediment source), turbidity currents (sediment supply), geostrophic currents (sediment reworking and distribution) and climate (sea level, and hence sediment supply and type). Today, sediment is shed from the collisional Southern Alps, part of the Pacific/Indo-Australian plate margin, and passes east across the adjacent shelf and into the Otago Fan complex at the head of the Bounty Trough. Paths of sediment supply, and locations of sediment deposition, are controlled by the bathymetry of the Bounty Trough, with axial slopes as high as 37 m/km (2°) towards the trough head, diminishing to around 3.5 m/km (0.2°) along the trough axis. The Bounty Fan is located 800 km further east, where the Bounty Channel debouches onto abyssal oceanic crust at the mouth of the Bounty Trough. The Bounty Fan comprises a basement controlled fan-channel complex with high leveed banks exhibiting fields of mud waves, and a northward-elongated middle fan. Channel-axis gradients diminish from 6 m/km (0.35°) or more on the upper fan to less than 1 m/km (<0.06°) on the lower fan. Parts of the left bank levee and almost the entire middle fan are being eroded and re-entrained within a Deep Western Boundary Current (DWBC), which passes along the eastern New Zealand margin at depths below 2000 m. The DWBC is the prime source of deep, cold water flow into the Pacific Ocean, with a volume of ca. 20 Sv and velocities up to 4 cm/s or greater. The mouth of the Bounty Channel, at a depth of 4950 m at the south end of the middle fan, acts as a point source for an abyssal sediment drift entrained northward under the DWBC at depths below 4300 m. The Bounty Fan probably originated in the early to middle Neogene, but has mostly been built during the last 3 Myr (Plio-Pleistocene), predominantly as climate-controlled sedimentary couplets of terrigenous, micaceous mud (acoustically reflective; glacial) and biopelagic ooze (acoustically transparent; interglacial), deposited under the pervasive influence of the DWBC.     

Sonar acoustic facies and sediment distribution on an area of the deep ocean floor

Long-range sidescan sonar can be used to map sediment distributions over wide expanses of deep ocean floor. Seven acoustic facies that arise from differing sediment or rock types have been mapped over the low-relief Saharan continental rise and Madeira abyssal plain. These have been calibrated with sampling, profiling and camera studies and the facies can be traced confidently on a regional scale using the sidescan data. The mapping of the sediment distribution shows that a complex interplay of turbidity current and debris flow processes can occur at a continental rise/abysaal plain transition over 1000 km from the nearest continental slope.     

海盆沉积“源-汇”系统分析:南海北部珠江海谷-西北次海盆第四纪深水浊积扇

运用近年来采集的高分辨率地震资料和多波束测深数据,在珠江海谷及西北次海盆深海平原区发现大规模发育的第四纪重力流沉积体系,该沉积体系沿珠江海谷以北西-南南东方向贯穿整个北部陆坡,进入西北次海盆后呈扇形展开,形成珠江海谷-西北次海盆大型深水浊积扇系统。据沉积体系空间展布特征差异,将珠江海谷划分为北、中、南三段,北段为过路侵蚀和水道下切,中段以水道充填和天然堤沉积为主,南段以水道-天然堤和朵叶体沉积共存为特征,揭示出北部陆坡珠江海谷是珠江口外陆缘物质输送海盆深海平原的主要通道;海盆区总体以朵叶体发育为特色,呈扇形展布。深水扇系统可分为三期次沉积体,其区域结构记录了重力流沉积物从侵蚀、卸载到南海海盆作为限制性盆地接收陆源沉积物的全过程,为“源-渠-汇”的研究构建了一个完美的范例。本文以珠江海谷-西北次海盆第四纪深水浊积扇沉积体系为例,完整地揭示了水道-扇体的组构和特征,清晰呈现了陆坡-海盆砂体展布的规律,可为建立南海北部新近纪早期深水扇形成模式提供参考,有助于指导南海深水油气勘探工作。     

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).     

Echo character of the East Brazilian continental margin and its relationship to sedimentary processes

The nature and regional distributions of various types of bottom echoes recorded on 3.5-kHz echograms from the East Brazilian continental margin (8–30°S) provide valuable information about sedimentary processes which have been active on a regional scale. The ten types of echoes observed fall into two major classes: distinct and indistinct. Indistinct echoes have two sub-classes; prolonged and hyperbolic. A qualitative correlation is observed between three types of distinct and indistinct-prolonged echoes and the relative abundance of coarse, bedded sediment (silt, sand, gravel) in piston cores. Regions returning distinct echoes with continuous parallel sub-bottoms contain little or no coarse sediment; regions returning indistinct very prolonged echoes with no sub-bottoms contain very high concentrations of coarse sediment; and regions returning indistinct semiprolonged echoes with intermittent sub-bottoms contain moderate or intermediate amounts of coarse sediment. Thus the regional distributions of these three echo types reflect the dispersal of coarse terrigenous sediment throughout the region. High concentrations of coarse sediment are restricted to relatively small areas which are generally proximal to large deep-sea channels, whereas very low concentrations occur in distal regions such as the lowermost continental rise and adjacent abyssal plain. Moderate concentrations of coarse sediment occur throughout most of the continental rise. Five of the six types of hyperbolic echoes observed are reflected from erosional/depositional bed forms. Although some of these bed forms (especially on the upper continental rise) have probably been produced by gravity-controlled mass flows (turbidity currents, slumps, etc.) the fact that the most extensive and widespread regions of hyperbolic echoes occur in distal regions beneath the present axis of flow of the Antarctic Bottom Water suggests that most of these bed forms are the result of sediment reworking by the contour-following bottom currents of this water mass.     

Sedimentary processes in the Stromboli Canyon and Marsili Basin, SE Tyrrhenian Sea: results from side-scan sonar surveys

 Sedimentary processes in the Stromboli Canyon and in the Marsili Basin are studied on the basis of side-scan sonographs. The basin margins are characterized by slump scars, gullies, channels, and large debrites on the Calabrian slope and by straight chutes of fast downslope sediment transport and blocky–hummocky avalanche deposits on the flanks of the Stromboli volcano. In the Stromboli Canyon and in minor deep-sea channels, sediment transport by turbidity currents generates sediment waves. Between the basin margins and the abyssal plain, the outcropping volcanic basement traps part of the sediment coming from the marginal areas. The abyssal plain is characterized by low relief lobes and ponded sediments.     

The Meriadzek Terrace: A marginal plateau

The Meriadzek Terrace forms part of the continental margin in the Bay of Biscay at a depth between that of the Continental shelf and the abyssal plain. Reflection profiles show that it is bounded on either side by basement ridges with a sediment infill between the ridges. It was probably formed by downfaulting of the continental shelf, possibly connected with the opening of the Bay of Biscay.     

Variation of sediment geotechnical properties between the greater antilles outer ridge and the nares abyssal plain

Abstract

It is clear from morphology alone that distinctly different dynamic and sedimentary processes can be expected to be associated with the Greater Antilles Outer Ridge relative to those of the adjacent Nares Abyssal Plain. This difference is further substantiated by seismic reflection data which show the ridge to be a very large prism of acoustically transparent sediment in contrast to the stratified deposits of the abyssal plain. An examination of the geotechnical properties of the near‐surface (0 to 2.4m) deposits of the two areas also reveals distinct differences in their sedimentological characteristics. The outer ridge sediments, of more or less homogenous clay‐size material, display much higher water contents, porosities, sensitivities, plasticity, and organic carbon contents in contrast to the abyssal plain deposits which are much less homogenous owing to the presence of turbidite sequences. The turbidites themselves are uniquely contrasted to the other abyssal plain sediments by their higher silt content, wet bulk density, shear strength, and sensitivity.