Submarine Valleys Research Articles

Dismantling of an isolated tropical carbonate platform through flank collapse and canyon erosion, Coral Sea, Northeast Australia

The steep slopes of carbonate platforms frequently display large-scale sediment destabilization features like rockfalls, mass transport complexes, and slope erosion. The processes and factors triggering such instabilities and how they interact are a matter of ongoing discussion. We use hydroacoustic, sedimentological, and seafloor imaging data to map and characterize slope instabilities and potential controlling factors at the flank of the isolated Tregrosse carbonate bank in the Coral Sea, northeast Australia. Erosion of gullies and submarine valleys is concentrated in slope segments with the platform rim at several 10s of meters of water depth, i.e. where there is potential for sediment transfer from the bank interior to the slope. Gravity core data indicate that most sediment export from the platform occurs during sea-level fall. The toe of slopes neighboring segments with a shallower platform rim are mostly characterized by mass-transport complexes of platform rim and upper slope rocks forming extended block fields. Distal slope areas are dismantled through submarine landslides resulting in scalloped head scarps. The basal detachment surface of these submarine landslides appears to be rooted in several 100 s of meters in the subsurface at a lithological heterogeneity, which is documented by a gamma-ray peak in the downhole logging data from Ocean Drilling Program Site 817. Our findings show that (1) canyon erosion, (2) platform rim and upper slope destabilization as well as (3) lower slope dismantling, largely act independently of each other to destabilize the flanks of the carbonate bank. The complexity of the carbonate platform dismantling processes and the corresponding controlling factors shown in this study should also be considered when interpreting seismic morphological data.

Giant sediment wave fields adjacent to debris-flow filled deep sea valleys: New evidence of cohesive flows transforming into dilute turbidity currents

Sediment waves are subaqueous sedimentary figures belonging to the supercritical flow domain and are of growing interest to the scientific community and industry. They are ubiquitously observed on the seafloor of world's oceans, as well as in the stratigraphic record imaged by marine seismic datasets. In this study we focus on the Cenozoic strata offshore Ivory Coast, where giant sediment waves developed at the base of slope range in height and wavelength: 10–100 m and 1–6 km, respectively. Sediment waves fields in this study developed simultaneously and adjacent to wide, rectilinear valleys, filled by mass-transport deposits. Thus, sediment waves serve as a rare example of large-scale deep-water cyclic steps formed through phase transformation (water entrainment and dilution) of laminar debris flows.The lithological nature of sediment waves can be estimated through the observation of polygonal faulting affecting the sediment waves fields, which suggest a dominant abundance of fine-grained material (clay and silt-prone). This study also shows that wide submarine valleys flanked by sediment waves do not necessarily correspond to sand-prone depositional systems, and that their potential to hold reservoir units for hydrocarbon exploration or CO2 storage should be evaluated with caution when in lower resolution datasets are used.

The Paleo-Lupar: Bathymetric and seismic evidence for a submerged Late Pleistocene river valley on the Sunda Shelf, off Sarawak

This article presents bathymetric and seismic evidence of a submerged Late Pleistocene river valley on the Sunda Shelf, off Sarawak. The submarine feature identified as the Paleo-Lupar is detectable in digital elevation models (DEM) at water depths of about 40 m near the Lupar river mouth to more than 100 m to the north nearer the shelf edge. Bathymetric profiles revealed an ~30 km-wide submarine valley, with individual channels that, in places, can be more than 80 m deeper than the surrounding seabed. The Paleo-Lupar is one of several major paleo-rivers that drained the Sunda Shelf when sea level was lowest during the Last Glacial Maximum (LGM) around 21 ka BP and were subsequently submerged due to the sea level rise that has since occurred.

Paleovalleys and paleodepressions of the Kara Sea Vostochno-Prinovozemelsky area

U-shaped and V-shaped palleovalleys and paleodepressions are identified on multifrequency seismic data in near-surface section (NSS) within three study areas in Vostochno-Prinovozemelsky area. Valleys of the Late Cretaceous and Quaternary ages and of different genesis are found in the first area of study. The palleovalleys of the Late Cretaceous age are V-shaped and U-shaped by its characteristic morphological properties and are likely formed as a result of the rivers erosional activity. Burried valleys filled with Quaternary glacial sediments are similar to U-shaped paleovalleys, genesis of which is usually connected to glacial plucking in the Late Quaternary glaciation. The tunnel valleys of Quaternary age result from impact of ice sheet existing to the east of the Novaya Zemlya archipelago in the Late Quaternary were revealed in the second study area. In contrast to the first two sites, isometric depressions of Quaternary age and controversial genesis were revealed in the third one. The direction of the identified paleovalleys in some cases coincides with the submarine valleys indicated on the USSR Northern Seas submarine valleys map [Lastochkin, 1977] and the Kara Sea geomorphological map [Miroshnikov et al., 2021].

Geochemistry of the Laptev and East Siberian seas sediments with emphasis on rare-earth elements: Application for sediment sources and paleoceanography

Deciphering the origin of sedimentary material in the Arctic can contribute to a deeper understanding of modern transport and sedimentation processes. Here, the application of rare earth elements (REEs) is particularly promising, as they can serve as indicators of sources as well as chemical and physical conditions and processes. This study aimed to determine the distribution of REE sources in the bottom sediments and the forms of their occurrence on the East Siberian Arctic Shelf. The concentrations and distributions of the REEs and major and trace metals, grain size, and mineral composition of 72 surficial sediment samples and two sediment cores that were collected from the Laptev and East Siberian seas were studied. Increasing REE content, the predominance of light REE, and close correlation with Zr, Nb, Y, Sc, and Ti were revealed in the Laptev and western East Siberian sea sediments in the zone of influence of the Lena River runoff. The source of the REEs was the eroded Yedoma complex along the coast. The REE content in the sub-colloidal fraction of the sediments was comparable to the REE concentration of the bulk bottom sediments and varied in the East Siberian Sea from 104 to 220 mg kg−1, with the maximum content around the submarine valley of the Indigirka River, in the Laptev Sea, ranging from 124 to 197 mg kg−1. The findings could be used to study ice dynamics and to conduct lithostratigraphic and paleoceanographical reconstructions of the Central Arctic.

Analysis of Engineering and Geological Conditions of International Submarine Optical Fiber Cable Routing in the East China Sea Section

The East China Sea section of the Chinese international submarine optical fiber cable generally starts from the Shanghai coast, crosses the East China Sea shelf and continental slope to the east, and intersects with the international optical fiber cable system in Okinawa Trough. In this paper, the regional tectonic, seismic, and other environmental conditions of the East China Sea section of the international submarine optical cable are briefly described based on the previous data. Additionally, the geological engineering conditions of the international optical cable routing in the East China Sea area will be analyzed based on the field investigation data such as multibeam, side-scan sonar, shallow stratigraphic profile, sediment sampling, and cone penetration test (CPT). It is found that the main geological factors in the East China Sea shelf area are submarine scouring, sandwave, shallow gas, and hard seabed; in the land slope area are steep slope, submarine valley, rock, landslide, and shallow gas (pockmark); and in the Okinawa Trough area are gully, sea mound/seamount, rock, shallow gas, fault, and hard strata outcrop. This paper briefly analyzes the impact of each geological disaster factor on the submarine cable project and proposes solutions for providing essential information and some scientific basis for designing, constructing, operating, and maintaining international submarine optical fiber cables in the East China Sea area.

Glacial Troughs Enhance Shelf/Slope Exchange in the Barotropic Limit

AbstractGlacial troughs are flat‐bottomed, steep‐sided submarine valleys that incise the shelf and significantly alter coastal circulation. We examine how these features drive exchange between the shelf and the slope in the barotropic and linear limits. When the alongshore flow is in the Kelvin‐wave (downwave/downwelling favorable) direction, the troughs move transport from the shelf upwave of the trough to the slope downwave of the trough, diminishing wind‐driven alongshore transport on the shelf downwave of the trough. Conversely, when the alongshore flow is against the Kelvin wave direction (upwave/upwelling favorable), the troughs move transport from the slope downwave of the trough to the shelf upwave of the trough. These cross‐shelf flows are driven by the acceleration and curvature of the flows induced by the narrowing and turning isobaths around the trough, and the bottom friction experienced by these accelerated flows. These dynamics are quantified by examining the along‐isobath evolution of potential vorticity in the model's limits.

Structural characteristics of the Chiwei Platform and their implications for the back-arc rifting of the Okinawa Trough, East China Sea

The Chiwei Platform is an isolated submarine topographic high located on the northwestern slope of the southern Okinawa Trough. It is separated from the East China Sea Shelf by a wedge-shaped submarine valley corresponding to a deep basin (Chiwei Depression). Newly acquired seismic reflection profiles and P-wave velocity models demonstrate that the acoustic basement of the platform is shallow and only covered by thin sediments or is even exhumed. Similarities in seismic reflection and P-wave velocity structures between the Chiwei Platform, East Chiwei Seamount and Diaoyudao Uplift suggest that the Chiwei Platform is a high-standing relict basement high formed during the back-arc rifting of the Okinawa Trough. It lies in the transition zone in which the directions of the back-arc rifting centers clockwise rotated from the NE-SW to E-W southwestwardly. Southwest of the transition zone, the southern Diaoyudao Uplift is left-laterally shifted relative to the middle Diaoyudao Uplift and might be disconnected from the latter due to the presence of the Chiwei Depression. The original southern Diaoyudao Uplift exhibited a basement high resembling the middle and northern Diaoyudao Uplift. However, this NE-SW-trending basement high was destroyed and collapsed due to oblique dissections of the E-W-trending rifting centers along with westward propagation of the southern Okinawa Trough. Since nonmagnetic prerifted basement rocks have not been drilled on the Diaoyudao Uplift, the Chiwei Platform is a preferential drilling site where the prerifted basement rocks can readily be obtained due to its high-standing position and thin sedimentary cover. This work can provide supplementary constraints on the basement properties of the Ryukyu back-arc region and help to better understand the evolution of the southern Okinawa Trough.

СОВРЕМЕННЫЕ АСПЕКТЫ ТЕКТОНИЧЕСКОГО СТРОЕНИЯ ЮЖНО-ВЕРХОЯНСКОГО СИНКЛИНОРИЯ

Problems of the origin and tectonic pattern of the South Verkhoyansk sinclinorium are discussed. It is suggested that in the Late Paleozoic the South Verkhoyansk basin inherited the structure of the Sette-Daban rift graben and, as a submarine valley, was positioned across the Vtrkhoyansk passive margin. Lateral motions of the Okhotsk indentor in course of Late Mesozoic orogenic events led to transformation of the South Verkhoyansk basin into the same name two-stade sinclinorium of which the lower cleavage stage represents as ore-bearing metamorphic layer.

BLACK SEA SUBMARINE VALLEYS – PATTERNS, SYSTEMS, NETWORKS

The article presents a detailed analysis of the underwater morphology of the entire Black Sea basin beyond the shelf break. The focus is on submarine valley systems on continental slope and rise zones, and partially in the abyssal plain area. The present research is among the very few studies that have undertaken a morphological analysis on a regional scale, for an entire marine basin. This achievement was possible by using the publicly available EMODnet bathymetric map of the Black Sea. The Black Sea submarine valleys networks are presented in a map-sketch. It includes 25 valley systems, 5 groups of simple first order channels, and other number of simple, not associated channels. The 25 valley systems are adding up more than 110 main channels and tributaries. Morphological description and analysis of each of the mapped systems is given – shape and plan view morphology, dimensions (length and surface) and slope gradient. Some considerations about the amount of sediments supplied by the valleys from the shelf to the basin floor, forming the deep-sea fans, are presented. For a more detailed and precise image of the Black Sea network of submarine valleys additional work would be necessary to cover the entire basin with a minimum standard network of modern bathymetric mapping and of high resolution seismic survey lines. To take into account the composition and geological structure of different sectors, as factors that influence significantly the morphology of the seabed, would be also particularly useful.

Linking carbonate sediment transfer to seafloor morphology: Insights from Exuma Valley, the Bahamas

AbstractThe depositional record of carbonate slopes provides a valuable archive of past environmental and climatic changes. Modern carbonate slopes reveal morphological variabilities (for example, gullies and canyons) shaped by episodic slope collapses and turbidity currents. Furthermore, climate‐induced fluctuations in sea level regulate sediment availability and delivery to the deep‐sea. Morphological and climatic controls on calciclastic sediment transfer are often complex to decipher. The aim of this study is to link seafloor morphology and depositional processes in an active carbonate submarine channel (Exuma Valley, the Bahamas) over the last 40 kyr. The dataset includes multibeam and seismic surveys, and two sediment cores retrieved from the valley axis. A series of abrupt slope‐breaks, called knickpoints, occurs along Exuma Valley, and plays a key role in sediment transport and accumulation. Initiation processes proposed for knickpoint formation include bank‐collapse, side gully erosion and loss of confinement. Slope collapses detected on the bathymetry prevail in the upstream muddy section of the submarine valley, as attested to by a planktic‐rich debrite–turbidite couplet in the first core. In contrast, the second core collected downstream of the knickpoints train, includes 32 bioclastic sandy event‐beds (i.e. turbidites). Hydrodynamic sorting generates grain segregation (for example, Halimeda‐rich base versus planktic‐rich top) and geochemical contrasts (Sr/Ca) in turbidites. Turbidite frequency and grain composition within beds reflect the variation of carbonate sources during glacial–interglacial periods. This research allows to link slope morphology with deposits of a modern large‐scale carbonate factory, and to deduce sea‐level changes over that last 40 kyr in the Bahamas. These results can provide new perspectives on the understanding of ‘source to sink’ mechanisms in carbonate systems.

Abrupt Holocene ice-sheet thinning along the southern Soya Coast, Lützow-Holm Bay, East Antarctica, revealed by glacial geomorphology and surface exposure dating

Geological reconstruction of the retreat history of the East Antarctic Ice Sheet (EAIS) since the Last Glacial Maximum (LGM) is essential for understanding the response of the ice sheet to global climatic change and the mechanisms of retreat, including a possible abrupt melting event. Such information is key for constraining climatic and ice-sheet models that are used to predict future Antarctic Ice Sheet AIS melting. However, data required to make a detailed reconstruction of the history of the EAIS involving changes in its thickness and lateral extent since the LGM remain sparse. Here, we present a new detailed ice-sheet history for the southern Soya Coast, Lützow-Holm Bay, East Antarctica, based on geomorphological observations and surface exposure ages. Our results demonstrate that the ice sheet completely covered the highest peak of Skarvsnes (400 m a.s.l.) prior to ∼9 ka and retreated eastward by at least 10 km during the Early to Mid-Holocene (ca. 9 to 5 ka). The timing of the abrupt ice-sheet thinning and retreat is consistent with the intrusion of modified Circumpolar Deep Water (mCDW) into deep submarine valleys in Lützow-Holm Bay, as inferred from fossil foraminifera records of marine sediment cores. Thus, we propose that the mechanism of the abrupt thinning and retreat of the EAIS along the southern Soya Coast was marine ice-sheet instability caused by mCDW intrusion into deep submarine valleys. Such abrupt ice-sheet thinning and retreat with similar magnitude and timing have also been reported from Enderby Land, East Antarctica. Our findings suggest that abrupt thinning and retreat as a consequence of marine ice-sheet instability and intrusion of mCDW during the Early to Mid-Holocene may have led to rapid ice-surface lowering of hundreds of meters in East Antarctica.

Did an earthquake located off Jersey trigger a mudflow preserving the only loess outcrop known under the seas?

A 5 m long core sample, retrieved from between Jersey and the Cotentin (France) has been studied using different techniques. Between the lower part and the top, the core is a yellowish loess, a black mud and a grey shelly formation. The basal formation, which displays a typical loess granulometry, is the only loess found anywhere in situ on the subtidal seabed worldwide. Elsewhere, this fragile sediment has always been washed out by the successive Quaternary transgressions. The overlying mud formation, which displays characteristics of a landslide, sealed the loess deposit. Because this core has been sampled on the rim of a submarine valley located in front of a major onshore geological disruption, clearly associated with an active seismic zone, it is possible that an earthquake triggered a slope failure causing a gravity collapse of materials that protected the loess deposit from subsequent erosion, although we cannot completely rule out the effects of a large flood. Supplementary material: Tables showing the results of the pollen analysis and distribution of mollusc shells and other fossils in borehole O-VC15, and a figure showing the distribution of mollusc species in core O-VC15 and a possible reconstruction of the palaeoenvironment during sediment deposition are available at https://doi.org/10.6084/m9.figshare.c.4991753

First ROV Exploration of the Perth Canyon: Canyon Setting, Faunal Observations, and Anthropogenic Impacts

This study represents the first ROV-based exploration of the Perth Canyon, a prominent submarine valley system in the southeast Indian Ocean offshore Fremantle (Perth), Western Australia. This multi-disciplinary study characterises the canyon topography, hydrography, anthropogenic impacts, and provides a general overview of the fauna and habitats encountered during the cruise. ROV surveys and sample collections, with a specific focus on deep-sea coral, were conducted at six sites extending from the head to the mouth of the canyon. Multi-beam maps of the canyon topography show near vertical cliff walls, scarps, and broad terraces. Biostratigraphic analyses of the canyon lithologies indicate Late Paleocene to Late Oligocene depositional ages within upper bathyal depths (200-700 m). The video footage has revealed a quiescent ‘fossil canyon’ system with sporadic, localised concentrations of mega- and macro-benthos (~680-1800 m), which include corals, sponges, molluscs, echinoderms, crustaceans, brachiopods, and worms, as well as plankton and nekton (fish species). Solitary (Desmophyllum dianthus, Caryophyllia sp., Vaughanella sp., and Polymyces sp.) and colonial (Solenosmilia variabilis) scleractinians were sporadically distributed along the walls and under overhangs within the canyon valleys and along its rim. Gorgonian, bamboo, and proteinaceous corals were present, with live Corallium often hosting a diverse community of organisms. Extensive coral graveyards, discovered at two disparate sites between ~690-720 m and 1560-1790 m, comprise colonial (S. variabilis) and solitary (D. dianthus) scleractinians that flourished during the last ice age (~18 ka to 33 ka BP). ROV sampling (674-1815 m) spanned intermediate (Antarctic Intermediate Water) and deep waters (Upper Circumpolar Deep Water) with temperatures from ~2.5-6°C. Seawater CTD profiles of these waters show consistent physical and chemical conditions at equivalent depths between dive sites. Their carbonate chemistry indicate supersaturation (Ωcalcite ~1.3-2.2) with respect to calcite, but mild saturation to undersaturation (Ωaragonite ~0.8-1.4) of aragonite; notably some scleractinians were found living below the aragonite saturation horizon (~1000 m). Seawater δ13C and nuclear bomb produced Δ14C compositions decrease in the upper canyon waters by up to ~0.8‰ (< 800 m) and 95‰ (< 500 m) respectively, relative to measurements taken nearby in 1978, reflecting the ingress of anthropogenic carbon into upper intermediate waters.

Fine gold in the bottom deposits of the Dnieper submarine valley (The NW Black sea shelf): prospects, morphology and genesis

Fine gold in the bottom deposits of the Dnieper submarine valley (The NW Black sea shelf): prospects, morphology and genesis

Nepartak Typhoon Influenced Bottom Sediments From the Yangtze River Estuary and Adjacent East China Sea‐Foraminiferal Evidence

AbstractForaminiferal taphocoenose characteristics, notably allochthonous tests in surface sediment samples from the Yangtze River Estuary and adjacent areas in the East China Sea, were examined during (July 2016) and after typhoon Nepartak (February 2017) crossed this region. A comparison of transport indices from five sample stations reveals that this typhoon significantly strengthened sediment transport, although the transport in predominate direction (TWCnorth branch) occurs all year. Moreover, findings from a comprehensive impact of currents, topography, test size, and shape, as well as other abiotic factors, show that transport occurs in the order (from stronger to weaker): south‐north path ≫ southeast‐northwest path &gt; nearshore‐northeast path. Along the south‐north path, the TWCnorth branch transports larger tests of taxa Ammonia compressiuscula and Cavarotalia annectens as bed load along the gentle seabed in a rolling or saltating process. Results from the southeast‐northwest path show smaller and bulging tests of the dominant species Epistominella naraensis and Bolivina robusta may be transported as suspension load along the steep submarine valley by the TWCnorthwest branch. On the nearshore‐northeast path, only part of allochthonous tests Ammonia beccarii vars. was transported, probably due to distance traveled and weakening of the typhoon. Furthermore, we also infer the northeastern area as a sink zone that accumulates grains of allochthonous tests predominantly from the southern source zone, and a selection of tests from littoral species originating from the nearshore source zone. It is important to identify and independently count allochthonous test samples if a large number of these tests are recorded. Findings from this study provide a reference to identify and investigate typhoon modern effects in areas with complex currents and frequent strong physical dynamic events, as well as their geological records.

Coastal circulation in the southwestern Yellow Sea in the summers of 2008 and 2009

The coastal ocean circulation off the Chinese Subei (north Jiangsu Province) coasts in the southwestern Yellow Sea in summer is studied using hydrographic data from four research cruises, moored current meter data in the Lunan (south Shandong Province) trough, and Lagrangian trajectories of satellite-tracked ARGOS surface drifters in the summers of 2008 and 2009. The hydrographic data indicate a fresh water plume extending offshore in the northeastward direction from the Sheyang River mouth and a belt of relatively cold surface temperature from the surrounding areas located offshore of the Subei coasts to the south of the plume in summer. The drifter trajectories show that the Subei coastal current, which is traditionally thought to flow southward year round, flows northward in June through July under the forcing of the southerly monsoon. A northward intrusion along the submarine valley off the Changjiang mouth is indicated by the movement of the drifters, which is persistently strong and independent of the winds. Associated with the northward flow, coastal upwelling is generated offshore of the Subei coast to the south of the Sheyang River plume, below which upwelling is generated but is shut down by the buoyancy of the fresh water at the surface. Coastal upwelling is also observed off the coast of Qingdao city in middle July through early August of 2008, which is forced by the along-shore component of the southerly and southwesterly winds. The Qingdao upwelling in summer is reported for the first time in history.

Bottom currents observed in and around a submarine valley on the continental slope of the northern South China Sea

Bottom currents at about 1000 m depth in and around a submarine valley on the continental slope of the northern South China Sea were studied by a 14-month long experiment from July 2013 to September 2014. The observations reveal that bottom currents are strongly influenced by the topography, being along valley axis or isobaths. Power density spectrum analysis shows that all the currents have significant peaks at diurnal and semi-diurnal frequencies. Diurnal energy is dominant at the open slope site, which is consistent with many previous studies. However, at the site inside the valley the semi-diurnal energy dominates, although the distance between the two sites of observation is quite small (11 km) compared to a typical horizontal first-mode internal tide wavelength (200 km). We found this phenomenon is caused by the focusing of internal waves of certain frequencies in the valley. The inertial peak is found only at the open slope site in the first deployment but missing at the inside valley site and the rest of the deployments. Monthly averaged residual currents reveal that the near-bottom currents on the slope flow southwestward throughout the year except in August and September, 2013, from which we speculate that this is a result of the interaction between a mesoscale eddy and the canyon/sag topography. Currents inside the valley within about 10 mab basically flow along slope and in the layers above the 10 mab the currents are northwestward, that is, from the deep ocean to the shelf. The monthly mean current vectors manifest an Ekman layer-like vertical structure at both sites, which rotate counter-clockwise looking from above.

Cascades and plunge pools on the continental slope of the Gulf of Alaska

More than 321 400 km2 of seafloor in the Gulf of Alaska was mapped in 2005 (Gardner & Mayer 2005). Both bathymetry and high-resolution backscatter data were collected from the R/V Kilo Moana . The resulting digital terrain models and backscatter mosaics provide a remarkable view of the processes on a tectonically active, glaciated margin. In the course of these surveys several morphological features, similar to cascades and plunge pools found beneath terrestrial waterfalls, were identified on the continental slope. Although several cascade-like features are seen along the continental slope of the southeastern Alaskan margin, the most prominent is found SW of Yakutak Bay and south of the Malispina Glacier which emanates from the St Elias Mountain Range (Fig. 1a, b). The St Elias Range includes some of the highest mountains in North America with remarkable vertical relief, for example Mount St Elias rises to 5489 m above sea level in less than 16 km horizontal distance (Fig. 1a, b). Offshore of Yakutak Bay is the Yakutak Valley, a large, glacially excavated, submarine valley that cuts NE–SW across the continental shelf. Yakutak Valley originates in Yakutak Bay with a width of approximately 50 km and a depth of 120 m as it reaches the shelf break 90 km from the mouth of the bay (Fig. 1a). Fig. 1. Multibeam bathymetry and backscatter along the SE Alaskan margin. ( a …

Mid-latitude complex trough-mouth fans, Laurentian and Northeast fans, eastern Canada

Submarine fans seaward of mid-latitude glacial ice streams in cross-shelf troughs show a complex evolution. Unlike high-latitude fans where glacigenic debris-flows predominate, meltwater discharge appears to play a larger role in fan evolution, reflected in the growth of high channel-levees and the development of broad, flat-floored submarine valleys. Where canyons are deeply incised, large mass-transport events may be triggered leading to much of the mid-fan area being constructed of mass-transport deposits (MTDs). The Laurentian and Northeast fans are two large submarine fans that lie seawards of major glacial cross-shelf troughs on the continental margin south of Nova Scotia, Canada (Fig. 1a). Both fans span more than 300 km from the shelf break to the Sohm Abyssal Plain. The fans have well-developed channel-levee systems with the western levee of the Laurentian Fan forming a major constructional feature on the margin (Fig. 1c, e). Multibeam and side-scan data coverage of the fans is more complete for the lower fan of Northeast Fan and for the middle and upper fan of Laurentian Fan. Fig. 1. Multibeam bathymetry, backscatter, side-scan and seismic-reflection data over large mid-latitude trough-mouth fans south of …