Canyon Head Research Articles

Demise of a submarine canyon? Evidence for highstand infilling on the Waipaoa River continental margin, New Zealand

Submarine canyons are major geomorphologic features on the Earth's surface. Their formation has received considerable debate, but their demise has received less attention. Research of modern canyons with cores and moorings has documented active sediment transport and deposition, but extrapolation of these local observations over larger areas is precluded by complex canyon geomorphology. High‐resolution multibeam and chirp data presented here provide convincing evidence of an infilling canyon head on the Waipaoa River margin of New Zealand. Tens of meters of Holocene sediment have accumulated on the outer shelf and in Lachlan canyon as a result of off‐shelf sediment transport. Regardless of the ultimate fate of this system over geological time scales, this research demonstrates highstand sedimentation as a possible mechanism for canyon burial and cause of canyon demise, which has important implications for the evolution of canyons globally.

Late Glacial to Preboreal sea-level rise recorded by the Rhône deltaic system (NW Mediterranean)

A unique late Glacial–Preboreal record of changes in sea-level and sediment fluxes originating from the Alps is recorded in the Rhône subaqueous delta in the Western Mediterranean Sea. The compilation of detailed bathymetric charts, together with high-resolution seismic profiles and long cores, reveals the detailed architecture of several sediment lobes, related to periods of decreased sea-level rise and/or increased sediment flux. They are situated along the retreat path of the Rhône distributaries, from the shelf edge and canyon heads up to the modern coastline. They form transgressive backstepping parasequences across the shelf, the late Holocene (highstand) deltas being confined to the inner shelf. The most prominent feature is an elongated paleo-shoreface/deltaic system, with an uppermost sandy fraction remolded into subaqueous dunes. A long piston core into the bottomsets of this prograding unit allows precise dating of this ancient deltaic system. In seismic data, it displays aggradation, starting at ∼ 15 cal kyr BP, followed by progradation initiated during the first phase of the Younger Dryas, a period of reduced sea-level rise or stillstand. The delta kept pace with resumed sea-level rise during the Preboreal (which is estimated at about 1 cm/yr), as a result of increased sediment supply from the Alps (melting of glaciers and more humid climate “flushing” the sediment down to the sea). Abandonment of the delta occurred around 10,500 cal yr BP, that is to say about 1000 yr after the end of the Younger Dryas, probably because of decreased sediment flux.

Evidence of sediment resuspension by nonlinear internal waves on the western Portuguese mid-shelf

In-situ observations of nonlinear internal waves (NIWs) propagating over the northern shelf of Portugal, near the Nazaré submarine canyon (39°47.4′N/009°11.4′W), and evidence of their action on the bottom, are presented and discussed. Synthetic Aperture Radar (SAR) images from this margin suggest the generation of NIWs over the canyon head (39°35′N/009°25′W) and the subsequent propagation directly to the mid-shelf. An observational program was conducted in this area in the summer of 2004, between August 29th and September 1st, to study these short-period internal waves. Temperature and current high resolution measurements were made with thermistor chains and acoustic Doppler current profilers (ADCPs), moored along the canyon generated NIW track, at the 82 m isobath. A simultaneous ENVISAT SAR image was obtained during the experiment. These observations were complemented by a CTD survey and bottom sediment sampling. Collected data show the propagation of large amplitude internal solitons forcing strong bottom current pulses. Turbidity profiles and ADCP echo variability suggest the remobilization of the mid-shelf bottom sediments induced by these short-period events. Calculations of bottom shear stress, induced by the observed canyon generated NIWs, reveal an energetic internal wave activity that can complement, during summer, the role of winter swell in sediment resuspension over the mid-shelf. This work is a contribution to the European project EUROSTRATAFORM, whose objectives include the study of the specific canyon systems dynamics and related sedimentary impacts on the European continental margin.

Sinuous deep-water channels: Genesis, geometry and architecture

Sinuous deep-water channels display a wide range of geometries and internal architectures. Most modern examples have been documented from large passive-margin fans, supplied by major rivers carrying huge volumes of dominantly fine-grained sediments, e.g. Amazon, Mississippi, Zaire, Bengal, Indus, Rhône and Nile Fans. However, similar examples have also been documented from tectonically active margins, e.g. Magdalena Fan. In most cases, modern sinuous channels comprise the core element of laterally extensive channel–levee systems that often aggrade significantly above a low-gradient (0.1–0.5°) fan surface; individual channels may extend downslope for 100s of kilometres. Typically, channels are subject to frequent avulsions, with only one channel active at any given time. Present highstand conditions have ensured that activity in many modern sinuous channels is much reduced due to the disconnect between fluvial feeder system and canyon head, and some have even been heavily modified or destroyed by major mass-transport deposits. Exceptions include Zaire Fan, where recent activity has provided useful insights into flow processes. The majority of detailed studies relating to sinuous channels in the subsurface originate from offshore west Africa, where channels typically occur within incisional (confined) slope–channel complexes and often represent the latter stages of channel complex fill. Both dominantly aggradational and laterally migrating styles are recognised, while modern seafloor channels in this region display a similar incisional character, e.g. Cap Timiris Canyon. Morphologic expression of sinuosity is harder to recognise at outcrop, but there are an increasing number of documented examples of lateral accretion deposits, representing point-bar growth, that are currently thought to be diagnostic of sinuous channel forms. Sinuous channel lateral migration, and point-bar growth, appears to be driven by sustained flow of fluvial-sourced (probably hyperpycnal) low-density turbidity currents, although there does appear to be variation in energy conditions, with some outcrop examples showing switches in erosive and depositional phases of activity. Previous studies have frequently focussed on the obvious gross planform morphologic similarities between fluvial and deep-water sinuous channels, e.g. nature of sinuosity, presence of point bars and cut-off loops. However, we suggest here that the differences between submarine and river channels are of greater significance, in terms of geometry, flow processes, migration style and deposit character. Sinuous deep-water channels typically form initially by a moderate amount of incision followed by rapid initial bend growth (associated with bypass of sediment). Channels that show significant aggradation then reach a point where there is a near cessation of planform movement (ossification), and growth is dominated by vertical aggradation. A new process model is proposed for this developmental sequence that synthesises observations and experiments that were previously paradoxical.

Sedimentary particle distribution and dynamics on the Nazaré canyon system and adjacent shelf (Portugal)

In the framework of the EUROSTRATAFORM project 108 bottom sediment samples (1st cm) from the Nazaré continental shelf have been analysed for grain-size, mineralogy (X-ray diffraction) organic carbon and carbonate contents. The particulate matter (PM) from intermediate (INL) and bottom nepheloid layers (BNL) was also analysed in 84 stations for grain-size and composition. The particle analysis was related to the present day sedimentary processes directly promoted (or intensified) by the presence of the Nazaré submarine canyon. The Nazaré canyon intersects the entire continental shelf and may act as a catchment area for material which is transported over the shelf, but its complex dynamics will extend to sediment distribution on the adjacent shelf. Major areas of erosion and resuspension and calmer areas where fine muddy sediments can settle on a temporary or permanent timescale were identified by their mineralogical and organic carbon (C org) distribution pattern. According to this data the Victoria tributary and the Nazaré canyon head (280–300 m) seem to act as permanent depocenters. A conceptual model was proposed to delineate the mechanisms acting in this particular area, taking into account three different sources of material to the Nazaré canyon: 1. during upwelling conditions, resuspension of middle shelf deposits by internal wave activity and transport to the canyon in the BNL; 2.erosion of northern/southern beach and sea cliff materials transported by littoral drift to the canyon head and 3. fine sediment input from the southern river and transport to the canyon, during downwelling conditions (winter).

Provenance of organic matter and nutrient conditions on a river- and upwelling influenced shelf: A case study from the Portuguese Margin

Organic matter contained in surface sediments from four regions on the western Portuguese shelf, which are influenced by coastal upwelling and fluvial input, was analysed with respect to elemental organic carbon (C org) and nitrogen (N total) content and isotopic carbon and nitrogen ratios ( δ 13C org, δ 15N). C org/N total weight ratios and δ 13C org values are interpreted in terms of terrigenous or marine organic matter sources, supported by CaCO 3 content. Organic matter in the shelf sediments is mainly of marine origin, with increasing terrigenous components only close to rivers and estuaries. In the northern shelf region the data indicates significant terrigenous supply by the Douro River. North of the Nazaré Canyon organic matter composition implies a mainly marine origin, with a higher terrestrial influence close to the canyon head. Organic matter composition in the central shelf region, which is dominated by the Tagus Estuary and the Tagus prodelta, reveals a change from a continental-type signature within the estuary to a more marine-type signature further to the west and south of the estuary mouth. In the southern region near Cape Sines the geochemical properties clearly reflect the marine origin of sedimentary organic matter. Sedimentary δ 15N values are interpreted to reflect various degrees of assimilation of seasonally upwelled nitrate, in relation to the upwelling centres. In the estuarine environment, inputs of agriculturally influenced dissolved inorganic nitrogen are reflected in the sediments. No evidence for N 2-fixation or denitrification is found. On the central shelf north of the Nazaré canyon, sedimentary δ 15N values are close to marine δ 15NO 3 − and thus indicate a complete NO 3 − assimilation and N-limitation of marine production. Light δ 15N values in distal sediments off the Douro River mouth and in samples south of C. Sines reflect high NO 3 − supply and a close proximity to the seasonal upwelling centres. Particularly in sediments form the Sines region, light δ 15N values in southern samples reflect stronger upwelling further south.

Trophic relationships in deep-water decapods of Le Danois bank (Cantabrian Sea, NE Atlantic): Trends related with depth and seasonal changes in food quality and availability

The trophic relationships of decapod crustaceans on Le Danois bank (NE of Iberian Peninsula, NE Atlantic Ocean) were studied within the framework of the multidisciplinary project ECOMARG during two surveys, one in October 2003 and the other in April 2004. The diets of eleven species of decapods were analyzed and, within a rather continuous gradient of food source exploitation, 3 trophic groups were identified: (1) plankton feeders, comprising the shrimps Acanthephyra pelagica, Sergia robusta, and Pasiphaea tarda, which preyed on meso-bathypelagic taxa such as euphausiids and calanoids; (2) benthos feeders, comprising the crangonids Pontophilus norvegicus and Pontophilus spinosus, the crab Geryon trispinosus and the shrimp Aristeus antennatus; and (3) an intermediate group, including the rest of species, with mixed diets that included detritus. Among the third group, anomurans ( Munida tenuimana, Pagurus alatus, and Parapagurus pilosimanus) consumed phytoplanktonic detritus in April, suggesting a link with peaks of surface Chl a occurring between March and April in the study area. Gut pigment and isotopic (δ 13C/δ 15N correlations) analyses revealed that assemblages inhabiting the top of the bank (455–612 m) and the inner basin (642–1048 m, close to the Lastres canyon head) had different food sources, with species inhabiting the deepest region exhibiting a stronger dependence on marine snow derivatives. These results are consistent with the higher proportion of mud and sediment organic matter (OM) content in the inner basin (82.2% pellites; 6.3% OM at 1028 m) compared to the top of the Le Danois bank (only 13.9% pellites; 2.8% OM at 485 m), which is a hydrodynamically more active zone. Exploitation of different food sources is also consistent with differences in the trophic level of species, inferred from stable δ 15N isotope analyses, which yield values ranging from 6.88‰ for the hermit crab P. alatus to 13.52‰ for the crangonid shrimp P. norvegicus. Stomach fullness was higher in April 2004 than in October 2003, both between and within species of the dominant decapods, including detritus feeders ( M. tenuimana) and benthos feeders (e.g. G. trispinosus, P. norvegicus). Most species exhibited a parallel increase in their density in April 2004, with a significant positive correlation between density and stomach fullness. This increase coincides with a peak of surface Chl a concentration occurring in March–April.

Near-bottom horizontal transfer of particulate matter in the Palamós Submarine Canyon (NW Mediterranean)

From March to November 2001, six current meters equipped with turbidimeters were moored 12 meters above the bottom inside the Palamos submarine canyon and on the adjacent slope. Horizontal particle fluxes were calculated from current and suspended sediment concentration data. This work aims to evaluate the advective sediment fluxes taking place in the Palamos canyon region, and to discern the nature and time-scales of the processes involved in the across-margin transfer of particles over the study period. Near-bottom currents inside the canyon were constrained by the local topography and displayed a high spatio-temporal variability. The net near-bottom transport of suspended matter in the canyon was largely driven by sharp increases of sediment load and current speed, most of them attributed to sediment gravity flows. These energetic events dominated over the current-driven sediment transport, which was relatively weak due to periodical up-canyon/downcanyon inversions along the canyon axis, and to complex current patterns and the presence of low sediment loads for most of the time at the other sites. During this 8-month experiment, the mid-canyon ( 1200 m depth) acted as a by-pass zone, while at the canyon head, net sediment transport was directed persistently up-canyon. These patterns were further heightened during a major storm in November 2001. However, in this particular submarine canyon, substantial sedimentary activity and offshore export of particulate matter also occur in the absence of significant external forcings (storms, river floods). Human activities (deep trawling) induced sediment gravity flows during the dry and calm season (spring-summer) and enhanced the offshore transfer of particles during this time period.

Analysis of slope failures in submarine canyon heads: An example from the Gulf of Lions

To improve understanding of evolution of submarine canyons, a three‐dimensional slope stability model is applied to Bourcart Canyon in the western Gulf of Lions in the Mediterranean Sea. The model builds on previous work by Z. Chen and others, and it uses the upper bound theorem of plasticity to calculate the factor of safety of a kinematically admissible failing mass. Examples of three‐dimensional failure surfaces documented in the literature were used to test the model formulation. Model application to Bourcart Canyon employed the results of detailed stratigraphic analyses on the basis of data acquired by swath bathymetry, subbottom profiling, high‐resolution seismic reflection surveys, and piston coring. The sediment layers were also characterized using in situ geotechnical measurements and laboratory tests. The effects of three loading scenarios were analyzed: (1) earthquake shaking, (2) hemipelagic sedimentation, and (3) axial incision. These three mechanisms influenced the predicted volumes and shapes of slope failures along the flanks of Bourcart Canyon, and comparison of these predictions with failure geometries inferred from seafloor morphology showed that mass failures could account for the observed morphology along the canyon walls as well as a mechanism of canyon widening.

Pliocene – Pleistocene history of the Gippsland Basin outer shelf and canyon heads, southeast Australia

The Gippsland Basin on Australia's southeastern continental margin is host to a number of large shelf-breaching canyons that form part of the Bass Canyon system. Analysis of high-resolution bathymetry data and biostratigraphically controlled shallow seismic data across the shelf and upper slope and associated canyon heads shows that widespread erosion at the canyon heads began in the earliest Pleistocene (CN13a, 1.95 – 1.72 Ma). Deep V-shaped channels were eroded into pre-existing U-shaped channels. Erosion was not necessarily a response to Pleistocene lowstands but followed a period where (carbonate) sedimentation rates on the shelf were greatly elevated. Both carbonate production and erosion were likely to be amplified, if not caused, by intensification of the Bass Cascade density current during Late Pliocene eustatic highstands. During the Middle Pleistocene (CN14a, 0.95 – 0.47 Ma) outer shelf canyon-head channels were rapidly infilled. Canyon-head channels migrated laterally across the shelf, predominantly in a northeast direction, influenced by the prevailing northeast moving Bass Cascade. Upper slope canyon heads have remained largely non-depositional exposing Pliocene strata at the seafloor. A minor episode of deformation occurred during the Middle Pliocene (CN12, 3.70 – 1.95 Ma) that produced low-amplitude open folds in Upper Pliocene and older sediments, beneath the northern inner shelf. Beneath the central inner shelf and middle shelf, similar open folds developed during the Late Pliocene to at least the Middle Pleistocene (CN13 – CN14a, 1.95 – 0.47 Ma).

Ampelisca mississippiana: a new species (Crustacea: Amphipoda: Gammaridea) from the Mississippi Canyon (Northern Gulf of Mexico)

A new ampeliscid species (Amphipoda, Gammaridea) was recovered in high densities from a depth of about 480 m at the Mississippi Canyon (28 32' N, 89 49' W). Ampelisca mississippiana is described and illustrated based on specimens collected from the head of the canyon, where the species formed an ampeliscid mat with average densities of 12,000 ind. m -2 . Ampelisca mississippiana resembles A. pugetica Stimpson, 1864, and A. mexicana Barnard, 1954a, but differs in the shape of the head, absence of corneal lenses, relative lengths of antennae 1 and 2, shape of the segments of pereopods 7, and shape of the carina of the urosome. The new species also resembles Ampelisca uncinata Chevreux, 1887, but differs in the shape of the head, the dorsal carina of pleon segment 4, features of some segments of pereopod 7, the second uropod and the mouthparts.

Combining [ 3He] cosmogenic dating with U–Th/He eruption ages using olivine in basalt

This paper presents new U–Th/He crystallization and 3He exposure ages on olivine for a suite of basalt samples from the Snake River Plain, Idaho. The results provide the first demonstration that U–Th/He geochronology can be successfully applied to Late Quaternary basalts using measurements of olivine phenocrysts as opposed to U-rich minerals. The approach employs abrasion to remove the outer rinds of olivine crystals to avoid problems with 4He implantation from the groundmass, and a specially designed extraction system that allows U, Th and He concentrations and isotopic compositions to be measured on the same olivine grains. We demonstrate the viability of the U–Th/He technique on basalts with ages of 373 ka and 3400 ka previously dated by Ar–Ar, and show that concurrent measurements of U–Th– 4He ages and 3He cosmogenic exposure ages improve the accuracy of both methods. The combined U–Th– 4He– 3He technique is applied to a series of boulders and flows from Box Canyon near Hagerman, Idaho, to evaluate the ages of the basalts and to use exposure ages to make inferences about the age and mode of formation of the canyon. The U–Th/He ages of the Box Canyon basalts are between 86 ± 12 ka and 130 ± 12 ka (1-sigma errors) and suggest at least two periods of eruption. The ages are similar to a ca. 95 ka Ar–Ar age previously assigned, and distinguish the Box Canyon basalts from nearby ca. 395 ka basalt. Exposure ages of boulders within the canyon at the mouth, head and middle of Box Canyon are 21 ± 1, 19 ± 3 and 48 ± 3 ka; scoured bedrock at the canyon head has an exposure age of 45 ± 5 ka. These data indicate that the canyon was carved before the Bonneville Flood.

Highstand fans in the California borderland: The overlooked deep-water depositional systems

Contrary to widely used sequence-stratigraphic models, lowstand fans are only part of the turbidite depositional record; our analysis reveals that a comparable volume of coarse-grained sediment has been deposited in California borderland deep-water basins regardless of sea level. Sedimentation rates and periods of active sediment transport have been determined for deep-water canyon-channel systems contributing to the southeastern Gulf of Santa Catalina and San Diego Trough since 40 ka using an extensive grid of high-resolution and deep-penetration seismic-reflection data. A regional seismic-reflection horizon (40 ka) has been correlated across the study area using radiocarbon age dates from the Mohole borehole and U.S. Geological Survey piston cores. This study focused on the submarine fans fed by the Oceanside, Carlsbad, and La Jolla Canyons, all of which head within the length of the Ocean-side littoral cell. The Oceanside Canyon–channel system was active from 45 to 13 ka, and the Carlsbad system was active from 50 (or earlier) to 10 ka. The La Jolla system was active over two periods, from 50 (or earlier) to 40 ka, and from 13 ka to the present. One or more of these canyon-channel systems have been active regardless of sea level. During sea-level fluctuation, shelf width between the canyon head and the littoral zone is the primary control on canyon-channel system activity. Highstand fan deposition occurs when a majority of the sediment within the Oceanside littoral cell is intercepted by one of the canyon heads, currently La Jolla Canyon. Since 40 ka, the sedimentation rate on the La Jolla highstand fan has been >2 times the combined rates on the Oceanside and Carlsbad lowstand fans.

Acoustic structure and recent sediment transport processes on the continental slope of Yeşilırmak River fan, Eastern Black Sea

High-resolution acoustic data indicate that sediment erosion is a significant process in the continental slope of Yeşilırmak River Fan, occurring both related to the submarine canyons and in the open continental slope. The sediment processes observed along the slope consist of sliding and debris flows on the canyon walls together with the recent erosional surfaces at the canyon heads wherever the canyons exist, and slides and related scarp features structurally controlled by rotational faults in the central part of the open continental slope. Formation of slides both on the open slope and in canyon walls is a multi-phase sliding process, in which successive sliding events occur at the same location producing a vertically stacked sliding, resulting in several relict slide features deeper parts in the sediments. We conclude that gravitational loading on the steep slope is the main driving force for most of the erosional features, and in place to place, overpressured pore fluids due to gas accumulation can contribute the instability. Tributary canyon systems are observed on the slope, with slide scars in the proximal part. According to their different erosional structure, these canyons can be separated into two groups, as northern and southern sector canyon systems, where the sediment bypass and hence erosional processes are much more active on the canyon heads in the southern sector than those in the northern part. Terrigenous sediment load from Yeşilırmak River is transported along downslope by successive hyperpycnal fluxes and the activity of turbidity currents along the channel axis. Acoustic data indicate that the continental slope of Yeşilırmak River Fan is a classical deep-sea turbidite system, where we observe slide scars in proximal part, and channel levee systems with slide and debris flow deposits inside the channels in the distal part with some small sized gullies.

Marine geology of the Quaternary Bass Canyon system, southeast Australia: A cool-water carbonate system

The modern Bass Canyon is one of the world's largest submarine canyon systems that is entirely located within a cool-water carbonate environment. Five large shelf-breaching and three slope-confined tributary canyons coalesce on the lower slope and enter the massive deep-water Bass Canyon at 3000 m depth. Normal slope sediments consist predominantly of sandy calcilutite and muddy calcarenite interpreted as mud flow and mud-lubricated sandy debris flow deposits respectively, and hemipelagic foraminiferal calcilutite. Canyon head sediments consist of intraclast-rich calcarenite and calcirudite, probably deposited by mass-wasting events, and fine calcarenite, deposited by cohesionless sandy debris flows. Canyon filling sediments consist of well-sorted fine calcarenite deposited by (long term) semi-continuous cohesionless sandy debris flows, and rarer intraclast-rich mud, interpreted as cohesive sediment gravity flow deposits. Two types of tributary canyons are recognized: V-shaped canyons with steep walls (up to 35°) that are well-defined by backscatter acoustics; and broad U-shaped canyons (walls ≤ 10°) poorly defined by backscatter. It is most likely that tributary canyons developed from down-slope eroding sediment gravity flows triggered at the shelf break, which mature to develop dendritic canyon heads. Dendritic canyon heads entrain coarse-grained shelf sediment, which fuel erosive gravity flows and scour deep V-shaped canyon profiles. Once canyon heads stabilise, shelf-derived erosive sediment flows are reduced and the canyon profile switches from V- to U-shaped. This is aided by continual pelagic sedimentation and sediment gravity flows from the adjacent slope and canyon walls. Mud-free sandy debris flows from the lower slope converge with sandy outflows from tributary canyons and flow down the Bass Canyon floor. Sand cored from the Bass Canyon floor preserves cyclic fluctuations in magnetic susceptibility. High magnetic susceptibility zones are associated with increases in cool-temperature planktonic foraminiferal, and have been tentatively correlated to oxygen isotope stages 6.2–6.8 (135–185 ka) and 8.2–8.4 (245–278 ka). Based on this correlation, sedimentation rates on the slope and within the Bass Canyon were found to be highest during the highstand and regressive systems tracts, and lowest during the lowstand and transgressive systems tracts.

Comparing the smooth, parabolic shapes of interfluves in continental slopes to predictions of diffusion transport models

Computer models of stratigraphic development at continental margins often include elements in which sediment topography evolves according to a diffusion equation, representing effects of down-slope sediment transport. However, there has been little effort to verify the model, for example by showing whether deposit geometries are as expected from it or by considering whether transport is consistent with the model's assumptions. The issue is important for understanding transport, erosion and deposition in continental slopes more generally and the extent to which such processes are recorded in seabed morphology. Examples of slope topography illustrating diffusive-like characteristics are examined here. In the uppermost slope of the USA central Atlantic margin, sandy areas between canyon heads are smoothly convex-upwards and almost parabolic, a morphology similar to subaerial soil-mantled hillslopes known to evolve according to a diffusion equation. Other data of interfluves in muddy sediment from both Atlantic and Pacific USA margins also show this smooth convex-upwards form. In subaerial hill crests, the upwards-convex parabolic shape arises where steady erosion by streams along channels provide constant flux boundary conditions (i.e., the parabola is the steady-state solution to the diffusion equation). Although submarine processes will likely be different, the similar morphology of submarine interfluves prompts the question as to whether erosion along channels and movements of sediment between channels can also sometimes occur to cause a diffusive-like evolution of topography. The discussion therefore focuses on submarine processes affecting erosion and deposition, in particular to address whether the morphology results from true diffusion (which requires that sediment moves down-slope with a transport flux proportional to local bed gradient) or is fortuitous (sediment transport is instead unrelated to local bed gradient and the morphology arises from other effects on accumulation rates).

Massive sand beds attributed to deposition by dense water cascades in the Bourcart canyon head, Gulf of Lions (northwestern Mediterranean Sea)

Nowadays, the Gulf of Lions continental shelf and slope are under the influence of dense water cascading, wind-induced bottom currents and the geostrophic Northern Current. In order to characterize sedimentary activity at the shelf break, several interface and piston cores were taken in the Bourcart canyon head and a current meter equipped with temperature, conductivity, pressure and turbidity sensors was moored during the 2003–2004 winter season. Even if the canyon is not connected directly to continental sources since Last Glacial Maximum, detailed grain size, X-ray and sediment facies analysis of interface cores show that down to 350 m water depth, the canyon head is blanketed by up to 1.5 m of structureless muddy medium-grained sand. 210Pb exc activity measurements demonstrate present day sedimentary activity of the canyon head. Time series of currents with peak velocity reaching 0.37 m/s, suspended sediment concentration and temperature indicate that dense water cascading is the main process allowing the reworking, transport and accumulation of sand within the canyon head, even though winter 2003–2004 was characterized by low cascading events compared to other years. These sand beds, called “cascadite” constitute a new type of deposit that differs from other typical slope deposits (turbidites, hyperpycnites, contourites) in terms of flow duration and sedimentological characteristics. They are similar to shallow water contourites. They might be time-equivalent to the recent turbidites described throughout the deep western Gulf of Lions basin.

Spatial and temporal variability of downward particle fluxes on a continental slope: Lessons from an 8-yr experiment in the Gulf of Lions (NW Mediterranean)

A long-term experiment of downward particle fluxes and currents has been initiated in 1993 on the continental slope of the Gulf of Lions (NW Mediterranean) and pursued within the frame of several French and European projects (PNEC, Euromarge-NB, MTP II-MATER, EUROSTRATAFORM). Sediment traps and current meters were deployed at several locations on this slope deeply incised by numerous canyons, with an extensive spatial coverage for the first 2 years (canyons at the entrance, middle and exit of the gulf with respect to the general along-slope circulation, head and mid-canyon depths, adjacent open slope). From late 1995 onwards, this design was reduced to the two mid-canyon moorings at the entrance and exit of the gulf. Monthly fluxes and hourly temperatures and currents were recorded at 500 m (30 m above bottom, mab) in the canyon heads, at 500 and 1000 m (respectively 530 and 30 mab) nominal depths at the mid-canyon sites, and at 750 m (30 mab) open slope. This study aims at describing the spatial, seasonal and interannual variability of flux intensity and composition of settling particles, and at analyzing the role of diverse forcings in the control of particle exchange across the margin. Results from the first 8 years (1993–2001) show that total mass fluxes – in the 10 1–10 4 mg m − 2 d − 1 range – increase along slope, particularly for the near-bottom traps, between the NE (Planier Canyon) and the SW (Lacaze-Duthiers Canyon) limits of the Gulf of Lions, indicating an increased shelf export of particulate matter in the western part of the system. Bulk chemical composition (organic matter, carbonate, opal and lithogenic fraction) remained rather stable during the course of the experiment, tending towards values typical of superficial shelf sediments at higher mass fluxes. First-order calculations using a simple two-component mixing model suggest a decreasing contribution of primary particles settling out of the overlying waters to the total flux from the entrance towards the exit of the system. Particulate material transferred to the deeper slope in the southwestern part of the Gulf of Lions appears therefore to predominantly originate in resuspended shelf and/or upper slope sediment. Downward particle fluxes and potential forcing parameters exhibit a high seasonal variability, with higher values from late autumn to early spring. Furthermore, unprecedented winter flux peaks observed in 1999 dominated the interannual differences, which otherwise were quite limited. Correlations between sources of particulate material on the shelf (i.e., river and atmospheric inputs, phytoplankton biomass and sediment resuspension), cross-slope exchange mechanisms (derived from in situ temperature and current records) and flux data indicate a predominant effect of dense cold water cascading on the exchange of particulate matter between the shelf and the slope.

Sand bodies at the shelf edge in the Gulf of Lions (Western Mediterranean): Deglacial history and modern processes

The outer continental shelf of the Gulf of Lions is covered by thick, mainly regressive, sand deposits. A combination of seismic, sedimentological and geochronological methods allows us to demonstrate that a veneer of transgressive sand bodies, few m to less than 1 m thick, reworks these deposits. They take the form of sand ridges and transverse dunes that formed at different periods. In fact, the low-gradient outer shelf of the Gulf of Lions displays a complete record of deglacial history, including not only transgressive deposits that formed during sea-level rise, but also bedforms that still evolve under episodic high-energy events that occur under more highstand conditions. Core lithology, 14C dates and regional sea-level curve suggest that the formation of the sand ridges was favoured during a period of deceleration of sea-level rise, such as during the Younger Dryas event. The dunes are part of a “mobile carpet” (here U160). The deposition of this marine sand veneer is possibly related to the interplay between different factors that trigger the turning on/off for bottom currents sand deposition/transport on the outer shelf. In fact, the sands appear to have been reworked intermittently due to high-energy conditions in shallow water depth (currents, wave regime and wind-driven circulation) as well as they form current ridges superimposed to the transgressive deposits in the westernmost part of the studied area (at the Bourcart Canyon's head).

Past and present sedimentary activity in the Capbreton Canyon, southern Bay of Biscay

Located in the south-eastern part of the Bay of Biscay, the Capbreton Canyon incises the continental shelf up to the 30 m isobath contour, and acts as a natural conduit for continental and shelf-derived sediments. EM1000 multibeam bathymetry shows two main features characterising the canyon — a deeply entrenched meandering channel, bordered by fluvial-like terraces constituting large sediment traps. A dataset of cores and seismic profiles together with a multibeam bathymetry map has enabled the characterisation of recent sedimentary activity in the axial channel and on the terraces. Data analysis evidenced the major role of the canyon head in recent sediment dynamics. This part of the canyon is a temporary reservoir for sediments, accumulated by coastal hydrodynamic processes. Exceptional climatic, tectonic or hydrodynamic events can mobilise the sediments and generate gravity-driven flows. Under the present-day sea-level highstand conditions, these flows are not powerful enough to bring their bedload to the deep sea, and are confined mainly to the upper part of the canyon. Turbidity currents model the axial channel pathway and are at the origin of terrace formation. Terraces in the Capbreton Canyon are not typical but rather are reduced to confined levees. Three factors control the vertical growth of a terrace: (1) the amount of overspilled sediments brought by turbidity currents, (2) hemipelagic sedimentation and (3) terrace height. The amount of sediment spilling over a terrace decreases with increased terrace elevation. Concurrently, the proportion of hemipelagic fallout depositing on a terrace increases. Terraces are considered to be fossil when the height of the terrace prevents further deposition by overspilling. The terraces studied in this paper are interpreted as having formed during the Holocene, implying that the sediment dynamics of the Capbreton Canyon is continuous through time. Highstand periods differ from lowstand periods because they show a decrease in the energy of erosive processes. Temporal variations in erosive and depositional processes in the canyon are controlled by the Adour River, which delivers large amounts of sediment to the system.