Sediment Drift Research Articles

A new pathway for Deep water exchange between the Natal Valley and Mozambique Basin?

Although global thermohaline circulation pathways are fairly well known, the same cannot be said for local circulation pathways. Within the southwest Indian Ocean specifically there is little consensus regarding the finer point of thermohaline circulation. We present recently collected multibeam bathymetry and PARASOUND data from the northern Natal Valley and Mozambique Ridge, southwest Indian Ocean. These data show the Ariel Graben, a prominent feature in this region, creates a deep saddle across the Mozambique Ridge at ca. 28°S connecting the northern Natal Valley with the Mozambique Basin. Results show a west to east change in bathymetric and echo character across the northern flank of the Ariel Graben. Whereby eroded plastered sediment drifts in the west give way to aggrading plastered sediment drift in the midgraben, terminating in a field of seafloor undulations in the east. In contrast, the southern flank of the Ariel Graben exhibits an overall rugged character with sediments ponding in bathymetric depressions in between rugged sub/outcrop. It is postulated that this change in sea-floor character is the manifestation of deep water flow through the Ariel Graben. Current flow stripping, due to increased curvature of the graben axis, results in preferential deposition of suspended load in an area of limited accommodation space consequently developing an over-steepened plastered drift. These deposited sediments overcome the necessary shear stresses, resulting in soft sediment deformation in the form of down-slope growth faulting (creep) and generation of undulating sea-floor morphology. Contrary to previous views, our works suggests that water flows from west to east across the Mozambique Ridge via the Ariel Graben.

Chapter 23 Continental shelves as sediment capacitors or conveyors: source-to-sink insights from the tectonically active Oceanside shelf, southern California, USA

Abstract Continental shelves are the key interfaces between terrestrial sediment source areas and deep-sea depositional systems, promoting the transfer of sediment across continental margins. Work on shelves in the context of entire continental-margin sediment-routing systems has focused on their importance as capacitors of sediment during several to tens of thousands of years of post-glacial shoreline transgression and sea-level highstand. We demonstrate that the tectonically active Oceanside shelf offshore southern California has served as an efficient conveyor of sediment from land to the deep sea during millennia of significant climatic fluctuations. This conveyance is a result of littoral drift of sediment to canyon heads at narrow segments of the shelf. We compare insights from the Oceanside shelf to other shelves across the tectonically active Pacific margin of the United States, and demonstrate the importance of shelf width, climatic forcings and timescale of observation in assessing the role of shelves as sediment capacitors or conveyors.

Late Quaternary history of contourite drifts and variations in Labrador Current flow, Flemish Pass, offshore eastern Canada

Contourite drifts of alternating sand and mud, shaped by the Labrador Current, formed during the late Quaternary in Flemish Pass seaward of the Grand Banks of Newfoundland, Canada. The drifts preserve a record of Labrador Current flow variations through the last glacial maximum. A high-resolution seismic profile and a transect of four cores were collected across Beothuk drift on the southeast side of Flemish Pass. Downcore and lateral trends in grain size and sedimentation rate provide evidence that, between 16 and 13 ka, sediment was partitioned across Beothuk drift and the adjacent Flemish Pass floor by a strong current flow but, from 29 to 16 ka, sedimentation was more of a blanketing style, represented by draped reflections interpreted as being due to a weaker current. The data poorly resolve the low sedimentation rates since 13 ka, but the modern Labrador Current in Flemish Pass is the strongest it has been in at least the past 29 ka. Pre-29 ka current flow is interpreted based on reflection architecture in seismic profiles. A prominent drift on the southwestern side of Flemish Pass formed above a mid-Miocene erosion surface, but was buried by a mass-transport deposit after the penultimate glacial maximum and after drift deposition switched to eastern Flemish Pass. These findings illustrate the temporal complexity of drift sedimentation and provide the first detailed proxy for Labrador Current flow since the last glacial maximum.

Long-term evolution of a sand spit, physical forcing and links to coastal flooding

Aerial photographs, topographic field data and environmental forcing agents were used to assess historic and annual sand spit changes at Ginst Spit, Pendine Sands, West Wales. Aerial photographs highlighted that the spit shoreline facing the dominant wave direction had accreted steadily throughout the assessment timeframe (1947–2010). Stabilisation works in the form of rubble mounds aided recovery of the dune system thought to have been damaged due to training exercises carried out during the Second World War. Annual topographic surveys (1995–2010) showed a terminal end eastward migration extending into the channel of the 3 estuary complex. Dominant waves emanate from south southwest, and wave models suggest that longshore sediment drift is from west toward east. Sub-dominant southeast waves create a counter drift back toward the west forming the customary northward hook. Model results showed significant wave transformation from offshore model boundaries to the nearshore zone for both wave height and direction, which found agreement with qualitative assessment. Annually averaged wave components showed varying correlations with sand spit rates of shoreline change. Precipitation and spit growth were associated with flood events, which suggested that a combination of fluvial and coastal processes contribute to lowland inundation. By showing the major role the spit plays in protecting the town in its lee, this work provides a new understanding of sand spit evolution to inform coastal management.

Contourite drifts on early passive margins as an indicator of established lithospheric breakup

The Albian–Cenomanian breakup sequence (BS) offshore Northwest Iberia is mapped, described and characterised for the first time in terms of its seismic and depositional facies. The interpreted dataset comprises a large grid of regional (2D) seismic-reflection profiles, complemented by Industry and ODP/DSDP borehole data. Within the BS are observed distinct seismic facies that reflect the presence of: (a) black shales and fine-grained turbidites, (b) mass-transport deposits (MTDs) and coarse-grained turbidites, and (c) contourite drifts. Borehole data show that these depositional systems developed as mixed carbonate–siliciclastic sediments proximally, and as organic-carbon-rich mudstones (black shales) distally on the Northwest Iberia margin. MTDs and turbidites tend to occur on the continental slope, frequently in association with large-scale olistostromes. Distally, these change into interbedded fine-grained turbidites and black shales showing widespread evidence of deep-water current activity towards the top of the BS. Current activity is expressed by intra-BS erosional surfaces and sediment drifts. The results in this paper are important as they demonstrate that contourite drifts are ubiquitous features in the study area after Aptian–Albian lithospheric breakup. Therefore, we interpret the recognition of contourite drifts in Northwest Iberia as having significant palaeogeographic implications. Contourite drifts materialise the onset of important deep-water circulation marking the establishment of oceanic gateways between two fully separated continental margins. As a corollary, we postulate the generation of deep-water geostrophic currents to have had significant impact on North Atlantic climate and ocean circulation during the Albian–Cenomanian, with the record of such impacts being preserved in the contourite drifts analysed in this work.

Northward drift of suspended sediments in the Yangtze estuary in spring

The spatial distribution of suspended sediment concentration (SSC) in spring (February–April) from 2007 to 2013 in the Yangtze (Changjiang) estuary and coastal seas was analysed using remote-sensing data. The results indicated that areas of high SSC drifted northwards along the coast about 100 km away from the Yangtze estuary. SSC increased considerably in both mass and area in 2013. The first principal component analysis (PCA) pattern explained the dominant spatial distribution pattern of SSC in the study area. This highly clustered controlling pattern may have been caused by the joint effects of Yangtze diluted water (YDW), the Taiwan Warm Current (TWC), coastal currents, northeasterly winds, and the shallow coastal shelf. The SSC dispersed and extended into a wide area, and the main range of sediment concentration was located in an area the shape of a trapezium (31.9° N–33.8° N, 122° E–125.5° E, 30.7° N–32.6° N). YDW and the TWC could be the main reason for this northward drift of SSC. The long-term sinking of suspended sediment in clustered areas may have contributed considerably to the formation of the shallow coastal shelf above the 60 m isobath.

Contribution of swash processes generated by low energy wind waves in the recovery of a beach impacted by extreme events: Nha Trang, Vietnam

ABSTRACT Lefebvre, J.-P., Almar, R., Viet, N.T., Uu, D.V., Thuan, D.H., Binh, L.T., Ibaceta, R., Duc,N.V., 2014. Contribution of swash processes generated by low energy wind waves in the recovery of a beach impacted by extreme events: Nha Trang, Vietnam . In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 663–668, ISSN 0749-0208. Nha Trang beach experiences southerly sediment drift during winter monsoons and northerly sediment drift during summer monsoons. In addition, the area is likely to be impacted by tropical storms or typhoons. Due to the presence of islands at the south east border of the bay, the strongest impact on the shoreline apart from extreme events is due to NE swell from October to April. The mechanism responsible for the sediment drift generated by low energetic locally generated wind waves is insufficiently understood. It involves the functioning of the swash zone for w...

Relationships between sand spit evolution, environmental forcing and inland inundation

ABSTRACT Thomas, T., Lynch, S.K., Phillips, M.R., Williams, A.T., Duck, R.W. 2014. Relationships between sand spit evolution, environmental forcing and inland inundation.In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium(Durban, South Africa),Journal of Coastal Research, Special Issue No. 70, pp. 072–077, ISSN 0749-0208. Topographic field data and environmental forcing agents were used to assess morphological changes (1995–2010) at Ginst Spit, Pendine Sands, West Wales and flood potential at Laugharne, a town in the lee of the spit. Areal change between annual surveys showed variable distal end migration eastward extending into the channel of the Towy, Taf and Gwendreath estuaries. Dominant waves emanate from south toward southwest and suggest that the longshore sediment drift is from west toward east. However, sub-dominant waves from southeast are limited by fetch but engender a counter drift back toward the west at the distal end forming the customary northward h...

Effects of sprayers and nozzles on spray drift and terminal residues of imidacloprid on wheat

In pesticide application, it can be obtained by using appropriate nozzles and sprayers to reduce spray drift and pesticide residues. In this study, a field trail was conducted to determine the spray drift and pesticide residue of imidacloprid on wheat, using three types of flat fan nozzles from the LECHLER Company (LU120-02, AD120-02, IDK120-02) and two types of sprayers (guided-baffle shield sprayer (GBSS) and conventional sprayer (CS)). The spray drift (measured with both airborne and sediment samplers) and terminal residues on wheat were analyzed by liquid chromatography triple-quadrupole mass spectrometry. The results show that both the sediment drift and airborne spray profile were greatest for the LU120-02 nozzle, intermediate for the IDK120-02 nozzle, and lowest for the AD120-02 nozzle. The shielded sprayer gave one third of the drift recorded by airborne samplers and one half of the drift recorded by sediment samplers, compared with results from the CS. Airborne spray drift decreased with increasing height. The imidacloprid residues on wheat grain from different application methods were all below the maximum residue limit (MRL) suggested by EU (0.1 mg/kg) or China (0.05 mg/kg) and no imidacloprid residue was detected on wheat straw. Considering the factors on environment protection, food safety, and biological efficacy, the AD120-02 nozzle and GBSS performed better compared with other nozzles and sprayers. The combination of AD120-02 nozzle with a GBSS is likely to be the optimum application method on wheat.

Impacts of wave energy and littoral currents on shoreline erosion/accretion along the south-west coast of Kanyakumari, Tamil Nadu using DSAS and geospatial technology

The present study investigates the impact of wave energy and littoral current on shorelines along the south-west coast of Kanyakumari, Tamil Nadu, India. The multi-temporal Landsat TM, ETM+ images acquired from 1999 to 2011 were used to demarcate the rate of shoreline shift using GIS-based Digital Shoreline Analysis System. The statistical analysis such as net shoreline movement and end point rate were determined from the multi-temporal shoreline layers. Moreover, the wave energy and seasonal littoral current velocity were calculated for each coastal zone using mathematical equations. The results reveal that the coastal zones, which include Kanyakumari, Kovalam, Manavalakurichi and Thengapattinam coasts, consisting of maximum wave energy along with high velocity of littoral current, have faced continuous erosion processes. The estimated wave energy along these zones ranges from 6.5 to 8.5 kJ/km2 and the observed current velocity varies from 0.22 to 0.32 m/s during south-west and north-east monsoons. The cumulative effect of these coastal processes in the study area leads to severe erosion that is estimated as 300.63, 69.92, 54.12 and 66.11 m, respectively. However, the coastal zones, namely Rajakkamangalam, Ganapathipuram, Muttam and Colachel, have experienced sediment deposits due to current movement during the north-east monsoon. However, the trend changes during the south-west monsoon as a result of sediment drift through backwash. The spatial variation of shoreline and its impact on wave energy and the littoral current have been mapped using the geo-spatial technology. This study envisages the impact of coastal processes on site-specific shorelines. Hence, the study will be effective for sustainable coastal zone management.

20th century sediment budget trends on the Western Gulf of Lions shoreface (France): An application of an integrated method for the study of sediment coastal reservoirs

This paper presents a shoreface sediment budget established for the 20th century (1895–1984–2009) along the microtidal wave-dominated coast of the western Gulf of Lions (Languedoc-Roussillon, Mediterranean Sea, SE France). The implementation of a diachronic bathymetric approach, coupled with the definition of sand reservoirs (upper sand unit — USU) by very high-resolution seismic surveys and the results of LiDAR investigations, offers a new means of defining precisely the magnitude and change trends of the sediment budget. The aim of this study is to link the Large Scale Coastal Behaviour (LSCB) of the littoral prism (expressed in terms of shoreface sediment budget, shoreface sediment volume and spatial distribution pattern of cells) to climatic change, river sediment input to the coast, longshore sediment transport distribution, impact of hard coastal defence structures and artificial beach nourishment. The results show a significant reduction of the volume of the western Gulf of Lions littoral prism over 114years (−26.1±4.6×106m3). From 1895 to 1984, the overall budget is slightly positive, with a volume estimated at 4.1±3.5×106m3. For 1984–2009, however, the estimated sediment budgets clearly indicate that erosion is dominant over the last 25years, with a volume loss of −30.2±4.2×106m3. In relation to the long-term sediment budget and longshore drift pattern, the long-term trend of the USU volume distribution displays strong spatio-temporal contrasts linked to longshore sediment drift, spatial distribution of fluvial sediment inputs and hard engineering structures. Locally, the sedimentary reservoir is significantly eroded within a century (−80% of USU), since the initial amount present was low and not sustainable. The emphasis is on the importance of considering the volume changes of available sediment reservoirs rather than their losses and gains. Erosion of the Languedoc-Roussillon shoreface is likely to continue in the future due to the “natural” decrease of river sediment input and the sand removal for human purposes. Consequently the littoral sand prism results in sedimentary reservoirs that are gradually being used up.

Stratigraphic gaps at northern South China Sea margin reflect changes in Pacific deepwater inflow at glacial Termination II

To constrain short-term changes of climate and oceanography in the northern South China Sea (SCS) over interglacial marine isotope stage (MIS) 5.5, we studied planktic and benthic δ 18O records of seven marine sediment cores with a time resolution of 70–700 yr. Using 6-8 tie points the planktic records were tuned to the U/Th chronology of speleothem δ 18O records in China and Europe. The last occurrence of pink Globigerinoides ruber marks the top of Heinrich stadial 11 (HS-11) near 128.4 ka. HS-11 matches a 2300-yr long positive δ 18O excursion by 1.5/0.8‰ both in planktic and benthic δ18O records. Hence half of the planktic δ18O signal was linked to increased upwelling of 18O- and 12C-enriched deep waters in the southwestern SCS. The increase was possibly linked to a strengthened inflow of Pacific deep waters through the Bashi Strait, that form a boundary current along the northern slope of the SCS, building a major sediment drift. At its lower margin near 2300–2400 m water depth (w.d.) Parasound records reveal a belt of modern erosion. At the end of glacial termination 2, stratigraphic gaps deleted HS-11 in core MD05-2904 and subsequent peak MIS 5.5 at ODP Site 1144. Likewise hiatuses probably earmarked all preceding glacial terminations at Site 1144 back to 650 ka. Accordingly, boundary current erosion then shifted ∼300 m upslope to ∼2040–2060 m w.d. These vertical shifts imply a rise in boundary current buoyancy, that in turn may be linked to transient events of North Pacific deepwater formation similar to that traced in SCS and North Pacific paleoceanographic records over glacial termination 1.

Current and sea-level signals in periplatform ooze (Neogene, Maldives, Indian Ocean)

Periplatform ooze is an admixture of pelagic carbonate and sediment derived from neritic carbonate platforms. Compositional variations of periplatform ooze allow the reconstruction of past sea-level changes. Periplatform ooze formed during sea-level highstands is finer grained and richer in aragonite through the elevated input of material from the flooded platform compared to periplatform ooze formed during the episodes of lowered sea level. In many cases, however, the sea floor around carbonate platforms is subjected to bottom currents which are expected to affect sediment composition, i.e. through winnowing of the fine fraction. The interaction of sea-level driven highstand shedding and current impact on the formation of periplatform ooze has hitherto not been analyzed. To test if a sea-level driven input signal in periplatform ooze is influenced or even distorted by changing current activity, an integrated study using seismic, hydroacoustic and sedimentological data has been performed on periplatform ooze deposited in the Inner Sea of the Maldives. The Miocene to Pleistocene succession of drift deposits is subdivided into nine units; limits of seismostratigraphic units correspond to changes or turnarounds in grain size trends in cores recovered at ODP Site 716 and NEOMA Site 1143. For the Pleistocene it can be shown how changes in grain size occur in concert with sea-level changes and changes of the monsoonal system, which is thought to be a major driver of bottom currents in the Maldives. A clear highstand shedding pattern only appears in the data at a time of relaxation of monsoonal strength during the last 315ky. Results imply (1) that drift sediments provide a potential target for analyzing past changes in oceanic currents and (2) that the ooze composition bears a mixed signal of input and physical winnowing at the sea floor.

A GLACIAÇÃO STURTIANA (~750 MA), A ESTRUTURA DO RIFTE MACAÚBAS-SANTO ONOFRE E A ESTRATIGRAFIA DO GRUPO MACAÚBAS, FAIXA ARAÇUAÍ

This paper is a contribution to the knowledge of the Sturtian glaciation (~800-750 Ma) and the gravitationalsedimentation of the Macaúbas and Santo Onofre Groups. The primary goal of this paper is: (1) todescribe glaciogenic deposits on the eastern edge of the São Francisco craton; (2) to present stratigraphic,sedimentologic and the tectonic setting of the Macaúbas-Santo Onofre rift. An extensional phase is markedby the intrusion of mafic dike swarms at 1100 – 900 Ma and alkaline rocks (750-670 Ma). The Sturtianglacial event (800-750 Ma), with formation of ice-caps in Rodinia supercontinent, is well exposed in theJequitaí area. Glaciomarine facies developed on the border of the craton grade laterally into debris-flowand turbidites. The Neoproterozoic Macaúbas – Santo Onofre rift evolved into a wide embayment limitedby normal faults and a transfer zone. The sedimentary sequence consists of the diamictites, conglomerates,sandstones,, graywackes, pelites deposited by sediment gravity flows. The Neoproterozoic Macaúbas Grouprepresents rift to drift sedimentation related to the break-up of Rodinia. Macaúbas- Santo Onofre rift isregarded as intracontinental with a limited oceanization located at southern of the basin.

Palaeoenvironmental records from the West Antarctic Peninsula drift sediments over the last 75 ka

Abstract We present results of a multi-proxy study on marine sediment core JR179-PC466 recovered from the crest of a sediment drift off the West Antarctic Peninsula at approximately 2300 m water depth. The 10.45 m-long core consists dominantly of glaciomarine terrigenous sediments, with only traces of calcium carbonate (<1 wt%). Despite the very low abundance of calcareous foraminifera, planktonic shell numbers are sufficient for stable isotope analyses in two-thirds of the samples studied. The core chronology is based on oxygen isotope stratigraphy and correlation of its relative palaeomagnetic intensity (RPI) with a stacked reference curve. According to the age model, core PC466 spans the last 75 ka, with average sedimentation rates of between about 4 and 25 cm ka −1 . Planktonic foraminifera abundances fluctuate between 0 and 30 individuals per gram throughout the core, with minima observed during Marine Isotope Stage (MIS) 2 (14–29 ka before present, BP) and MIS4 (57–71 ka BP). Planktonic foraminifera are present in the Holocene but more abundant in sediments deposited during MIS3 (29–57 ka BP), owing to less dilution by terrigenous detritus and/or better carbonate preservation. During MIS3, foraminifera maxima correlate with Antarctic warming events as recorded in the δ 18 O signal of the EPICA Dronning Maud Land (EDML) ice core. They indicate higher planktonic foraminifera production and better carbonate preservation west of the Antarctic Peninsula during that time. The abundance of ice-rafted detritus (IRD) in core PC466 increased during the last deglaciation between about 19 and 11 ka BP, when numerous icebergs drifted across the core site, thereby releasing IRD. During this time, sea-level rise destabilized the Antarctic Peninsula (APIS) and West Antarctic (WAIS) ice sheets that had advanced onto the shelf during the sea-level low-stand of the Last Glacial Maximum (LGM; c . 19–23 ka BP). Overall, our results demonstrate that it is possible to establish an age model and reconstruct palaeoceanographical and climatic changes at high temporal resolution from sedimentary sequences recovered at 2300 m water depth from a West Antarctic drift.

Active gas venting through hydrate-bearing sediments on the Vestnesa Ridge, offshore W-Svalbard

Abstract Gas hydrate systems offshore western Svalbard are extensive and include the whole Vestnesa Ridge, an elongated sediment drift north of the Molloy Transform and just east of the Molloy Ridge, one of the shortest segments of the slow spreading North Atlantic Ridge system. The crest of the Vestnesa Ridge at water depth between 1200–1300 m is pierced with fluid flow features. Seafloor pockmarks vary in size up to 700 m in diameter. Pockmarks are generally larger at the eastern start of the Vestnesa Ridge than at its western termination. A recent cruise with R/V Jan Mayen discovered methane flares in the water column above the pockmark field at the onset of the Vestnesa Ridge. Over a period of two days at least 3 pockmarks were continuously active and methane flares in the water column reached a height of approximately 800 m. It is still unclear what has triggered the increase in gas expulsion from seafloor sediments. High-resolution 3D seismic data acquired in 2007 and 2010 show significant differences of the subseafloor expression of these fluid leakage systems. At the western end of the Vestnesa Ridge, sub-seafloor fluid flow features resemble well-described chimney structures. However, the seismic expression of the active fluid flow features is much broader, much more chaotic, dome-shaped and is not very similar to a typical chimney structure. The Vestnesa Ridge gas-hydrate and free-gas system occurs within few kilometers of a mid-oceanic ridge and transform fault, which makes this gas-hydrate system unique on Earth. The close proximity to the spreading center and its hydrothermal circulation system affects the dynamics of the gas-hydrate and free-gas system. The high heat flow together with the high tectonic activity of this region, a thick sedimentary cover, a shallow maturation window and an accelerated rate of biogenic and thermogenic gas production cause substantial disturbance to the free-gas system leading to high variability in gas supply, gas migration and gas hydrate build up and dissociation.

Variations in sediment transport at the central Argentine continental margin during the Cenozoic

The construction of the sedimentary cover at most passive continental margins includes gravitational downslope transport and along‐slope contourite deposition, which are controlled by tectonics, climate and oceanography. At the eastern continental margin of Argentina the history of deposition and erosion is intimately linked to the evolution of the South Atlantic and its water masses. Here we present a detailed seismic investigation of the mixed depositional system located between 41°S and 45°S. The study provides a northward complement to prior investigations from the southern Argentine margin and together with these may be used as background information for future ocean drilling in the region. Prominent features in our seismic cross sections are submarine canyons, mass wasting deposits, contourite channels, and sediment drifts. Four major seismic units above regional reflector PLe (∼65 Ma) are separated by distinct unconformities of regional extent. Using a dense grid of reflection seismic profiles, we mapped the depocenter geometries of the seismic units and derived a chronology of the depositional processes during the Cenozoic. While the Paleocene/Eocene (∼65–34 Ma) is characterized by hemipelagic sedimentation under relatively sluggish bottom water conditions, strong Antarctic bottom water (AABW) circulation led to widespread erosion on the slope and growth of a detached sediment drift during the Oligocene and early Miocene (∼34–17 Ma). After deposition of an aggradational seismic unit interpreted to represent the Mid‐Miocene climatic optimum (∼17–14 Ma), gravitational downslope sediment transport increased during the middle to late Miocene (∼14–6 Ma) possibly related to tectonic uplift in South America. The Pliocene to Holocene unit (<∼6 Ma) is very heterogeneous and formed by interactions of downslope and along‐slope sediment transport processes as indicated by the evolution of canyons, slope plastered drifts and channels.

Submarine mass wasting and associated tsunami risk offshore western Thailand, Andaman Sea, Indian Ocean

Abstract. 2-D seismic data from the top and the western slope of Mergui Ridge in water depths between 300 and 2200 m off the Thai west coast have been investigated in order to identify mass transport deposits (MTDs) and evaluate the tsunamigenic potential of submarine landslides in this outer shelf area. Based on our newly collected data, 17 mass transport deposits have been identified. Minimum volumes of individual MTDs range between 0.3 km3 and 14 km3. Landslide deposits have been identified in three different settings: (i) stacked MTDs within disturbed and faulted basin sediments at the transition of the East Andaman Basin to the Mergui Ridge; (ii) MTDs within a pile of drift sediments at the basin-ridge transition; and (iii) MTDs near the edge of/on top of Mergui Ridge in relatively shallow water depths (< 1000 m). Our data indicate that the Mergui Ridge slope area seems to have been generally unstable with repeated occurrence of slide events. We find that the most likely causes for slope instabilities may be the presence of unstable drift sediments, excess pore pressure, and active tectonics. Most MTDs are located in large water depths (> 1000 m) and/or comprise small volumes suggesting a small tsunami potential. Moreover, the recurrence rates of failure events seem to be low. Some MTDs with tsunami potential, however, have been identified on top of Mergui Ridge. Mass-wasting events that may occur in the future at similar locations may trigger tsunamis if they comprise sufficient volumes. Landslide tsunamis, emerging from slope failures in the working area and affecting western Thailand coastal areas therefore cannot be excluded, though the probability is very small compared to the probability of earthquake-triggered tsunamis, arising from the Sunda Trench.

Anatomy of the tidal scour system at Minas Passage, Bay of Fundy, Canada

Strong currents have eroded thick Quaternary sediments to create a scour trough at Minas Passage, in the Bay of Fundy, Canada, site of Earth's largest tides. We describe this trough in the context of a larger system that comprises a range of elements, viz: 1) Scour troughs extending 170m below mean sea level are incised into thick glaciomarine sediments and have exhumed bedrock over wide areas. The flanking uneroded terrain has low relief and a winnowed surface. 2) Sets of sand banner banks off Cape D'Or and Cape Chignecto. 3) The atypical set of banner banks at Cape Split, consisting of the Scots Bay dune field and its counterpart, a large gravel bank trapped in the Minas Passage scour trough. 4) Low-relief banks with sand ribbons and barchan dunes alongside some banner banks, and termed ‘shadow banks’. 5) A large (0.8km3) sediment drift at the entrance to Minas Channel (without large bedforms). The location of troughs and banks can be correlated with tidal-current patterns: trenches are located in regions of very strong bi-directional currents; banner banks near headland-sited tidal gyres; shadow banks in areas of maximum mean bottom stress asymmetry; and the sediment drift at the entrance to Minas Channel in an area of weak bottom stress at all stages of the tides. Previous work has argued that the scour system formed after 3400 14Cyrs BP (radiocarbon years before present) following collapse of a barrier system across Minas Passage. We speculate that formation of the scour trough system may have released vast quantities of sediment that have not been accounted for in previous sediment budgets, and that much of this released sediment has been sequestered in the late-Holocene salt marshes at the head of the Bay of Fundy.

Structure of mud volcano systems and pockmarks in the region of the Ceuta Contourite Depositional System (Western Alborán Sea)

New high-resolution seismic and swath bathymetry data reveal the detailed structure of nine mud volcanoes and two large fields of pockmarks at water depths of 400–1100m in the western Alborán Sea. These features are related to episodes of focused fluid flow through Pliocene-Quaternary contourite deposits of the Ceuta Drift. We describe nine mud volcano systems, composed of two key structural elements: (i) extrusive edifices comprising cones, bicones and low-dip mudflows interfingering with adjacent drift sediments and (ii) intrusive feeder complexes comprising pipes and unstratified downward-tapering cones underlying the extrusive edifices. The mud volcano systems are grouped into four types: I: Complex edifices; II: Stacked single edifices; III: Single edifices fed by buried bicones and IV: Single edifices. All except the last record episodic extrusive activity since the Pliocene, the timing of which is constrained by correlation between the main mud extrusion episodes and dated unconformities within the Ceuta Drift. Complex mud volcanoes composed of twin to multiple seabed cones are the longest-lived systems, with at least six extrusive episodes since the mid-Pliocene; most of the mud volcanoes appear to have been reactivated in the mid-Pleistocene (~0.9Ma, MPR discontinuity) and all at MIS12 (0.4Ma). While the onset of focused fluid flow was triggered by mid-Pliocene tectonically-driven diapirism of Lower Miocene shales that form the rooting zone of the mud volcanoes, we suggest that some of the Pleistocene phases of mud extrusion might be related to erosion of the contourite drift during major sea-level falls.In addition to vertical focused fluid flow, we propose that leakages from the mud volcano feeder complexes into high-permeability contourite sediments may cause lateral fluid migration and form gas accumulations. Lateral fluid flow in the near surface sediments could be responsible of forming subsurface reservoirs and consequently, random pockmarks by fluidification and/or liquefaction of the non-consolidated shallow sediments. Stacked buried paleo-pockmarks indicate that this mechanism has been persistent along recent times, at least after the mid-Pleistocene.