Sediment Wave Fields Research Articles

The role of sediment gravity flows on the morphological development of a large submarine canyon (Taiwan Canyon), north‐east South China Sea

AbstractHigh‐resolution multibeam bathymetric and multichannel seismic data are used to investigate the morphology of a submarine canyon (Taiwan Canyon), and its surrounding strata, in the north‐east South China Sea. This submarine canyon shows two main branches at its head, and changes its orientation from north‐west/south‐east to east–west due to the effect of a tectonically active seamount. The asymmetry of the submarine canyon’s banks in its middle reach is due to the combined action of recurrent slope instability and turbidity currents. In addition, two fields of sediment waves were identified in the study area. Field 1 is located on the south‐west levée of the canyon and is fed by turbidity currents from one of its branches, being also associated with marked hydraulic jumps. Field 2 is observed in the southern bank of the lower canyon reach and was formed by the overspill of turbidity currents within the Taiwan Canyon due to the effect of inertial centrifugal forces. Turbidity currents sourced from Dongsha Channel also contributed to forming Field 2. Importantly, trains of plunge pools have been identified along the thalweg of the lower canyon reach, generated by turbidity currents deriving from the submarine canyons in the north of the Taiwan Canyon. These results not only provide a very detailed account of submarine bedforms within and around a large submarine canyon, but also contribute to a better understanding of their origin and development. The high‐resolution bathymetric and seismic data in this work reveal how gravity flows can drive erosion and deposition in submarine canyons.

Depositional architecture and evolution of basin-floor fan systems since the Late Miocene in the Northwest Sub-Basin, South China Sea

The sediment budget of the Northwest Sub-basin, South China Sea since the Late Miocene (11.6 Ma, average thickness > 1000 m) accounts for more than two-thirds of the total infill since the initial ocean spreading of the sub-basin (32 Ma). The sediment sources and architectural pattern of these deposits, however, are poorly known. Using high-resolution 2D reflection seismic data with age constraint from IODP boreholes, we have documented two interdigitating basin-floor fan systems that developed since the Late Miocene. These were fed by two of the largest deep-water canyon systems worldwide, from the west (the Central Canyon/Xisha Trough) and the northeast (the Pearl River Canyon), as well as from smaller headless canyons and gullies across the surrounding slopes. Based on careful analysis of seismic facies, their geometry and occurrence, we identify the principal deep-water architectural elements, including the multi-scale channels, channel-levee complexes, lobes, sheets and drapes, mass-transport deposits, volcanic intrusions, turbidity-current sediment-wave fields, and a contourite drift/terrace.Tentative reconstructions show that the development of these Late Miocene-Quaternary basin-floor fan systems was dominated by changes of sediment supply. The Xisha fan reached its largest extent during the Late Miocene, while the Pearl River fan was most active during the Late Miocene to Quaternary. During the Late Miocene, both the conduits of the Central Canyon and the Pearl River Canyon were active with abundant sediment supply, generating the two incipient fan systems. Sediment supply from the west via the Central Canyon persisted throughout the Late Miocene, being coarser-grained than that of the Pearl River fan. With the demise of the Central Canyon during the Pliocene and consequent sharp decrease in sediment supply, the Xisha fan size reduced significantly. By contrast, supply of mud-rich sediments from the Pearl River and northern slope increased through the Pliocene and into the Quaternary, leading to the modern sedimentary pattern of interdigitating basin-floor fans. Insights into the evolution of sediment supply and fan development through time derived in this study contribute to a better understanding of how source to sink systems feed marginal oceanic basins such as the South China Sea.

Stratigraphic framework and sediment wave fields associated with canyon-levee systems in the Huatung Basin offshore Taiwan Orogen

Newly compiled bathymetry, sub-bottom profiling and seismic data are used to illustrate seafloor morphology, submarine canyon networks and stratigraphic framework in Huatung Basin (HB) and an adjacent sub-basin, named as East Luzon Arc Basin offshore east Taiwan. Based on seismic facies characteristics and isopach maps, three major stratigraphic sequences are recognized and we suggest that they could be resulted from pre- and syn-Taiwan arc-continent collision. Based on the sediment thickness and sedimentation rates, ages of the HB strata could be estimated. Pelagic sediments formed the lower HB sequence which follows the HB basement reliefs since 50–102 Ma. Then, the initiation of Taiwan orogen and Luzon Arc volcanic activities changed the deposition pattern after 5 Ma, and mass transport deposits developed in middle HB sequence. After rapid uplift of the Taiwan mountain belt since about 1.5 Ma, complex canyon-levee systems and thick strata formed the upper sequence in the northern HB. In contrast, sediment thickness is thin and no submarine canyon developed in southern HB. The seafloor in South Huatung Basin is flat, except in its eastern part where couple basement ridges appeared. In North Huatung Basin, canyon-levee complex and sediment waves are the most distinct morphological features in the drainages of two submarine canyons. We propose a three-stage process for sediment wave development: (1) sediment gravity flow can initiate a base of sediment wave; (2) wavy geometry begins to develop; (3) sediment waves continuously aggrade due to overbank sediments in the canyon-levee system. This study presents the evolution of sedimentary processes in a deep-sea basin and how the deposition patterns were affected by the convergent tectonics.

Neogene to recent evolution of the Southern Gulf of Mexico basin: Tectonic controls on deep‐water sediment dispersal systems

AbstractNewly available two‐dimensional (2D) and limited three‐dimensional (3D) reflection seismic data coupled with publicly available well and core data were used to generate the first comprehensive regional basin evolution model for the deep‐water Neogene to recent southern Gulf of Mexico (GoM). This evolution is presented in the context of contemporaneous onshore tectonic drivers and predecessor basin tectonic history. Dynamic uplift to the west in North America, transpression to the south and tectonically influenced salt tectonics to the east served to collapse the margins of the southern Gulf of Mexico Basin throughout the late Neogene and into the modern. This collapse and salt inflation served to shut off efficient sediment transport into the axial deep basin. Newly developed nearshore extensional accommodation along both the western margin of the basin and salt‐rooted orogenic structures to the south and east appear to have baffled coarser‐caliber sediment input from southern (Mexican) sediment sources into the deep Gulf of Mexico Basin beginning in the late Miocene. This sequence is recorded in the southern GoM basin by a transition from large early–mid Miocene submarine channel belts to late Miocene–recent sediment wave fields and mass transport deposits. Improved resolution of new 2D and 3D subsurface seismic reflection data shows that sediment waves of the southern Gulf of Mexico are supercritical bedforms, long wavelength antidunes and cyclic steps, not contourites. These supercritical bedforms are among the most spatially expansive and morphologically largest documented globally. The location and evolution of these bedforms appears closely tied to the tectonically driven history of margin collapse, salt inflation and the starving of the deep basin of coarse‐grained sediment. Cumulatively, the tectonic drivers that intuitively should have increased sediment flux into the deep southern Gulf of Mexico acted to ‘pinch off’ the basin, starving it of sediment supplied from southern sediment sources.

Sediment waves on the Santa Catarina Plateau (western South Atlantic)

High-resolution seismoacoustic and multibeam data revealed a large field of sediment waves covering the Santa Catarina Plateau on the Brazilian Southern Margin. Within the field several zones were determined: a zone of large sediment waves (up to 70 m in height), a zone of smaller sediment waves (15–30 m in height) and a zone of buried sediment waves. The correlation of the seismoacoustic and multibeam data with the results of numerical modeling of bottom current velocities showed a strong influence of bottom currents on the formation of sediment waves. The process of wave burial in the northern part of the field can be caused by the interplay between gravity flows from the continental slope and bottom currents.

Late Quaternary multi-genetic processes and products on the northern Gulf of Cadiz upper continental slope (SW Iberian Peninsula)

On continental margins, the upper slope to shelf break environment forms a critical region where sediment supply, hydrographic activity and gravitational processes determine how and when sediments are partitioned between the shallow- and deep-marine realm. On the SW Iberian margin, relatively few studies have addressed the dynamics of this region, although it holds key information regarding the link between the sedimentary evolution of the continental shelf and the contourite depositional system on the middle slope. This work therefore presents a high-resolution analysis of the morphological and stratigraphic expressions of late Quaternary (dominantly last glacial and present-day interglacial) sedimentary processes on the upper slope and shelf margin sector between 7° and 7°30’ W.The integration of seismic, bathymetric and hydrographic data reveals the presence of alongslope processes and products (a bottom current-related plastered drift, moat, erosional surface and terrace, an internal wave-/tide-controlled sediment wave field), downslope (gravitational) processes and products (an upper slope – shelf margin valley system, slumps, debrites, gullies), neotectonic elements (diapirs) and fluid flow features (pockmarks, bright spots). The spatial distribution of these features indicates that the study area becomes increasingly alongslope-dominated towards the W, and oppositely, more downslope-dominated towards the E, because sediment supply to the latter area is enhanced under the dominant eastward dispersal of fluvially supplied sediments on the shelf. In addition, glacial-interglacial variations in the amount of sediments supplied to the shelf edge and the intensity of oceanographic processes in the study area also generate a distinct temporal variability, with glacial and interglacial intervals respectively recording principally downslope- and alongslope-controlled morphological elements. Finally, regardless of these overall spatial and temporal patterns, diapirism and fluid flow are inferred to locally destabilize sediments and induce small-scale mass wasting in the study area. These findings are not only relevant to the northern Gulf of Cadiz, but also to the general understanding of sedimentary dynamics and controls in mixed downslope- and alongslope-controlled upper slope to shelf margin settings worldwide.

Seismic evidence of bottom current controlled sedimentation in the Ceará Rise region (central Atlantic)

This study is based on the seismoacoustic data (3.5 kHz) collected during cruise 9209 of the RV Maurice Ewing (1992), organized within the framework of preparation for ODP drilling – leg 154. The analysis of the acquired data helped distinguish major acoustic facies and their spatial distribution in the study area. It was found out that almost the entire northern segment of the Ceará Rise is covered by a vast field of contourite sediment waves. The distribution of sediment wave heights is suggested to be controlled by geostrophic currents of AABW and NADW as well as by processes on the boundaries between water masses. Contourite erosional features are not widespread and generally resulted from the interaction of the bottom current with prominent topographic obstacles like western scarp of the Ceará Rise or basement outcrops.

Features and Origin of Turbidity Current Sediment Waves in the Huatung Basin off the Eastern Taiwan Island

Based on numerous high‐resolution seismic profiles, sediment waves and their distribution, morphological characteristics, internal structure, and potential origins were revealed in the eastern waters of Taiwan. The sediment waves are located at the junction between the Taitung Canyon and other canyons in the slope. The wave length and the wave height of a single waveform ranged from 0.8 to 7.2 km and from 18 to 75 m, respectively (NE‐SW direction). Sediment waves, located inside the bend of the Taitung Canyon, were characterized by an upward migration and showed mass transport deposits (MTDs) at the bottom, while the inner curve of the bend was subdivided into lower and upper wavy transition units. The sediment waves on the outer curve of the bend were characterized by vertical accumulation, and there was no mass flow deposit at the bottom. According to the geometry of the sediment waves, the calculated flow thicknesses across the entire wave field ranged from 196 to 356 m, and the current velocity ranged from 15 to 21 cm/s. The morphological characteristics, the internal structure, and the distribution of sediment waves, as well as the numerical calculations, evidenced that these sediment waves had formed by turbidity currents. The development of the sediment wave field in eastern Taiwan was found to be similar to that in southwestern Taiwan. It was the sedimentary response of the tectonic movement between 3 and ∼1 Ma which created the sedimentary systems where gravity flow processes predominated. Turbidity current sediments settled in the place of less topographical constraints or overflowed in the bend section of the Taitung Canyon, which resulted in the formation of sediment wave fields.

Gravitational, erosional, sedimentary and volcanic processes on the submarine environment of Selvagens Islands (Madeira Archipelago, Portugal)

Volcanic oceanic islands are the emerged portions of large volcanoes that rise abruptly from the ocean floor. Although the geological record of these islands provide some insights into the evolution of their entire volcanic edifices, for a complete picture, it is essential to study the islands' submarine pedestals and surrounding settings. In this study we document the submarine morphology of the Selvagens Islands (Madeira Archipelago, Portugal) and relate it with their subaerial evolution, in order to gain a more complete insight on the evolutionary history of these islands. To this purpose, we employed bathymetric surfaces, backscatter mosaics and subbottom profiler data integrated with geological observations of the subaerial part of the islands. Our study reveals the presence of a complex submarine morphology composed mainly of two deeply eroded central volcanoes – the edifices of Selvagem Grande and Selvagem Pequena – and a less eroded saddle-like ridge related to a fissural volcanic system located between them. Other secondary landforms generated by volcanism, erosion, gravitational collapses, and possible faulting activity, are also present. Whilst the building of the two central volcanoes relates to the older volcanic stages of both islands, the fissural system, with a general NE-SW trend, is probably contemporaneous of the younger volcanic stages of Selvagem Grande. The shelf of Selvagem Grande also features the eroded remains of numerous relatively young volcanic cones, seemingly forming several NW-SE and NE-SW lineaments, the same direction observed onshore for the younger volcanic stage of this edifice. The shelves' break of both Selvagens was affected by multiple landslide scars and channel headwalls. The channels constitute a large drainage network that fans out in a radial pattern along the flanks of both central edifices. In the deeper parts of this submarine drainage, large sediment wave fields are observed in some locations with wavelengths of 300–1,900 m and 2–45 m in height; these tend to form cyclic steps that change gradually downslope to antidunes, and their origin is attributed to sediment density flows that suffer hydraulic jumps when their channel confinement ends and the slope gradient decreases from ~7° to ~2.5°. Several scour features are also found at 3,300–4,000 m water depth, being 3–20 km in width, 10–20 km in length, rectangular or U-shaped in cross-section and 40–100 m deep. They occur around the base of the submarine edifices, where seafloor gradients change to 3.0°–0.5°. Their origin is also related to hydraulic jumps of unconfined sediment density flows, which are able to erode the seafloor and create these features. These morphologies are a testimony to how volcanic, erosional and depositional processes conjugate to shape the flanks of a very long-lived volcanic island system, one of the oldest in the North Atlantic Ocean.

The role of internal waves in the late Quaternary evolution of the Israeli continental slope

Internal waves are widely present in world's oceans propagating in stratified ocean layers such as the seasonal pycnocline. At continental shelves and slopes, breaking of internal waves may create turbulence of sufficient energy to erode and transport sediments. In this study, we investigate the late Pleistocene to Recent impact of internal waves on the morphology of the Israeli Mediterranean continental slope. Based on mooring data, we show that the internal wave spectrum on the upper slope contains significant energy at near-inertial and semidiurnal tidal frequencies. Sediment subbottom profiler data show two fields of nearly contour parallel sediment waves located in water depths of 80 m–130 m and 190 m–350 m, respectively. A zone of flat seafloor morphology and remarkably constant slope inclination of 1.0°–1.2° separates both sediment wave fields. The entire stretch of seafloor between 80 m and 350 m corresponds to depths of major ocean density stratification, supporting the propagation of internal waves. In addition, this area is critically inclined with respect to semidiurnal internal tides and supercritically inclined with respect to near-inertial waves, two scenarios previously related to internal wave breaking. We propose that breaking internal waves control the inclination of the Israeli continental slope and contribute to sediment starvation within the zone of flat seafloor morphology. The formation of sediment waves is attributed to high-frequency trailing internal waves of similar wavelengths as sediment waves, observed within a seismic image of the water column. We correlate subbottom profiler data to a radiocarbon dated sediment core. No sediment waves formed from 16 cal ka–8 cal ka, indicating that the seafloor impact of internal waves ceased during a time of rapid sea level rise and fresh water input in the Eastern Mediterranean realm. The end of this period coincides with the formation of sapropel S1.

Enigmatic holocene sand ridges: complex meandering to anastomosing bedforms in the North Sea (German Seabight)

An intricate field of NNW-SSE trending sand ridges was discovered for the first time in 2010 and mapped in detail over the past 6 years during multiple campaigns in the vicinity of the islands of Helgoland and Dune in the German Seabight. The tramline-shaped, elongate, linear and meandering sand ridges are up to 1 km in length, over 10 m in height and spaced 10–50 m apart. The ridges exhibit complex anastomosing patterns, converging and disappearing to the south of the study area and diverging, becoming more subdued and disappearing further to the north. Seismic profiling, surface samples and multibeam data show that the ridges are stationary, while the sediment wave fields to the north and west are migrating westwards. Based on our mapping results, the most likely hypothesis for the development and maintenance of these enigmatic structures is a combination of tidal currents, tides and sediment influx. Whether these structures are associated with the Mesozoic subcrop or are glaciofluvial in origin and have been modified by the last glacial-interglacial intervals or are sedimentary structure linked to Holocene sea level rise in the southern North Sea is a matter of debate. Further studies, including numerical modelling of these sand ridges structures, would provide clues to past sedimentary and hydrodynamic conditions and future changes in the highly dynamic and complex tidal environment of the southern North Sea.

Origin of spectacular fields of submarine sediment waves around volcanic islands

Understanding how large eruptions and landslides are recorded by seafloor morphology and deposits on volcanic island flanks is important for reconstruction of volcanic island history and geohazard assessment. Spectacular fields of bedforms have been recognised recently on submerged flanks of volcanic islands at multiple locations worldwide. These fields of bedforms can extend over 50 km, and individual bedforms can be 3 km in length and 150 m in height. The origin of these bedform fields, however, is poorly understood. Here, we show that bedforms result from eruption-fed supercritical density flows (turbidity currents) in some locations, but most likely rotational landslides at other locations. General criteria are provided for distinguishing between submarine bedforms formed by eruptions and landslides, and emphasise a need for high resolution seismic datasets to prevent ambiguity. Bedforms associated with rotational landslides have a narrower source, with a distinct headscarp, they are more laterally confined, and internal bedform structure does not suggest upslope migration of each bedform. Eruption-fed density currents produce wide fields of bedforms, which extend radially from the caldera. Internal layers imaged by detailed seismic data show that these bedforms migrated up-slope, indicating that the flows that produced them were Froude supercritical. Due to the low density contrast between interstitial fluid and sediment, the extent and dimensions of submarine eruption-fed bedforms is much greater than those produced by pyroclastic density currents on land.

Morphometric analysis of plunge pools and sediment wave fields along western Great Bahama Bank

A systematic assessment of morphometric parameters of over 200 plunge pools along the steep escarpment base of western Great Bahama Bank and ~2000km2 of sediment wave fields of upslope migrating cyclic steps documents the importance of cascading density currents for sedimentation on the slope. The abrupt downslope termination of the Holocene mud wedge with the cyclic steps shows that strike parallel bottom currents interacting with the off-bank currents are responsible for slope sediment distribution. The plunge pools and sediment waves were generated by density-cascading and both are related to a hydraulic jump caused by two gradient changes along the slope profile. The bedforms highlight both the erosional and depositional nature of this current regime, which is controlled by slope gradient and the directional confinement by either grooves on the steep escarpment or gullies on the middle to lower slope. The >20m deep plunge pools are ellipsoidal, with an average width of 211m and length of 283m, but variations between 80–360m and 100–650m for width and length occur. They are caused by an average gradient break of 16° on the cemented upper slope. Gradient breaks of as low as 2° are reported to be sufficient to scour a plunge pool like the ones in the study area, but higher gradient breaks between 10 and 20° are quite common for consistent plunge pool development. A second gradient break further down slope changes flow conditions for the generation of cyclic steps, with asymmetric waveforms of 4m high (average) in large wave fields in the entire the study area. Orientation is mainly orthogonal to the main slope direction but wave lengths and sinuosity vary, depending on the morphologic location of the wave field. The comparison with sediment wave fields with similar geometries or in similar environments shows that flow reconstruction based on the wave height and wave length alone along with a characterization of sediment grain size, however, yields non-unique results and is site specific.

Gravitational, erosional and depositional processes on volcanic ocean islands: Insights from the submarine morphology of Madeira Archipelago

The submarine flanks of volcanic ocean islands are shaped by a variety of physical processes. Whilst volcanic constructional processes are relatively well understood, the gravitational, erosional and depositional processes that lead to the establishment of large submarine tributary systems are still poorly comprehended. Until recently, few studies have offered a comprehensive source-to-sink approach, linking subaerial morphology with near-shore shelf, slope and far-field abyssal features. In particular, few studies have addressed how different aspects of the subaerial part of the system (island height, climate, volcanic activity, wave regime, etc.) may influence submarine flank morphologies. We use multibeam bathymetric and backscatter mosaics of an entire archipelago – Madeira – to investigate the development of their submarine flanks. Crucially, this dataset extends from the nearshore to the deep sea, allowing a solid correlation between submarine morphologies with the physical and geological setting of the islands. In this study we also established a comparison with other island settings, which allowed us to further explore the wider implications of the observations. The submarine flanks of the Madeira Archipelago are deeply dissected by large landslides, most of which also affected the subaerial edifices. Below the shelf break, landslide chutes extend downslope forming poorly defined depositional lobes. Around the islands, a large tributary system composed of gullies and channels has formed where no significant rocky/ridge outcrops are present. In Madeira Island these were likely generated by turbidity currents that originated as hyperpycnal flows, whilst on Porto Santo and Desertas their origin is attributed to storm-induced offshore sediment transport. At the lower part of the flanks (−3000 to −4300 m), where seafloor gradients decrease to 0.5°–3°, several scour and sediment wave fields are present, with the former normally occurring upslope of the latter. Sediment waves are often associated with the depositional lobes of the landslides but also occur offshore poorly-developed tributary systems. Sediment wave fields and scours are mostly absent in areas where the tributary systems are well developed and/or are dominated by rocky outcrops. This suggests that scours and sediment wave fields are probably generated by turbidity currents, which experience hydraulic jumps where seafloor gradients are significantly reduced and where the currents become unconfined. The largest scours were found in areas without upslope channel systems and where wave fields are absent, and are also interpreted to have formed from unconfined turbidity currents. Our observations show that tributary systems are better developed in taller and rainy islands such as Madeira. On low-lying and dry islands such as Porto Santo and Desertas, tributary systems are poorly developed with unconfined turbidite currents favouring the development of scours and sediment wave fields. These observations provide a more comprehensive understanding of which factors control the gravitational, erosional, and depositional features shaping the submarine flanks of volcanic ocean islands.

Occurrence of submarine canyons, sediment waves and mass movements along the northern continental slope of the South China Sea

In this study, we reveal a series of newly discovered submarine canyons, sediment waves, and mass movements on a flat and smooth seafloor using high-resolution, multi-beam bathymetry and shallow seismic surveys along the northern slope of the South China Sea. We also describe their geomorphology and seismic stratigraphy characteristics in detail. These canyons display U-shaped cross sections and are roughly elongated in the NNW–SSE direction; they are typically 8–25 km long, 1.2–7 km wide, and form incisions up to 175 m into Pliocene–Quaternary slope deposits at water depths of 400–1000 m. Slide complexes and the sediment wave field are oriented in the NE–SW direction and cover areas of approximately 1790 and 926 \(\hbox {km}^{2}\), respectively. Debris/turbidity flows are present within these canyons and along their lower slopes. Detailed analysis of seismic facies indicates the presence of six seismic facies, in which Cenozoic strata located above the acoustic basement in the study area can be roughly subdivided into three sequences (1–3), which are separated by regional unconformities (Tg, T4, and T3). By combining these data with the regional geological setting and the results of previous studies, we are able to determine the genetic mechanisms used to create these canyons, sediment wave field, and mass movements. For example, frontally confined slide complexes could have been influenced by high sedimentation rates and high pore pressures. A series of very large subaqueous sediment waves, which record wavelengths of 1.4–2 km and wave heights of 30–50 m, were likely produced by interactions between internal solitary waves and along-slope bottom (contour) currents. Canyons were likely initially created by landslides and then widened laterally by the processes of downcutting, headward erosion, and active bottom currents and debris/turbidity flows on canyon floors. We therefore propose a three-dimensional model to describe the development of these mass movements, the sediment wave field, and canyons. The four stages of this model include a stable stage, followed by the failure of the slope, and subsequent formations of the sediment wave field and canyons.

Bottom currents and sediment waves on a shallow carbonate shelf, Northern Carnarvon Basin, Australia

The modern seafloor of the Australian Northwest Shelf between Exmouth and Dampier was analyzed for large scale sedimentary bedforms on 3D seismic reflection data. The Carnarvon MegaSurvey of Petroleum Geo-Services (PGS), a merged dataset of multiple industrial 3D seismic reflection surveys with a total size of 49,717km2, offers an extensive view of the continental shelf, slope and rise of the Northern Carnarvon Basin. Over the shelf two fields of large scale sediment waves were observed in water depths between 55–130m, where the seafloor may be influenced by different processes including internal waves, tides and storms. Based on the dimensions and orientations of the sediment waves the dominant direction and approximate strength of local bottom currents could be estimated. Information on local sediment grain-size distribution was provided by the auSEABED database allowing a classification of the observed sediment waves into sand- or mudwaves. The first sediment wave field is positioned northwest of the Montebello Islands where the shelf is comparatively narrow and local sediment is mainly sand-sized. It most likely formed by increased bottom currents induced by the diversion of tidal flows around the islands. The second sediment wave field is located north of the Serrurier and Bessieres Islands within a local seafloor depression. Local sediments are poorly sorted, containing significant amounts of mud and gravel in addition to the mainly sand-sized grains. The coarser sediment fraction could have been reworked to sandwaves by cyclone-induced bottom currents. Alternatively, the finer sediment fraction could form mudwaves shaped by less energetic along-slope oriented currents in the topographic depression. The sediment waves consist partially of carbonate grains such as ooids and peloids that formed in shallow water during initial stages of the post glacial sea-level rise. These stranded carbonate grains thus formed in a different environment than the sediment waves in which they were redeposited. In fossil examples of similar high-energy ramp systems this possible out-of-equilibrium relationship between grains and bedforms has to be taken into account for the interpretation of the depositional environment.

Submarine canyon-head morphologies and inferred sediment transport processes in the Alías-Almanzora canyon system (SW Mediterranean): On the role of the sediment supply

Canyon head regions are key areas for understanding the shelf-to-canyon sedimentary dynamics and assessing the predominant hydrodynamic and sedimentary processes shaping their morphology. High-resolution multibeam bathymetry surveys conducted on the shelf, upper slope and along the main thalwegs of the Alías-Almanzora submarine canyon system were used to recognize the specific morphological features of the various canyon heads and infer the main sediment transport mechanism operating through them. Off the Almanzora River, different canyon head morphologies were observed at 65–90m water depth, incising the outer prodeltaic deposits. A direct connection from the river supply is evidenced by the presence of a large radial field of sediment waves on the prodelta, presumably related to hyperpycnal flows during flash flood events, and their continuation towards trains of cyclic steps that merge into the Almanzora canyon head. Processes linked to seepage and groundwater sapping also seems apparent in the prodeltaic deposits, which results in pockmark alignments and formation of elongated narrow canyon heads. At the southern limit of the Almanzora prodelta, another type of canyon head results from the formation and merging of linear gullies. These have been interpreted as the morphological expression of the distal off-shelf transport of flood-related hyperycnal flows. However, most of the canyon heads of the main tributaries of this canyon system (i.e., Garrucha, Cabrera and Alías) are found at much shallower water depths (7–20m). They generally show a continuation in land with rivers or intermittent creeks (known as “ramblas”) that also experience flash flood events, but some of them are disconnected from any river source. These canyon heads cut into the infralittoral prograding wedge, displaying feeder channels that tend to oriented against the direction of the prevalent long-shore current. Some canyon head tributaries show sediment waves along their axis interpreted as the result of hyperpycnal flood events or storm-induced turbidity flows. This study focusses on the diversity of sediment transport processes that can co-exist in the various canyon tributaries of a given submarine canyon system, stressing on the importance of the canyon heads location with respect to the principal sedimentary source, which ultimately determines their main geomorphological traits.

Active tectonics of the Calabrian subduction revealed by new multi-beam bathymetric data and high-resolution seismic profiles in the Ionian Sea (Central Mediterranean)

The detailed morphology and internal structure of the Calabrian accretionary wedge and adjacent Eastern Sicily margin are imaged in unprecedented detail by a combined dataset of multi-beam bathymetry and high-resolution seismic profiles. The bathymetric data represent the results of 6 recent marine geophysical surveys since 2010 as well as a compilation of earlier surveys presented as a 2 arc-sec (60 m) grid. Several distinct morpho-tectonic provinces are identified including: the deeply incised Malta–Hyblean Escarpment, numerous submarine canyons, broad regions of relatively flat seafloor dominated by fields of sediment waves, the gently undulating anticlinal fold-and-thrust belts of the external Calabrian accretionary wedge and the adjacent portion of the Western Mediterranean Ridge. The Calabrian arc can be divided into 4 domains (from SE to NW): 1) the undeformed Ionian abyssal plain, 2) the external evaporitic wedge, 3) the internal clastic wedge, 4) the Calabrian backstop (Variscan crystalline basement). The Calabrian accretionary wedge can also be divided laterally into two major lobes, the NE- and the SW lobes, and two minor lobes. The kinematics of the limit between the two major lobes is investigated and shown to be sinistral in the external (evaporitic) wedge. A network of radial slip lines within the southernmost external wedge unequivocally demonstrate ongoing dextral displacement of a rigid indenter (representing the corner of the clastic wedge) into the evaporitic wedge thereby confirming the geodynamic model of an active lateral slab tear fault here off eastern Sicily. The slab tear produces a series of major sub-parallel dextral strike-slip faults offshore Mt. Etna and south of the Straits of Messina consistent with the relative motions between Calabria and the Peloritan domain (NE Sicily). Abundant strike-slip faulting, and wide-spread folding and thrusting observed throughout the entire accretionary wedge, indicate regional shortening between the Ionian abyssal plain (foreland) and the Calabrian–Peloritan backstop caused by active subduction.

A study on tectonic and sedimentary development in the rifted northern continental margin of the South China Sea near Taiwan

A series of Cenozoic rifted basins developed in the northern margin of the South China Sea (SCS). Tainan Basin is one of these rifted basins near Taiwan, lying in the outer margin. We have used reflection seismic data in the deepwater areas and boreholes drilled in the shelf of the Tainan Basin to understand the tectonic and sedimentary development in the northern SCS margin near Taiwan. Four key stratal surfaces (i.e., the base of the Pleistocene Series, the base of the Pliocene Series, the 17 Ma maximum flooding surface [MFS], and a breakup unconformity of approximately 30 Ma in age) and seven seismic facies (i.e., continuous- and parallel-layer seismic facies, wavy seismic facies, chaotic seismic facies, U-shaped canyon-cut seismic facies, imbricated-layer seismic facies, high-amplitude reflector package seismic facies, and extrusive volcanism seismic facies) are recognized from seismic data with ages constrained by borehole stratigraphy drilled in the shelf. We have established a model for Cenozoic tectonic and sedimentary development in the rifted northern margin of the SCS near Taiwan. The occurrence of Paleogene fault-bounded grabens/half-grabens topped by a breakup unconformity and draped by postrift sediments indicates that these deepwater rifted basins developed on the continental crust, attesting that a thinned continental crust underlies the deepwater study area, rather than oceanic crust as reported in some literature. Postbreakup extrusive volcanic bodies, of early Miocene age, were buried by thick deepwater sediments. Fairly continuous stratal surfaces of 17 Ma MFS reveal that volcanic activities ceased to be active since middle Miocene. A series of channel cut-and-fills is observed in late Miocene, Pliocene, and Pleistocene strata beneath and to the south of the modern Formosa Canyon. Two distinct fields of deepwater sediment waves developed since middle Pleistocene are found lying to the west of modern deformation front/Manila Trench and to the north and south of the Formosa Canyon, respectively.

Turbidity current activity along the flanks of a volcanic edifice: The Mafate volcaniclastic complex, La Réunion Island, Indian Ocean

Recent marine geophysical surveys reveal the existence of well-developed volcaniclastic deep-sea fans around La Réunion Island, Indian Ocean. The Mafate turbidite complex, located in the northwestern part of the island, is a large sedimentary system formed by two coalescent-like volcaniclastic deep-sea fans: the Mafate fan and the Saint-Denis fan. They are both connected to terrestrial rivers supplying sediment produced by erosion on the island, particularly during austral summer cyclonic floods. Through the integration of marine geophysical data (including bathymetry, backscatter multibeam sounder images, TOBI side-scan sonar images and seismic reflection profiles) and piston cores, a submarine morpho-sedimentary map of the surface architecture of the Mafate and Saint-Denis turbidite systems has been established. The systems are divided in three main domains: deep canyons in the proximal area, a channel network in the medial area, and distal depositional lobes on the abyssal sea floor. Two large sediment wave fields also formed as a result of the volcaniclastic turbidity currents. Three piston cores collected along the Mafate complex provide information on the sedimentary processes in this area over the last 25ka. The record of turbidite events in these cores is interpreted in terms of volcanic and climatic changes that could have controlled the sediment transfer to the deep ocean.