Sediment Waves Research Articles

Sequential bedform development in mixed turbidite–contourite systems: An example from the Cosmonaut Sea, East Antarctica

Mixed turbidite–contourite depositional systems are commonly found on continental margins, but their bedforms and associated sedimentary processes have not been studied in depth. In this work, we used multibeam echo-sounder, sub-bottom profiling, and multichannel seismic data from the continental rise of the Cosmonaut Sea, East Antarctica, to (1) identify primary bedforms in a combined-current (i.e., turbidity current + contour current) channel–levee system and (2) infer bedform-associated sedimentary processes. Within turbidite channels and on adjacent levees and distal overbank deposits, scours, furrows, and sediment waves of varying dimensions and trends were identified. These bedforms are interpreted to have formed in two steps, which have been likely repeated over and over again through time. First, scours and sediment waves within the channels were formed by turbidity currents, while sediment waves on adjacent levees were likely formed by synchronous interactions between overspilled unconfined turbidity currents and the westward Antarctic Bottom Water (AABW) contour current. Second, after waning of the episodic turbidity currents, AABW flow created a field of erosional furrows on a distal overbank, with these furrows truncating the large field of sediment waves earlier generated by the combined flow of interacting currents. Bedform locations, orientations, and truncating relationships are key for identifying the likely origins of mixed-system bedforms.

Tectonic and oceanographic controls on the slope-confined dendritic canyon system in the Dongsha Slope, South China Sea

Dendritic canyon system is the most widespread type of submarine canyons along continental margins and plays important roles in sediment delivery and deep-water ecosystem. Based on high-resolution multibeam bathymetric and seismic reflection data, this study investigates a slope-confined dendritic canyon system in the Dongsha Slope, South China Sea. The dendritic canyon system between water depths of 550–2200 m consists of one main canyon (C2) and three V-shaped tributaries (C1, C3, and C4), in association with clearly visible tectonic deformation (uplifting and faulting) at the upper segment. The main canyon is characterized by a concave-up longitudinal profile, while the tributaries show convex-up and/or linear profiles, probably indicating that they are affected by tectonic deformation to variable degrees. The alignments and locations of tributaries C3 and C4 are determined by the NE-SW trending faults, while the distribution of the tributary C1 and the upper segment of C2 is less affected by faults, which also indicates that tectonic controls on the dendritic canyon system vary spatially among tributaries. Widespread NE-SW oriented sediment waves and southeast-facing scarps and isolated scour depressions at canyon banks highlight the significant influence of northwestward-propagating internal tide/wave on the morpho-sedimentary processes. The internal tides/waves within canyons not only flushed canyon floor, but also contributed to the asymmetric distribution of sidewall gullies and failures. The obliquely shoaling internal waves, combined with other hydrodynamics (e.g., Kuroshio Current and mesoscale eddies), could significantly intensify along-slope bottom currents and erode the NE-SW-oriented trough and tributaries. Under the significant erosion of ocean currents, some of the sidewall gullies, troughs and sub-tributaries may finally evolve into new tributaries. This study reveals the coupling relationship between slope-confined dendritic canyon system and tectonic/oceanographic processes, which can shed light on the slope/canyon evolution of similar settings worldwide.

River response to mining-induced subsidence

Land subsidence is a phenomenon commonly associated with water withdrawal and mining. In river valleys it increases the risk of flooding, which is usually reduced by channel alignment and raising of flood dykes. In this work, we describe the natural, advanced processes of sedimentation and channel adjustment to ground level changes in the Chechło River reach within an about 30-year-old subsidence basin in southern Poland. Physicochemical properties of sediments, channel geometry and its changes over a 5-year period have been analyzed. These sediments of the ponded depression differ markedly from the laminated overbank deposits of the unaffected river reach in respect of their stratigraphy and accretion rate. Formation of a levee is the characteristic feature of the ponded reach. The levee deposits are silt-dominated and highly organic and overlay thick sandy strata. The ponded parts of the basin outside the levee zone are characterized by slow deposition of fine grained organic sediments. A local increase in a channel slope induces relatively intensive erosion at the knickpoint and the formation of sediment wave on the channel at the downstream end of the pond. These changes are associated with a rapid increase of the Width/Depth ratio. The sediment wave has a tendency to disperse in place due to channel downcutting and the formation of the over 100 m-long downstream reach with the geometry similar to that observed in unaffected river reaches. These investigations indicate that even small subsidence ponds can exist in a highly dynamic fluvial environment for decades before they disappear. They can enrich the local habitat and landscape for long enough to be important for biodiversity in regions heavily impacted by human activity. Decisions regarding the restoration of subsided lands should be site-specific and preceded by considerations for achieving the best possible benefits instead of mandatory liquidation of subsidence depressions.

Seismic geomorphology of submarine landslides in the Chalk Group of the Danish Central Graben: implications for reservoir potential

Abstract This study documents a variety of deposits created by submarine landslides within the Upper Cretaceous to lowermost Paleocene Chalk Group in the Danish Central Graben and investigates the impact of remobilization on porosity. Improved visualization of the landslides in 3D seismic data compared with previous studies was facilitated by better seismic data quality for the Chalk Group, the availability of a large stack of stratigraphy-consistent horizons and the use of spectral decomposition data. The illustrated examples are chosen to reflect the spectrum of deformation styles seen in the chalk and all have a well penetrating the affected succession. They include a large collapse (375 km 2 ) of an inversion ridge within the Kraka and Gorm formations, a field of large slide blocks (100–1000 m, 10–26 m) of likely lowermost Danian age embedded in the uppermost Ekofisk Formation, a debris flow system within the uppermost Tor Formation probably originating from the Ringkøbing–Fyn High and fine-grained bottom current sediment waves within the lowermost Danian Ekofisk Formation. In general, porosities are higher (10–25 porosity units) in the remobilized chalk compared with time-equivalent pelagic chalk in nearby reference wells. In earlier studies this has been linked to lack of bioturbation (resulting in limited grain repacking) in the remobilized chalks owing to high sedimentation rates, resulting in a relatively open fabric during initial burial. In contrast, surrounding and covering pelagic deposits could be much more effectively bioturbated, leading to tighter grain packing during burial. The insights of this study help in the seismic characterization of mud-grade carbonate oozes and have important applications in the reservoir modelling of mud-grade carbonate reservoirs (also in light of carbon capture and storage), and in palaeo-reconstructions of pelagic seafloors since submarine landslides provide kinematic indicators.

A review of the morphology, physical processes and deposits of modern straits

AbstractThis study reviews the morphology, hydrodynamics and sedimentology of 33 modern straits, including examples from diverse tectonic and climatic settings. Strait morphology ranges from short, simple straits to long, tortuous passages many hundreds of kilometres long; depths range from 10 m to >1 km. The morphological building block of strait sedimentation is a constriction flanked by open basins; a single strait can contain one or several of these. Currents accelerate through the constrictions and decelerate in the basins, leading to a spatial alternation of high- and low-energy conditions. Currents in a strait can be classified as either ‘persistent’ (oceanic currents or density-driven circulation) or ‘intermittent’ (tidally or meteorologically generated currents). Constrictions tend to be bedload partings, with the development of transport paths that diverge outward. Deposition occurs where the flow decelerates, generating paired subaqueous ‘constriction-related deltas’ that can be of unequal size. Cross-bedding predominates in high-energy settings; muddy sediment waves and contourite drifts are present in some straits. In shallow straits that were exposed during the sea-level lowstand, strait deposits typically occur near or at the maximum flooding surface, and can overlie estuarine and fluvial deposits. The most energetic deposits need not occur at the time of maximum inundation.

Bottom Current Modification of Turbidite Lobe Complexes

Submarine lobes form at the distal end of sediment gravity flow systems and are globally important sinks for sediment, anthropogenic pollutants and organic carbon, as well as forming hydrocarbon and CO2 reservoirs. Deep-marine, near bed or bottom currents can modify gravity flow pathways and sediment distribution by directly interacting with the flow or by modifying seafloor morphology. Deciphering the nature of gravity- and bottom currents interaction, particularly in ancient systems, remains a challenge due to the lack of integrated datasets and the necessary oceanographic framework. Here we analyse high-resolution 3D seismic reflection and core data from the Upper Cretaceous interval offshore Tanzania to reveal the interaction of turbidite lobes with fine-grained sediment waves and contourite drift deposits. Contourite drift morphology governs the large-scale confinement style and shape of lobes that range from frontally confined and crescent shaped, to laterally confined and elongated, to semi-confined lobes. Core data reveals massive to cross-laminated high density turbidites in the lobe axis position that show no direct interaction between gravity flows and contour currents. Lobe off-axis and fringe deposits consist of parallel- and ripple-laminated, low density turbidites, which are inter-bedded with bioturbated, muddy siltstones that represent the toes of contourite drifts. Starved ripples, and streaks of up to fine-grained sandstone above individual turbidite beds indicate reworking by bottom currents. This facies distribution reflects the temporal interaction of quasi-steady bottom currents and turbidity currents that interact with the topography and build lobes over short periods of time. Frontally confined turbidity currents form lobes in a fill-and-spill fashion, in which the confinement of turbidity currents causes rapid deposition and obscures any bottom current signal. Lateral confinement causes increased turbidity current runout length, and promotes the development of lobe fringes with a high proportion of bottom current reworked sands. During times when sediment gravity flows are subordinate, contourites accumulate on top of the lobe, confining the next flow and thus modifying the overall stacking pattern of the lobe complex. Although sediment volumes of these bottom current modified lobe complexes are comparable to other deep-marine systems, bottom currents considerably influence facies distribution and deposit architecture.

Submarine Channel Mouth Settings: Processes, Geomorphology, and Deposits

Observations from the modern seafloor that suggest turbidity currents tend to erode as they lose channel-levee confinement, rather than decelerating and depositing their sediment load, has driven investigations into sediment gravity flow behaviour at the mouth of submarine channels. Commonly, channel mouth settings coincide with areas of gradient change and play a vital role in the transfer of sediment through deep-water systems. Channel mouth settings are widely referred to as the submarine channel-lobe transition zone (CLTZ) where well-defined channel-levees are separated from well-defined lobes, and are associated with an assemblage of erosional and depositional bedforms (e.g., scours and scour fields, sediment waves, incipient channels). Motivated by recently published datasets, we reviewed modern seafloor studies, which suggest that a wide range of channel mouth configurations exist. These include traditional CLTZs, plunge pools, and distinctive long and flared tracts between channels and lobes, which we recognise with the new term channel mouth expansion zones (CMEZs). In order to understand the morphodynamic differences between types of channel mouth settings, we review insights from physical experiments that have focussed on understanding changes in process behaviour as flows exit channels. We integrate field observations and numerical modelling that offer insight into flow behaviours in channel mouth settings. From this analysis, we propose four types of channel mouth setting: 1) supercritical CMEZs on slopes; 2) plunge pools at steep slope breaks with high incoming supercritical Froude numbers; 3) CLTZs with arrays of hydraulic jumps at slope breaks with incoming supercritical Froude numbers closer to unity; and, 4) subcritical CLTZs associated with slope breaks and/or flow expansion. Identification of the stratigraphic record of channel mouth settings is complicated by the propagation, and avulsion, of channels. Nonetheless, recent studies from ancient outcrop and subsurface systems have highlighted the dynamic evolution of interpreted CLTZs, which range from composite erosion surfaces, to tens of metres thick stratigraphic records. We propose that some examples be reconsidered as exhumed CMEZs.

Turbidity currents on the open slope of the Fraser Delta

Turbidity currents have been observed and measured in submarine channels and canyons where the flows are confined, but with the exception of one serendipitous direct measurement, are only inferred to occur on unconfined delta slopes. In this paper, direct measurement of turbidity currents on the Fraser delta slope are reported and their dynamics compared to turbidity currents in channelized settings. Twelve turbidity current events occurring between May 2014 and June 2018 were identified from Acoustic Doppler Current Profiler instruments deployed at two sites (109 and 159 m water depth). All the events occurred in the months of May and June, in association with high spring river flow. Five of the events were recorded at both sites and the timing of first arrivals gives estimates of the current speed ranging from 0.4 to 3.6 m s−1. Direct current measurements confirm that most flow speeds exceed 1.5 m s−1, although one weak event did not exceed 0.4 m s−1. Current profiles indicate that the flows are of variable thickness, ranging from less than 3 m to 15 m. The turbidity current events varied in duration from 5 to 66 min and, in the cases where transit speeds were available, the durations were proportionally shorter when the transit speed was high. In stronger flows, the initial stage of the flow is highly variable, suggesting extreme turbulence, and the velocity structure of the flow is similar to previous experimental and field observations, with a distinctive relatively thin, high speed lower region being evident at times. Downslope translation of the instrument platform during two of the events suggest the presence of a dense basal layer up to 2.75 m thick, as suggested in the travelling wave model of Heerema et al. (2020). The fact that only approximately half the flows measured at the shallower site are observed at the deeper site suggest that the runout distances of open slope flows are limited. Weaker flows are barely distinguishable in speed and direction from ambient tidal velocities. Flow directions suggest that the open slope flows are unconfined and initiate in the upper slope area that is approximately the same as the area of upslope sediment waves. It is hypothesized that shallow gullies in the same area may be important for initiation and ignition of the flows that become unconfined on the lower part of the slope.

Cretaceous to Eocene mixed turbidite-contourite systems offshore Nova Scotia (Canada): Spatial and temporal variability of down- and along-slope processes

The identification of several Cretaceous to Paleogene mixed turbidite-contourite systems along the upper to middle continental slope of Nova Scotia provides a unique opportunity to investigate the spatial and temporal variability of their morphological elements. The mixed systems were studied using 3D seismic reflection data and well-established chronostratigraphy from five exploration wells. Seismic interpretations and correlations suggest four main evolutionary stages for the Late Jurassic – Paleogene sedimentary record, characterized by distinctive, diagnostic features at large (>1–10 km) to small (<100 m) scales: 1) initial turbidite systems developed between 160 and 125 Ma with extensive tributary channel networks along the upper continental slope that fed submarine channels and channel-levees further down-slope; 2) onset and growth of mixed systems at 125–78 Ma, characterized by a seaward progradation and NE migration of down-slope elongated mounded drifts and wide, U-shaped channels formed under potentially synchronous interactions between a SW-flowing bottom current and SE-directed turbidity flows; 3) shift of mixed systems at 78–50 Ma with SW migration of the mounded drifts and submarine channels, associated with a potential switch of the bottom current direction at 78 Ma, from SW towards NE; and 4) burial stage post-50 Ma, characterized by several stages of hemipelagic deposition followed by gully erosion at approximately 50, 40, and 35 Ma. The gradual transition from turbidite systems to fully developed mixed systems is associated with the tectonic and sedimentary background of the Nova Scotian margin during the Early to Late Cretaceous, as well as the establishment of a Cretaceous to Eocene paleoceanographic circulation. Comparisons with other ancient and modern mixed systems reveal similar morphological features (e.g., mounded drifts and submarine channels) formed under synchronous and asynchronous interactions between along- and down-slope processes. Their lateral distribution and vertical variability reflect a gradual change between the most influential process, from down-slope turbidity currents to along-slope bottom currents. Secondary depositional features along the mounded drifts, such as N–S oriented sediment waves, result from along-slope sediment redistribution and remobilization near the seafloor. Mixed depositional systems form unconventional plays and may become future targets for energy geosciences in a search for hydrocarbons or carbon capture and storage.

Numerical simulations of wave climate in the Baltic Sea: a review

Efforts towards the numerical simulation of the Baltic Sea wave properties, started in the 1950s, have reached maturity by the implementation of contemporary third generation spectral wave models, such as WAM and SWAN. The purpose of this paper is to give an overview of the relevant efforts since the beginning of numerical wave simulations. The Sverdrup-Munk-Bretschneider (SMB) type models are still valuable tools for rapid estimates of some properties of wave climate in single locations. The spatial resolution of spectral wave models for the entire sea has increased from about 20 km to 1 km, and to 100–200 m in specific areas. The number of directional bins has increased from 10–15 to 24–36 and the number of spectral frequency bins from about 15 to 35–42. The models replicate all main features of the wave climate of the Baltic Sea, such as an overall mild but intermittent wave climate, the predominance of short windseas and the scarcity of long swell, east-west asymmetry, the strong impact of seasonal ice, and the specific properties of wave growth in some areas. The wave climate changes involve variations in regional wave intensity, core properties of wave-driven sediment transport and wave set-up. Reconstruction of wave properties in the nearshore, archipelago areas, and in narrow subbasins remains a challenge. These areas require finer spatial resolution and possibly advancement of wave physics to account for changes in the spectral composition of wave fields and specific features of wave growth in narrow basins. Progress in these fields is a pillar for a number of applications, from the quantification of sediment transport to proper input into management issues of the coastal zone.

Cretaceous to Cenozoic controls on the genesis of the shelf-incising Perth Canyon; insights from a two-part geomorphology mapping approach

Perth Canyon is Australia's second largest submarine canyon, and its shelf-incising morphology contrasts with the more prolific slope-confined canyons that typify Australia's passive continental margin. The canyon has a sinuous course that extends 120 km from the shelf break (~180 m depth) to its fan at the foot of the continental slope (~4500 m). Though the canyon initiates only 50 km offshore from a major city, its genesis and geomorphic stability have not been well understood. Bathymetry data acquired in 2015 by the Schmidt Ocean Institute enabled the application of a new two-part seafloor classification approach to objectively map the complexity of the system in unprecedented detail. Part 1 used a semi-automated approach to classify the seafloor bathymetry into morphological categories, and Part 2 defined these units as geomorphological features through the interpretation of sub-bottom and seismic images, sediment samples and acoustic backscatter datasets. The resulting geomorphic map reveals an array of aggradational (cyclic steps and sediment waves), incisional (entrenched canyon floor and nick-points) and mass movement (slump and slab failures) features that for the first time provide detailed insights into the canyon's formative processes. Large faults and the Cretaceous palaeobathymetry appear to have strong influence on the canyon's planform, its depth of incision, and the distribution and types of mass failure that characterise its flanks. These data also reveal the Perth Canyon to be a predominantly relict feature; a large Late Cretaceous infilled incised valley (subaerial) beneath the canyon headwall likely initiated the canyon's development and represents its initial and most active phase. Two more infilled incised valleys are stacked above the first, and demonstrate a progressive decrease in scale, and presumably also canyon activity. Each incised valley represents lowstand incisions of the palaeo-Swan River, and their timing is linked to pronounced Late Cretaceous to Cenozoic sea level regression events, palaeoclimatic change, and onshore catchment enlargement. The disconnection of the modern Perth Canyon from the present day Swan River, and the low rates of sediment accumulation on the adjacent shelf and slope, ensure low rates of sediment supply to the canyon, and only infrequent ignition of turbidity currents. Low rates of sediment supply can similarly account for the entrenched morphology of the modern fan and only minimal headwall movement coincident with seismic events in 2018. However, additional core and bathymetry data for the lower canyon reaches are required to conclusively determine the extent of recent canyon activity.

Seafloor pockmarks on the South Westland margin of the South Island/Te Waipounamu, Aotearoa New Zealand

ABSTRACT Enclosed depressions, termed pockmarks, are widespread seafloor morphologies, commonly associated with fluid seepage. This study provides the first detailed documentation of pockmarks offshore the South Westland margin of the South Island/Te Waipounamu, Aotearoa New Zealand. Pockmarks are identified from multibeam bathymetry (25-m grid) through manual and semi-automated selection in water depths of 100–2600 m. Pockmarks are most concentrated at 400–850 m water depth on continental slope areas between submarine canyons. A continuum of pockmark morphologies includes – (1) large (>0.5 km2 area) and irregularly shaped pockmarks above partially infilled channels; (2) small and circular pockmarks (∼100–200 m diameter; ∼0.008–0.03 km2 area) occurring between canyons; and (3) elongated and intermediate size pockmarks, generally oriented along-slope and often occurring above buried sediment waves. Elongated pockmarks appear to have been modified by near-seafloor oceanographic and/or turbidity current flows. Pockmark features occur across many locations around Aotearoa, including both the eastern and western margins. Some similar pockmark morphologies are identified in these different tectonic, sedimentary, and oceanographic settings, suggesting that there may be some similarity in formative mechanisms, but clear mechanisms leading to their formation remain enigmatic.

The Influence of Channel Planform and Slope Topography on Turbidity Current Overbank Processes: The Example of the Acquarone Fan (Southeastern Tyrrhenian Sea)

Overbank deposits provide a potentially valuable record of flows that have passed through a submarine channel. The architecture of overbank deposits has generally assumed to relate to autogenic processes related to channel construction. In previous models, which are largely based on passive margins, the distribution and geometry of these deposits is relatively simple, and hence generally predictable. Here, we show how the interaction of different flow types with the complex morphology on a highly-tectonically modified margin can profoundly affect overbank depositional processes, and hence also the resultant deposit geometry and architecture. Our case study is the Acquarone Fan, located in the intraslope Gioia Basin in the south-eastern Tyrrhenian Sea, whose topography is mainly controlled by the presence of the Acquarone structural ridge, which results in the confinement of the left south-west side of the channel-levee system. The research is carried out through analysis of multibeam bathymetric and high-resolution Chirp sub-bottom profiler data. Seven depositional units (Units I-VII) record the recent depositional history of the fan; their thickness has been mapped and their parent flow-types have been interpreted through their seismic response. According to unit thickness maps, two main patterns of deposition are recognized in the overbank area. Their depocenters coincide with different extensive sediment wave fields developed in specific tracts of the right levee and in the frontal splay area. We show that the location of the depocenters varies in time according to the prevalent flow-type and by its interaction with the surrounding seafloor topography and channel planform. We interpret that the lateral confinement of the channel by the structural high generates episodic rebound of the overspilling flow and the inversion of the channel asymmetry. The vertical stratification of the flow strongly influences the overbank deposition where the channel planform has a non-linear shape such as bends and knick-points. In particular, the vertical stratification influences the hydraulic jump size that conditions the amount of overspill and thus the location of overbank depocenters. This study highlights that variations in the sediment distribution and composition on the overbank can be related to the way different flows interact with tectonic setting.

ARCHITECTURAL COMPLEXITIES AND MORPHOLOGICAL VARIATIONS OF THE SEDIMENT WAVES OF PLIO-PLEISTOCENE CHANNEL LEVEE BACKSLOPE OF THE INDUS FAN

The architecture of the turbidity current sediment waves exhibits intricate morphologies and patterns on the Indus Fan channel levee backslope. The sediment waves are present on the channel levee of Plio-Pleistocene age and are absent in the deeper sections of the study area. The architecture of channel levee backslope on the Indus Fan is poorly understood. We used seismic interpretation techniques and modelling by utilizing high-resolution seismic data to approach this problem. The morphological variations in wavelength, crest dimensions and potential wave formation patterns suggest the autogenic and allogenic processes associated with wave development. Wavelengths reach up to 1473 m with an average of 486.84 m and the height of the levee ranges between 10 m and 60 m (average 30 m). The angle of the channel levee and dimension of the sediment wave here are independent of each other. Low angle levees have accommodated high dimension sediment waves and vice versa at multiple points downslope. Characteristically, the waves have formed on the outer levee (usually left) of the channels marked by steep margins suggesting that flow overspill caused the development of the waves. Generally, the younger sediment waves followed the patterns of older sediment waves, but the varying trends are often observed in the study area. The patterns of the sediment waves towards the younger sections of the levee indicate the modified and varying architectural style of growth. Sediment waves are generated by downslope turbidity currents. However, the deformation features have also possibly triggered the development of sediment waves.

Anatomy and dynamics of a mixed contourite sand sheet, Ryukyu Island Arc, northwestern Pacific Ocean

Contourites are well-known from many continental margins under the influence of bottom currents but have been little reported from the Pacific Ocean. This paper documents a new area of contourite-controlled sedimentation in the NW Pacific Ocean, which we call the Ryukyu Sand Sheet. This contourite sand sheet has an area of around 35,000 km2 and extends from the narrow island shelves to over 1500 m water depth. It comprises mainly moderate to well-sorted fine-grained sands, with current ripples and giant sediment waves and is also associated with small drifts. It is formed under the influence of three principal current systems – the Kuroshio Current, the Kuroshio Countercurrent and the Ryukyu Current. The interaction of these currents with each other and with a complex seafloor topography, spawns a series of meso-scale gyres, eddies and vortices that shape the seafloor and lead to deposition of an extensive sandy substrate, locally with gravels and exposed seafloor. Strong surface currents, as well as deep-water thermohaline circulation, both influence the depositional and erosional processes of deep-sea sediments. The role of the modern Kuroshio Current in this context supports earlier work that proposed an ancestral Kuroshio Current for the deposition of Miocene contourites onshore Japan. Sediment supply to the Ryukyu Sand Sheet is by a mixed process of seafloor polishing and sand spillover that involves combined oceanographic and gravitational processes.

Current-controlled sedimentation in the north-western Weddell Sea

The sedimentary basins of the north-western Weddell Sea are characterized by a variety of contourite drifts. This study is aimed at their identification, spatial mapping and temporal evolution and based on the integration of a large amount of seismic data collected by different countries including the recent data of the Russian Antarctic Expedition. Most of the drifts in the region being studied are classified as separated, confined, plastered or sheeted. The chain of sediment wave fields is mapped in the western and northern Powell Basin. The earliest contourite drifts started to form in the Early Miocene or, possibly, in the Late Oligocene. The changes in the depositional pattern in the Middle Miocene and then in the Late Pliocene are thought to have resulted from successive intensification of the bottom currents.

Coastal wave processes numerical modeling for large valley-type reservoirs

The problem of modeling sediment transport and wave processes of large valley-type reservoir under non-stationary conditions of the hydrological cycle active phase (spring-autumn period) is considered. Coupled 2D sediment transport model and 3D wave hydrodynamics was considered to describe these processes, which uses the Navier-Stokes equations. The wave hydrodynamics model is applied to large reservoir of the valley type, such as Tsimlyansky reservoir. Detailed numerical experiments were performed taking into account the real coastline geometry and the bottom relief of the Tsimlyansk reservoir southwestern part. The developed complex of models and programs allows to predict reshaping the bottom relief and coastline under various hydrometeorological conditions. The results of modeling can be in demand when planning water management activities in valley-type reservoirs.

Sedimentary response of the deep eastern Mediterranean basin to the north African desertification, sea level variation and regional tectonics

AbstractThe buildup of a continental rise and its morphological patterns depend on sediment transport and deposition via downslope slumps, turbidity currents, along‐slope contourites and other deepwater currents. These delivery mechanisms are central to source‐to‐sink reconstructions, yet untangling their individual roles is not straightforward. The current study focuses on the sink section of the Nile system at the northern Levant continental rise (eastern Mediterranean). During the Pliocene, fluvial systems from northern Africa and the Levant margin supplied sediments to the study area. This supply decreased significantly during the Quaternary because of two processes: (a) The aridification of North Africa made the Nile River the predominant contributor to the basin; (b) the topographic rise of the Levant landmass severely limited fluvial supply. Meanwhile, on‐going counter‐clockwise marine currents became the prevalent supply system to the Levant margin, and downslope sediment‐transport became the only source to the northern Levant continental rise, which became a significant sink of the larger Nile system. These conditions provide an excellent natural laboratory for understanding the individual role of downslope sediment‐transport processes in building a continental rise. This research is based on single‐channel sparker seismic data, multibeam bathymetry and four 7–8 m long piston‐cores collected over the northern Levant continental rise at water depths of 1200–1800 m, together with available multibeam bathymetry and industrial multi‐channel seismic reflection data. The results show a major depositional changeover during the Pliocene‐Quaternary transition from concordant aggradation to repeated deposition of &gt;12 sediment wave subunits, interpreted as upslope migrating cyclic steps. Quantitative analysis of the sediment wave morphology at the seafloor and subsurface, along with core data, indicates that the immediate sediment source was the nearby shelf. The supply was regulated by basinward‐landward shifts of the shore‐parallel marine currents during lowstand‐highstand conditions (respectively). Hence, this case study highlights the connection between sea‐level change and sedimentation patterns on a continental rise.

Stream Suspended Mud as an Indicator of Post-Mining Landform Stability in Tropical Northern Australia

Mining can cause environmental disturbances and thus mined lands must be managed properly to avoid detrimental impacts in the future. They should be rehabilitated in such a way that post mining landforms behave similarly as the surrounding stable undisturbed areas. A challenge for government regulators and mine operators is setting closure criteria for assessment of the stability of the elevated post-mining landforms. Stability of a landform is often measured by the number and incision depth of gullies. This can assess mass stability and bulk movement of coarse material. However, there is a need for a more sensitive approach to assess catchment disturbances using the concept of waves of fine suspended sediment and thus determine the dynamics of recovery of a post mining landform. A more environmentally meaningful approach would be to assess the fine suspended sediment (FSS, silt + clay (0.45 µm &lt; diameter &lt; 63 µm)) leaving the system and entering downstream waterways. We propose assessing stability through relationships between rainfall event loads of FSS and event discharge (Q) in receiving streams. This study used an innovative approach where, instead of using instantaneous FSS concentration, it used total FSS load in waves of sediment driven through the system by rainfall runoff events. High resolution stream monitoring data from 2004 to 2015 in Gulungul and Magela Creeks, Northern Territory, Australia, were used to develop a relationship between sediment wave and event discharge, ∑FSS α f(Q). These creeks are adjacent to and receive runoff from Ranger Mine. In 2008, a 10 ha elevated waste rock landform was constructed and instrumented in the Gulungul Creek catchment. The earthworks required to build the landform created a considerable disturbance in the catchment, making a large volume of disturbed soil and substrate material available for erosion. Between 2008 and 2010, in the first two wet seasons immediately after construction, the downstream monitoring site on Gulungul Creek showed elevated FSS wave loads relative to discharge, compared with the upstream site. From 2010 onwards, the FSS loads relative to Q were no longer elevated. This was due to the establishment of vegetation on the site and loose fine sediment being trapped by vegetation. Large scale disturbance associated with mining and rehabilitation of elevated landforms causes elevated FSS loads in receiving streams. The predicted FSS loads for the stream as per the relationships between FSS and event discharge may not show a 1:1 relation with the observed loads for respective gauging stations. When downstream monitoring shows that FSS wave loads relative to rainfall runoff event discharge reduce back to pre-construction catchment levels, it will indicate that the landform is approaching equilibrium. This approach to assess landform stability will increase the sensitivity of assessing post-mining landform recovery and assist rehabilitation engineers to heal the land and benefit owners of the land to whom it is bestowed after rehabilitation.

Genesis and evolution of large-scale sediment waves in submarine canyons since the Penultimate Glacial Maximum (ca. 140 ka), northern South China Sea margin

Sediment waves are common on the seafloor, but large-scale ones developed in submarine canyons are rarely reported. In this study, we document for the first time the Quaternary deep-water canyon-confined large-scale sediment waves developed on the northern South China Sea margin based on the analysis of a high-quality 3-D seismic reflection volume combined with a 2-D multichannel seismic reflection profile and three wells. Results show that the onset of these canyon-confined large-scale sediment waves can be dated back to the Penultimate Glacial Maximum (PGM)(ca.140 ka). This was the last period for the Pearl River Delta to prograde over the shelf edge. And these sediment waves have continued to develop and migrate upslope until present. The large-scale sediment waves pertaining to the PGM have a dominant down-slope asymmetrical 2-D morphology with wavelengths of 1.059–6.090 km and wave heights of 3.3–32.5 m. The present large-scale sediment waves show an up-slope asymmetrical 2-D morphology with dimensions of 0.667–5.628 km wavelength and 2.7–14.0 m wave height, which are smaller relative to those at the PGM. Grain sizes of the sediment waves at the PGM and present are interpreted to be coarse, inferred from seismic reflection data by high amplitude reflections (HARs). After a comprehensive analysis of the types of sediment-laden flows, the sediment provenances, the seafloor topography and the features of the sediment waves, these large-scale sediment waves might be interpreted as cyclic steps generated by the down-slope flowing supercritical turbidity currents and associated internal hydraulic jumps along high gradient (approximately greater than 1.28°) canyon thalwegs characterized by numerous slope breaks. The evolution of the large-scale sediment waves from partially depositional at the PGM to erosion-dominated at the present is controlled by variations of the sediment supply and the submarine slope accommodation along the canyon thalweg, which is manifested in the co-evolution of the sediment waves with the canyon. The findings of this study tell us that in addition to axial channels and mass-transport complexes, large-scale sediment waves can also develop on the canyon floor, which helps to improve our understanding of the complex canyon processes. The ‘large-scale sediment waves’ described here may be a new potential deepwater-reservoir element for the deep-water hydrocarbon exploration associated with submarine canyons.