Submarine Channel Levee Research Articles

Paleogene to Neogene sequence stratigraphy and deepwater depositional systems in Kribi-Campo/Rio-Muni margin (Offshore Cameroon-Equatorial Guinea Margin)

2D seismic data and Gamma Ray logs were interpreted to document the Tertiary deepwater depositional system observed in the southern part of the Kribi-Campo sub-basin and the northern part of the Rio Muni basin, both basins separated by the Campo high. The sedimentary filling of both basins occurred with cyclicity during the Tertiary. The Paleogene filling exhibits at least three Seismic Unit Packages (SUP1 to SUP3) interpreted as some Feeder Channels-Lobe Complexes (FCLC). The units with Mass Transport Complexes (MTCs) are low amplitude and chaotic to transparent seismic facies. They are interpreted as mud-rich complexes. The units related to FCLC, features high to moderate amplitude with shingled oblique progradational clinoforms. The Neogene filling is dominated by some submarine Leveed-Channels Complexes associated to Lobes (LCCL) with low to moderate amplitude and concave-up mounded configuration. The well log analysis revealed three main electrofacies namely, shale, siltstone, and sandstone that were deposited in various depositional environments. Shale lithology are underlined by irregular, bell and bow gamma ray log motifs which may be deposited in inner shelf, tidal channels and channel-levees systems that are associated with transgressive to regressive depositional sequences. Sandstone and siltstone lithology are typified by funnel and cylindrical gamma ray log motifs that are deposited in prograding marine shelf fans and slope channels. Influenced by the high slope of paleotopography around the Campo High, the submarine channel-levee are confined to a western part of Kribi-Campo sub-basin. The FCLC shows large planar distribution, while the MTC are poorly structured.

Modified Cam‐Clay Model for Large Stress Ranges and Its Predictions for Geological and Drilling Processes

AbstractWe modify the Modified Cam‐Clay (MCC) model for large stress ranges encountered in geological applications. The MCC model assumes that the friction angle (ϕ) and the slope of the compression curve (λ) of a mudrock are constant and thus predicts constant values for the lateral effective stress ratio under uniaxial, vertical strain (K0) and undrained strength ratio ( ). However, experimental work shows that λ, ϕ, and decrease and K0 increases substantially with stress over large stress ranges (e.g., up to 100 MPa). We incorporate the stress dependency of λ and ϕ into the MCC model and use the new model to predict and K0 ratios. The modified model, with only one additional parameter, successfully predicts the stress dependency of these ratios. We encode the modified model and use it for finite‐element analysis of a salt basin in the deepwater Gulf of Mexico. The stresses that the new model predicts around salt differ significantly from those predicted using the original model. We incorporate the stress dependency of the friction angle into the analytical models developed for critical tapers, wellbore drilling, and the stability of submarine channel levees. We show that the decrease of the friction angle with stress (1) results in a concave surface for critical wedges, (2) shifts the drilling window to higher mud weights and makes it narrower for a vertical wellbore, and (3) causes deep‐seated failure of submarine channel levees at lower angles. Our study could improve in situ stress and pore pressure estimation, wellbore drilling, and quantitative understanding of geological processes.

Supercritical flows overspilling from bypass‐dominated submarine channels and the development of overbank bedforms

AbstractOverbank deposits of submarine channels are typically thin‐bedded, fine‐grained and predominantly characterized by a series of sedimentary structures interpreted to record a relatively simple history of waning flow. Here, a new type of bedform indicative of Froude‐supercritical flow is reported from successions of thin‐bedded turbidites interpreted as channel overbank deposits in the Upper Cretaceous Rosario Formation, Baja California, Mexico. A link is demonstrated between the development of overbank deposits in the form of depositional terraces or internal levees and contemporaneously active sediment transport, bypass and deposition of coarser‐grained material in a channel. The overbank bedforms overlie an erosion surface and contain a suite of sedimentary structures indicative of initially Froude‐supercritical flow conditions and a progressive waning of flow strength. In some cases, a stacked repetition of facies is interpreted to record a rejuvenation of flow energy. The characteristic sedimentary sequence observed is as follows: (a) long wavelength, low amplitude erosional surface with superimposed scours; (b) antidune backsets; (c) upper stage plane‐parallel lamination; (d) subcritical climbing ripples; (e) supercritical climbing ripples; (f) lower stage planar laminated tops; (g) a sharp upper surface. The exact vertical sequence of sedimentary structures encountered varies depending on the point of observation with respect to the bedform crest and distance from the parent channel. The recognition of these distinctive bedforms allows for interpretation of sediment bypass and proximity to a channel thalweg. These bedforms have not hitherto been described and provide a further example of the range of flow processes operating in submarine channel–levee systems, which aids depositional environment interpretation in both subsurface and outcrop studies.

Turbidity Current Dynamics: 1. Model Formulation and Identification of Flow Equilibrium Conditions Resulting From Flow Stripping and Overspill

This study expands the widely used one‐dimensional, four‐equation model for turbidity currents to account for mass, momentum, and energy sinks associated with flow stripping and overspill processes acting upon the turbidity current suspension cloud. The suspension cloud is defined as any portion of the flow extending above the channel levees. The expanded model allows steady turbidity currents to evolve to a uniform, or equilibrium, state where mass, momentum, and energy gained through sediment and clear‐water entrainment processes are balanced by the mass, momentum, and energy lost to flow stripping and overspill. We perform a sensitivity analysis of the expanded model to assess how changes in channel (e.g., slope, channel height, width, bed friction, and radius of curvature) and flow (e.g., sediment grain size, suspension cloud concentration, and turbulence) properties affect the equilibrium flow conditions. By varying the model inputs from half to double their base case values, we find that the equilibrium values can change by up to a factor of 2. We find that the equilibrium conditions, including the flow Richardson number, were generally most sensitive to changes in slope, channel height, grain size, and suspension cloud concentration. Additionally, we identify model parameter space where Richardson subcritical equilibrium flow is possible. The fact that turbidity currents can attain equilibrium, and particularly subcritical equilibrium, might help elucidate the long turbidity current runout distances on low slopes inferred from extensive submarine channel levee systems.

Linking submarine channel–levee facies and architecture to flow structure of turbidity currents: insights from flume tank experiments

AbstractSubmarine leveed channels are sculpted by turbidity currents that are commonly highly stratified. Both the concentration and the grain size decrease upward in the flow, and this is a fundamental factor that affects the location and grain size of deposits around a channel. This study presents laboratory experiments that link the morphological evolution of a progressively developing leveed channel to the suspended sediment structure of the turbidity currents. Previously, it was difficult to link turbidity current structure to channel–levee development because observations from natural systems were limited to the depositional products while experiments did not show realistic morphodynamics due to scaling issues related to the sediment transport. This study uses a novel experimental approach to overcome scaling issues, which results in channel inception and evolution on an initially featureless slope. Depth of the channel increased continuously as a result of levee aggradation combined with varying rates of channel floor aggradation and degradation. The resulting levees are fining upward and the grain‐size trend in the levee matches the upward decrease in grain size in the flow. It is shown that such deposit trends can result from internal channel dynamics and do not have to reflect upstream forcing. The suspended sediment structure can also be linked to the lateral transition from sediment bypass in the channel thalweg to sediment deposition on the levees. The transition occurs because the sediment concentration is below the flow capacity in the channel thalweg, while higher up on the channel walls the concentration exceeds capacity resulting in deposition of the inner levee. Thus, a framework is provided to predict the growth pattern and facies of a levee from the suspended sediment structure in a turbidity current.

Integrating outcrop and subsurface data to assess the temporal evolution of a submarine channel–levee system

The morphological evolution of submarine channel systems can be documented using high-resolution three-dimensional seismic data sets. However, these studies provide limited information on the distribution of sedimentary facies within channel fills, channel-scale stacking patterns, or the detailed stratigraphic relationship with adjacent levee-overbank deposits. Seismic-scale outcrops of unit C2 in the Permian Fort Brown Formation, Karoo Basin, South Africa, on two subparallel fold limbs comprise thin-bedded successions, interpreted as external levee deposits, which are adjacent to channel complexes, with constituent channels filled with thick-bedded structureless sandstones, thinner-bedded channel margin facies, and internal levee deposits. Research boreholes intersect all these deposits, to link sedimentary facies and channel stacking patterns identified in core and on image logs and detailed outcrop correlation panels. Key characteristics, including depth of erosion, stacking patterns, and cross-cutting relationships, have been constrained, allowing paleogeographic reconstruction of six channel complexes in a 36-km2 (14-mi2) area. The system evolved from an early, deeply incised channel complex, through a series of external levee-confined and laterally stepping channel complexes culminating in an aggradational channel complex confined by both internal and external levees. Down-dip divergence of six channel complexes from the same location suggests the presence of a unique example of an exhumed deep-water avulsion node. Down-dip, external levees are supplied by flows that escaped from channel complexes of different ages and spatial positions and are partly confined and share affinities with internal levee successions. The absence of frontal lobes suggests that the channels remained in sand bypass mode immediately after avulsion.

Time-Transgressive Confinement On the Slope and the Progradation of Basin-Floor Fans: Implications For the Sequence Stratigraphy of Deep-Water Deposits

Abstract Observations from outcrop and subsurface datasets indicate that key stratigraphic surfaces in ancient submarine slope successions are diachronous and form during periods of seascape degradation and sediment bypass. Evidence for time transgressive confinement of submarine channel–levee systems includes composite basal erosion surfaces, cut-off bends and hanging valleys, and external levees overlying lobe deposits. After the onset of a sediment supply cycle, progressive confinement will develop on the submarine slope, through a combination of incision and external levee construction, such that successive sediment gravity flows will maintain their downslope energy farther into the basin. This way, frontal lobe deposits are incised by channel systems and overlain by external levee deposits as the channel–levee system becomes more entrenched and propagates farther into the basin. The stratigraphic response on the related basin floor is fan growth and net progradation until a maximum basinward extent is reached, which corresponds to the time of maximum through-channel sediment transfer (bypass). At this time a maximum regressive surface forms, although in reality this is challenging to identify in the rock record. Conceptually, this basin-floor process response to progressive slope confinement at a point could be autocyclic, but would be amplified with an allogenically driven waxing-then-waning sediment supply cycle. The coupled progressive confinement of the submarine channel–levee system and basin-floor fan growth will result in a diachronous lithological basal surface to the system. This challenges the idea of the deep-water sequence boundary being isochronous and passing into a single correlative conformity at the base of the basin floor fan, or that there is temporal distinction between deposition by high- and low-density turbidity currents.

A Polynomial Model for Lateral Thickness Decay of Submarine Channel-Levees

Abstract Active and relic channels with extensive levees are common on the seafloor adjacent to the continental margin. Exponential, power-law, and logarithmic functions have been proposed by others to describe the lateral thickness decay of submarine levees. In the present study, experiments have been conducted to study submarine levee growth by flow stripping at a channel bend. Various functions have been fitted to the data. The results show that a third-order polynomial function best describes the levee thickness in the cross-stream direction near a submarine channel bend. Generalized model coefficients are derived as functions of deposit thickness adjacent to the channel bank. The model is applied to experimental data obtained by others as well as levee shapes derived from seismic data. In both cases, satisfactory agreements are found. The proposed model can be a useful tool for stratigraphic modeling of submarine channel–levee systems and fill gaps in field data.

The possible role of Coriolis forces in structuring large-scale sinuous patterns of submarine channel–levee systems

Submarine channel-levee systems are among the largest sedimentary structures on the ocean floor. These channels have a sinuous pattern and are the main conduits for turbidity currents to transport sediment to the deep ocean. Recent observations have shown that their sinuosity decreases strongly with latitude, with high-latitude channels being much straighter than similar channels near the Equator. One possible explanation is that Coriolis forces laterally deflect turbidity currents so that at high Northern latitudes both the density interface and the downstream velocity maximum are deflected to the right-hand side of the channel (looking downstream). The shift in the velocity field can change the locations of erosion and deposition and introduce an asymmetry between left- and right-turning bends. The importance of Coriolis forces is defined by two Rossby numbers, RoW=U/Wf and RoR=U/Rf, where U is the mean downstream velocity, W is the width of the channel, R is the radius of curvature and f is the Coriolis parameter. In a bending channel, the density interface is flat when RoR∼-1, and Coriolis forces start to shift the velocity maximum when |RoW|<5. We review recent experimental and field observations and describe how Coriolis forces could lead to straighter channels at high latitudes.

Well-balanced two-dimensional coupled modelling of submarine turbidity currents

Submarine turbidity currents may wreak havoc on deep sea facilities and also affect the formation of many morphological features, which are closely linked to hydrocarbon reserves. Yet most previous understandings are based on the analyses of geological and geophysical data or decoupled modelling. Here a two-dimensional coupled mathematical model for turbidity currents over erodible beds is developed. The governing equations are built upon the complete layer-averaged mass and momentum conservation laws. The well-balanced slope limiter centred scheme is deployed, which facilitates a reasonable balance between the flux gradients and the bed source terms. This renders the model applicable to both regular and irregular topographies. The coupled model is verified against both one- and two-dimensional experimental turbidity currents. When applied to turbidity currents near a submarine canyon–-fan transition and over submarine fans, coupled modelling is found to be critical, which incurs negligible increase in the computational cost. Computational case studies demonstrate that bed slope, sediment particle size and upstream boundary condition are key factors dictating the formation of submarine channel–levee morphology. Appropriate bed slope and sediment particle size as well as larger inlet Richardson number may favour formation of submarine channel–levee morphology.

Submarine slope degradation and aggradation and the stratigraphic evolution of channel–levee systems

Abstract: Two seismic-scale submarine channel–levee systems exposed in the Karoo Basin, South Africa provide insights into slope conduit evolution. Component channel fills in a levee-confined channel system (Unit C) and an entrenched channel system (Unit D) follow common stacking patterns; initial horizontal stacking (lateral migration) is followed by vertical stacking (aggradation). This architecture is a response to an equilibrium profile shift from low accommodation (slope degradation, composite erosion surface formation, external levee development, sediment bypass) through at-grade conditions (horizontal stacking and widening) to high accommodation (slope aggradation, vertical stacking, internal levee development). This architecture is likely common to other channel–levee systems. Supplementary material: A detailed correlation panel (presented schematically in Figure 2 ) is available at www.geolsoc.org.uk/SUP18456 .

Influence of the Coriolis force on the velocity structure of gravity currents in straight submarine channel systems

Large‐scale turbidity currents in submarine channels often show a significant asymmetry in the heights of their levee banks. In the Northern Hemisphere, there are many observations of the right‐hand channel levee being noticeably higher than the left‐hand levee, a phenomenon that is usually attributed to the effect of Coriolis forces upon turbidity currents. This article presents results from an analog model that documents the influence of Coriolis forces on the dynamics of gravity currents flowing in straight submarine channels. The observations of the transverse velocity structure, downstream velocity, and interface slope show good agreement with a theory that incorporates Ekman boundary layer dynamics. Coriolis forces will be important for most large‐scale turbidity currents and need to be explicitly modeled when the Rossby number of these flows (defined as Ro = ∣U/Wf∣, where U is the mean downstream velocity, W is the channel width, and f is the Coriolis parameter defined as f = 2Ω sin(θ), with Ω being the Earth's rotation rate and θ being the latitude) is less than order 1. When Ro ≪ 1, the flow is substantially slower than a nonrotating flow with the same density contrast. The secondary flow field consists of frictionally induced Ekman transports across the channel in the benthic and interfacial boundary layers and a return flow in the interior. The cross‐channel velocities are of the order of 10% of the along‐channel velocities. The sediment transport associated with such transverse flow patterns should influence the evolution of submarine channel levee systems.

Architectural Style and Quantification of a Submarine Channel-Levee System Located in a Structurally Complex Area: Offshore Nile Delta

Abstract A Pleistocene channel–levee system located in a structurally complex area of the offshore Nile Delta is studied using a high-resolution three-dimensional (3D) seismic dataset. The seismic facies and stacking patterns are characterized and quantitative analysis of its morphology (e.g., thalweg longitudinal profile, relief, width, and levee thickness) is utilized to understand the controls on channelized-flow processes and resultant architecture. Overall a change downslope from a degradational to aggradational style is observed, which can be related to the concave-up slope profile. In comparison to other channel–levee systems the Noor has a shorter length scale and displays a steepened thalweg slope. This is interpreted to be influenced by structural movements and an associated knickpoint, which controlled a change in flow processes (e.g., velocity, turbulence, and thickness) and the associated location of the canyon to channel–levee transition zone. An unusual feature of downslope levee thickening is observed. Instead of the normal reduction in overbank sedimentation downslope, it is suggested that structural modification causing relative steepening of seabed topography resulted in increased turbidity-current velocity, fluid volume discharge, and associated sediment load, which overcame the normal downslope loss of sediment due to overbank sedimentation. This study shows that architectural style is strongly linked to slope topography, and that structural movements can influence the slope profile and flow process, resulting in modification of the morphology and dimensions of channel–levee systems.

A model of submarine channel-levee evolution based on channel trajectories: Implications for stratigraphic architecture

Channel-levee systems are frequently interpreted as having a long history of cut-and-fill by channel-shaped features of different scales. Results from a simple geometric model based on a centerline migration algorithm combined with a vertical channel trajectory show that an incising-to-aggrading trajectory of a single channel can produce realistic morphologies similar to systems observed on the seafloor and subsurface, including features such as a basal erosional surface, coeval inner and outer levees, internal erosional boundaries, and terraces draped by inner levee deposits. Channel migration results in composite erosional surfaces that are distinct from topographic surfaces, and their formation does not require larger than usual erosional flows. Many submarine channels interpreted as underfit were probably carved by flows similar to the ones that eroded and deposited the entire channel system. We suggest that the features of most submarine channel-levee systems do not require large temporal variations in flow magnitude but can be explained by a simpler model whereby incision, migration and aggradation of a single channel form over time results in an apparently complex system.

Sedimentological criteria to differentiate submarine channel levee subenvironments: Exhumed examples from the Rosario Fm. (Upper Cretaceous) of Baja California, Mexico, and the Fort Brown Fm. (Permian), Karoo Basin, S. Africa

Two scales of levee confinement are commonly recognised from submarine channel-levee systems on the seafloor and in the subsurface. Large-scale external levees bound the entire system whilst smaller-scale internal levees bound individual thalweg channels within the channel-belt. Although thin beds are commonly identified in core and well logs, their origin, and consequently their stratigraphic significance is currently poorly understood. This knowledge gap stems, in part, from the lack of unambiguously identified outcrop analogues of channel-levees, and in particular the lack of identifiable internal and external levees. Here we report from two exhumed channel-levee systems where both scales of confinement can be recognised: the Rosario Fm. of Baja California, and the Fort Brown Fm. of South Africa. A suite of characteristic sedimentary features are recognised from internal and external levees respectively: internal levees are characterised by structures indicative of complexity in the waxing-waning style of overspill, interactions with topography and flow magnitude variability; in contrast, external levees are characterised by structures indicative of simple surge-like waning flows, relatively uniform flow directions, laterally extensive beds, and a lack of erosive events. Using these observations, together with published literature, we propose a simple nomenclatural scheme for levee sub-environments, and criteria to differentiate between levee sub-environments in core or outcrop.

On the Origin of Paleocurrent Complexity Within Deep Marine Channel Levees

Abstract Complex patterns of temporal and spatial variation in paleoflow within individual turbidites have been inferred from studies of both recent and ancient submarine channel–levee sands and sandstones. Previous interpretations of these paleocurrents have invoked “shifting patterns of meandering streamlines” in essentially unconfined flows, and in some cases topographic deflection of flows as they spilled from the channel. However, other flow indicators, such as current ripples, suggest that overbank flow may exhibit more uniform flow directions. Here we re-examine the spatial and temporal evolution of overbank flow on submarine channel levees, using grain fabric data from the Cretaceous Rosario Formation and extant data from the recent Amazon channel. We integrate these observations with our theoretical understanding of the long-term evolution of overbank flow processes to propose a process-based model of evolution of overbank flow across submarine channel levees. Overbank flow over levees is inferred to be dominated by three main processes: (1) gravity forces that attempt to direct flow down the local maximum slope; (2) inherited momentum from the channelized flow, which in many cases is closer to the axial flow direction; (3) development over time of an overbank wedge of fluid that moves broadly parallel to the direction of the main channel. The interaction among these processes varies both in space and time, and collectively accounts for the observed paleoflow variations inferred from deposits. This study may allow better interpretation of depositional sub-environments within levee turbidites, both in core and in outcrop.

Submarine channel levee shape and sediment waves from physical experiments

Abstract Submarine channel levees aggrade through repeated overspill events from the channel axis. The shape of the levees may therefore reflect some characteristic(s) of the overspilling flow. It has been noted that basin floor levees typically have a relatively low-relief and taper exponentially to their termination; in contrast slope channel levees may be much steeper close to the channel. A simple physical experiment was performed where a surge-like sediment-laden current flowed through a curved channel. Significant overspill occurred and generated a deposit flanking the channel on either side. The experiment was repeated 25 times to build up low-relief channel-levees. It was found that in proximal areas, levees were steep and characterised by power-law decays, a transitional zone of logarithmically thinning levee was found a little further down-channel, followed by exponential decays in medial to distal areas. The style of levee decay is a function of spatial variation in overbank sedimentation rates. Where flows rapidly lose momentum and deposit across the grain-size spectrum, i.e., in proximal areas, levees tend to be steep; farther down the channel, the steep levee slope gives way to a more gradually tapering deposit. In more distal parts of the channel, deposition is directly related to sediment settling velocity (rather than the suspended load exceeding flow transport capacity as is the case in proximal areas), the deposit reflects this with relatively simple exponential thickness decays. Additionally, small-scale sediment waves developed under lee wave conditions on the inner-bend overbank. The waves initially migrated slightly towards the channel, but as the style of overspill evolved due to intra-channel deposition, flows moved out of the lee wave window and sedimentation became out of phase with the wavelength of the features and the topography was healed.

Interactions between submarine channel systems and deformation in deepwater fold belts: Examples from the Levant Basin, Eastern Mediterranean sea

Submarine channel levee systems form important hydrocarbon reservoirs in many deep marine settings and are often deposited within a structurally active setting. This study focuses on recent submarine channels that developed within a deepwater fold and thrust belt setting from the Levant Basin, eastern Mediterranean Sea. Compressional deformation within the study area is driven by the up-dip collapse of the Nile cone above the ductile Messinian Evaporites. Structures such as folds and strike slip faults exert a strong control on channel location and development over time. From this study four end-member submarine channel–structure interactions can be defined: Confinement, diversion, deflection and blocking. Each of these channel–structure interactions results in a distinct submarine channel morphology and pattern of development compared to unconfined channel levee systems. Each interaction can also be used to assess timing relationships between submarine channel development and deformation.

Organic matter budget in the Southeast Atlantic continental margin close to the Congo Canyon: In situ measurements of sediment oxygen consumption

Abstract A study of organic carbon mineralization from the Congo continental shelf to the abyssal plain through the Congo submarine channel and Angola Margin was undertaken using in situ measurements of sediment oxygen demand as a tracer of benthic carbon recycling. Two measurement techniques were coupled on a single autonomous platform: in situ benthic chambers and microelectrodes, which provided total and diffusive oxygen uptake as well as oxygen microdistributions in porewaters. In addition, sediment trap fluxes, sediment composition (Org-C, Tot-N, CaCO3, porosity) and radionuclide profiles provided measurements of, respectively input fluxes and burial rate of organic and inorganic compounds. The in situ results show that the oxygen consumption on this margin close to the Congo River is high with values of total oxygen uptake (TOU) of 4±0.6, 3.6±0.5 mmol m−2 d−1 at 1300 and 3100 m depth, respectively, and between 1.9±0.3 and 2.4±0.2 mmol m−2 d−1 at 4000 m depth. Diffusive oxygen uptakes (DOU) were 2.8±1.1, 2.3±0.8, 0.8±0.3 and 1.2±0.1 mmol m−2 d−1, respectively at the same depths. The magnitude of the oxygen demands on the slope is correlated with water depth but is not correlated with the proximity of the submarine channel–levee system, which indicates that cross-slope transport processes are active over the entire margin. Comparison of the vertical flux of organic carbon with its mineralization and burial reveal that this lateral input is very important since the sum of recycling and burial in the sediments is 5–8 times larger than the vertical flux recorded in traps. Transfer of material from the Congo River occurs through turbidity currents channelled in the Congo valley, which are subsequently deposited in the Lobe zone in the Congo fan below 4800 m. Ship board measurements of oxygen profiles indicate large mineralization rates of organic carbon in this zone, which agrees with the high organic carbon content (3%) and the large sedimentation rate (19 mm y−1) found on this site. The Lobe region could receive as high as 19 mol C m−2 y−1, 1/3 being mineralized and 2/3 being buried and could constitute the largest depocenter of organic carbon in the South Atlantic.

Late Weichselian fluvial evolution on the southern Kara Sea Shelf, North Siberia

Glaciations had a profound impact on the global sea-level and particularly on the Arctic environments. One of the key questions related to this topic is, how did the discharge of the Siberian Ob and Yenisei rivers interact with a proximal ice sheet? In order to answer this question high-resolution (1–12 kHz), shallow-penetration seismic profiles were collected on the passive continental margin of the Kara Sea Shelf to study the paleo-drainage pattern of the Ob and Yenisei rivers. Both rivers incised into the recent shelf, leaving filled and unfilled river channels and river canyons/valleys connecting to a complex paleo-drainage network. These channels have been subaerially formed during a regressive phase of the global sea-level during the Last Glacial Maximum. Beyond recent shelf depths of 120 m particle transport is manifested in submarine channel–levee complexes acting as conveyor for fluvial-derived fines. In the NE area, uniform draping sediments are observed. Major morphology determining factors are (1) sea-level fluctuations and (2) LGM ice sheet influence. Most individual channels show geometries typical for meandering rivers and appear to be an order of magnitude larger than recent channel profiles of gauge stations on land. The Yenisei paleo-channels have larger dimensions than the Ob examples and could be originated by additional water release during the melt of LGM Putoran ice masses. Asymmetrical submarine channel–levee complexes with channel depths of 60 m and more developed, in some places bordered by glacially dominated morphology, implying deflection by the LGM ice masses. A total of more than 12,000 km of acoustic profiles reveal no evidence for an ice-dammed lake of greater areal extent postulated by several workers. Furthermore, the existence of the channel–levee complexes is indicative of unhindered sediment flow to the north. Channels situated on the shelf above 120-m water depth exhibit no phases of ponding and or infill during sea-level lowstand. These findings denote the non-existence of an ice sheet on large areas of the Kara Sea shelf.