Sinuous Submarine Channels Research Articles

Modeling the Tilt of Bend‐Traversing Turbidity Currents: Implications for Sinuous Submarine Channel Development

AbstractThe controls on the development of submarine channel sinuosity are contested: slope gradient and Coriolis forcing have both been recognized as key governing factors: gradient via an inverse relationship (low sinuosity at high slope and vice versa), and Coriolis forcing through its effect on sedimentation patterns (reducing lateral bend migration, and hence sinuosity development, at high latitudes and/or in large channels). Using theoretical models to calculate the bulk properties of channelized turbidity currents, this study investigates the joint role of the Coriolis force and parameters including channel size, downchannel slope and turbidity current properties in the development of submarine channel sinuosity. Model validation is undertaken through the comparison of the calculated turbidity current tilting against the measured tilting of channel levees in the Northwest Atlantic Mid‐Ocean Channel; this approach is then used to evaluate the controls on channel sinuosity in nine other modern seafloor channels. The results indicate that the Coriolis force only becomes significant when the size of the channel, the slope gradient and flow conditions are within appropriate ranges instead of solely being dependent on latitude. Thus, thick and dense (≥1% bulk sediment concentration) flows traveling within steep‐gradient, small‐scale channels were shown to be relatively less susceptible to flow modification by Coriolis forcing even at high latitudes. On the other hand, thin and dilute (≪1% bulk sediment concentration) flows in shallow‐gradient, large‐scale channels showed susceptibility to Coriolis forcing at all latitudes. These results offer new insights into submarine channel evolution and intra‐channel sedimentation patterns.

Architecture and depositional processes of a submarine channel during the late Miocene in the Yinggehai basin, northwestern South China Sea

Submarine channels are the most extensive sediment transport systems on Earth. They transport sediment over hundreds of kilometers and form sediment deposits in the deep-sea, which can act as hydrocarbon reservoirs. During the Late Miocene, a sinuous submarine channel with sediment processes was discovered in the Yinggehai Basin, northwestern South China Sea. However, process interpretations of the factors that influence the stacking patterns, distribution, and morphology of these sinuous channel deposits in this region are not well understood. Based on the interpretation of high-resolution 3D seismic data and exploration well data, and insights from flow dynamics, we analyzed the factors influencing channel morphology, the architecture of the channel, and its associated geomorphic evolution. The channel can be divided into three sections (Sections Ⅰ, Ⅱ, and Ⅲ) in terms of its planform, and into two seismic units (Unit 1 and Unit 2) in terms of its vertical structure. The sinuous submarine channel originated from a combination of inherent structural features from secondary faults and strong erosion from turbidity currents, which occurred due to a sharp decline in sea level during the Late Miocene. Changes in the channel's orientation led to erosion and channel-margin failures, causing an increase in channel width and a decrease in margin gradient. The distribution and stacking patterns of the deposits are greatly influenced by the provenance, sea level changes, and the morphology of the channel. In Sections Ⅰ and Ⅲ, a river-like secondary flow with a limited lateral density gradient leads to vertical deposits. In Section Ⅱ, changes in the channel's orientation altered its morphology and induced topographic forcing, producing a strong river-like secondary flow. This flow consistently caused sediment to accumulate on the inner bank and erode on the outer bank, leading to lateral stacking of deposits in Unit 2. During the filling of Unit 2, diapiric activity uplifted the strata, resulting in thinner deposition in Section Ⅲ, while deposition thickness increased in Section Ⅱ with sufficient provenance. Unit 1 of Section Ⅲ displays desirable properties for a hydrocarbon reservoir, including thick sandstone with good sorting and roundness. This study provides valuable insights into the diverse architectures of spatially and temporally linked submarine channels, and establishes a correlation between the evolutionary process of the channel and the stacking patterns of its constituent elements.

The role of avulsion and splay development in deep-water channel systems: Sedimentology, architecture, and evolution of the deep-water Pliocene Godavari “A” channel complex, India

Abstract Combined seismic images and core data from shallow gas-charged, deep-water sands in the western Bay of Bengal provide a detailed record of channel formation and evolution. The KG-D6 Block, western Bay of Bengal, includes two main Pliocene deep-water channel complex sets, termed the A and B complex sets, currently under gas production by Reliance Industries. These represent sinuous channels developed at the base of the delta slope off of the Godavari River and at the head of a long continental slope into the deep Bay of Bengal. The A complex set includes two main channel complexes, the older A150 and younger A100 channel complexes. The A complex provides an excellent opportunity to explore the geometry, sedimentation, and evolution of a small submarine channel system because of shallow reservoir depth, excellent 3D seismic coverage, availability of cores, and the nearly complete system-wide gas charge of the sandstones. Relict sea-floor topography and the processes of avulsion and splay development figure importantly in the overall evolution of slope channel systems. In the KG-D6 area, a sand-rich avulsion splay derived from an older channel complex became the template upon which the A channel complex set evolved. The component A150 and A100 channel complexes developed along a single sinuous course, with the channel complexes building through coupled channel erosion and coeval levee construction. The Godavari slope channels are not confined by scalloped valley walls and did not migrate laterally during their evolution. Mass transport deposits (MTDs) occur in all parts of the channel-fill section. The A100 channel complex displays a well-developed fining-upward cycle in most sections apparently not reflecting shifts in channel position or meander cuts offs, as inferred for some other deep-water channel systems, but rather through changes in the activity within the channels and through advances and retreats of frontal splays up and down the channel system. The Godavari data demonstrate the importance of avulsion and splay development in the evolution of deep-water sedimentary deposits, even high on passive margin slopes, which has implications for understanding the evolution of sinuous submarine channels and the development and distribution of hydrocarbon reservoir compartments.

Using sea-floor morphometrics to constrain stratigraphic models of sinuous submarine channel systems

Constructing geologically accurate reservoir models of deep-water strata is challenging due to the reliance on incomplete or limited resolution datasets. Connecting areas of high-certainty across areas where data is sparse or non-existent (e.g., between wellbores) is difficult and requires numerous interpretations and assumptions. In this study, morphometric data from the Lucia Chica Channel System, offshore California, provides high-resolution 3-D information that is used to constrain correlation and characterization of ancient submarine channel fill deposits.A statistical relationship between cross-sectional asymmetry and planform morphology in sinuous submarine channels is determined from bathymetric data. Submarine channel cross-sectional asymmetry was quantified by calculating the ratio between the distance from the inner bend margin to the thalweg by the entire channel width. This metric was calculated for 243 cross-sections, from 27 channel bends ranging in sinuosity from 1.0 to 3.0. Three distinct channel geometries are classified based on their thalweg position; normal asymmetrical, symmetrical and inverse asymmetrical. Straight channel segments (sinuosities 1.0–1.05) exhibit the most symmetrical cross-sectional morphologies. Low (sinuosities 1.05–1.2) and high sinuosity (sinuosities >1.2) channel bends exhibit maximum cross-sectional asymmetries at bend apexes, with symmetrical cross-sectional morphologies at inflection points. As expected, asymmetry values systematically increase from straight to high sinuosity channel segments; however, the most significant increase occurs at the threshold from straight to low sinuosity channel segments, which suggests that even minor deviation from straight channels promotes development of asymmetrical cross-sectional morphologies. Defined relationships are utilized to inform correlation of channelform surfaces between two well-exposed outcrops of a sinuous deep-water channel system that are ∼1 km apart (Cretaceous Tres Pasos Formation, southern Chile). The developed methodology can be applied in the subsurface to model realistic channelform sedimentary bodies guided by channel planform interpretations from limited-resolution 3-D seismic data, augmented by well data.

Curvature-induced secondary flow in submarine channels

The curvature-driven secondary flow in sinuous submarine channels has been a subject of considerable interest and controversy. Here, results from numerical model studies involving saline flow in laboratory-scale channels are presented. A 3D finite volume model of density and turbidity currents is used and simulations are run with different inflow discharges and channel-axis slopes. The simulation results show strong influence of bend wave length, channel gradient, confinement and cross sectional shape on the structure of secondary flow in submarine channels. Major findings are: (i) reversal of secondary flow in submarine channels is strongly associated with a tight bend characterized by a smaller wave length to width ratio or larger wave number, (ii) for the same inflow condition and planform characteristics, a trapezoidal channel cross section is more favorable to secondary flow reversal than a rectangular cross section, (iii) lateral convection resulting from the interaction between in-channel and overbank flows leads to the reversal of secondary flow in an unconfined channel at a lower channel slope than in a confined channel with the same dimensions, (iv) flow discharge has only nominal effect on the secondary flow in submarine channels.

Three‐dimensional gravity‐current flow within a subaqueous bend: Spatial evolution and force balance variations

AbstractThe nature of three‐dimensional flow in submarine channel bends is poorly understood, largely due to the absence of detailed data from natural channels. Herein, data from density‐driven flows in a large reservoir on the Huanghe (Yellow) River are presented showing the spatio‐temporal variation of flow around a subaqueous bend. The data demonstrate for the first time that reversed helical flow, relative to that found in river channel bends, can occur from the centrifugal forcing of flow, even when the Coriolis force acts in the opposite direction. The data also suggest that reversed helical flow fields in submarine channels may be more frequent than currently estimated, notably for bends where Coriolis and centrifugal forces combine in the same direction. In addition, this study provides the first field evidence suggesting that sinuous submarine channels can exhibit an asymmetry in helical flow orientation between left and right‐turning bends, which will have major implications for the morphodynamics of submarine channels, their resultant patterns of sedimentation and, ultimately, the distribution of depositional units across submarine fan systems.

Global (latitudinal) variation in submarine channel sinuosity: REPLY

Current classifications of submarine channels and fans link channel sinuosity to gradient, and in turn to sediment caliber, with end members being high-sinuosity, low-gradient, fine-grained systems and low-sinuosity, high-gradient, coarse-grained systems. However, the most sinuous modern submarine channels, such as the Amazon, Bengal, Indus, and Zaire, along with ancient sinuous submarine channels, are located in equatorial regions. Here we quantitatively compare slope versus latitude controls on submarine channel sinuosity and show that the latitudinal control is strong, while that of slope is weak. Variation in sinuosity with latitude is shown to occur uniquely in submarine channels; no comparable relationship is observed for terrestrial river channels. Possible causal mechanisms for this latitudinal variation are explored, focusing on the influence of the Coriolis force, flow type, and sediment type. Although climate does not vary straightforwardly with latitude, climatic controls on flow and sediment type may explain some of the latitudinal variation; Coriolis force, however, varies with latitude alone and produces an excellent fit to the observed sinuosity-latitude distribution. Regardless of which control predominates, latitudinal global variation in channel sinuosity should have changed over geologic time. Since deposit architecture and facies are linked directly with sinuosity, submarine channel deposits should also systematically vary in space and time.

Experimental modeling of depositional turbidity currents in a sinuous submarine channel

Submarine channels with intricate meandering patterns and extensive levees are recognized as products of density-driven flows known as turbidity currents. Compared to the fluvial meandering channels, understanding of the flow and morphodynamics of submarine channels is limited. In this paper, we present experimental results on the morphodynamic and stratigraphic evolution of a submarine channel from sedimentation due to the passage of successive flow events. A pre-formed sinuous channel with multiple bends, a trapezoidal cross section, and an initial thalweg slope of 0.43° was emplaced in a large tank. A total of 29 runs, each lasting an hour, were made by releasing heavier fluids containing salt and silica powder at a constant rate in the tank filled with fresh water. The overbank flow was restricted to curvature-induced flow stripping.The following observations were made from the experimental measurements; (i) asymmetric channel cross sections developed due to higher deposition rates on the outer bank of the channel bends, (ii) an apron or sediment wedge with overlain bedforms appeared near the channel entrance that prograded and aggraded with successive runs, (iii) overbank flow due to flow stripping at bend apices resulted in lobe-shaped deposits, (iv) a higher concentration of solute resulted in flow confinement, and (v) the flow velocity during later runs increased due to channel narrowing and steepening of the gradient.

Turbidity current hydraulics and sediment deposition in erodible sinuous channels: Laboratory experiments and numerical simulations

This study explores the relationship between the hydraulics of turbidity currents in erodible sinuous channels and the resulting intra-channel sediment depocentres (channel bars). Four factors are considered to exert critical control on sedimentation in sinuous submarine channels: (1) the required equilibrium gradient of the flow versus the pre-existing channel gradient; (2) the wavelength of the flow rotation helicoid versus the channel curvature wavelength; (3) the degree of confinement of the channel-entering flow; and (4) the channel bank erodibility. The study verifies the role of these factors by testing a wide range of possible flow conditions and reveals the formative conditions for five types of intra-channel depocentres: point bars (meander bars), bars formed at the upslope or downslope bank in the channel-bend inflection zone, and the outer-bank bars formed directly upstream or downstream of the bend apex.Channel meandering is caused by turbidity currents that deposit point bars and possibly bars at the upslope bank of the channel-bend inflection zone. These are currents that result in no significant aggradation or degradation of the channel, spill out moderately and have a rotation helicoid rising against the bend inner bank. Channel aggradation disfavours meandering, but an established meandering channel can maintain its behaviour when forced to aggrade at a modest rate. Currents forming outer-bank bars and possibly bars at the upslope bank of the bend inflection zone are approximately in equilibrium with the channel gradient, spill out moderately, have a rotation helicoid rising outwards at the bends, and result in an overall straightening of the sinuous channel. Channel-aggrading currents that form outer-bank bars and the bypass currents that form bars at the downslope bank of the bend inflection zone spill out excessively and have an intricate velocity structure due to interaction with re-entering overbank flow. These currents tend to fill up or strongly modify the pre-existing sinuous channel.The study confirms and expounds on many previous laboratory inferences based on flows in solid-wall sinuous channels. It also demonstrates that numerical simulations can give considerable new insights into the sedimentation processes in submarine channels. The advantage of numerical simulations is that they allow monitoring of all crucial hydraulic parameters of the turbidity current and its responses to topography in three dimensions over full flow duration. Such simulations, when calibrated against laboratory experiments, can improve our understanding of both laboratory flows and natural turbidity currents.

The depositional characteristics of turbidity currents in submarine sinuous channels

Submarine channels with meandering patterns and extensive levee complexes are commonly found on continental slopes and fans. Here, a three-dimensional numerical model is applied to conduct a systematic study of poorly-sorted depositional turbidity currents in sinuous submarine channels with different side slope and sinuosity. Multiple runs with the same inflow conditions are made to study deposition patterns inside the channel and levees. Not only many known facts, such as the spilling and stripping of turbidity currents at channel bends as well as the different pattern of deposition at the inner and outer banks of channel bends, are produced, but also some important new findings, such as: i) small river-like circulation near the bottom corner of outer bank co-exists with predominant lateral convection as secondary flow; ii) the levee on the outer bank of a channel bend apex of steeper lateral slope shows larger variation in grain size distribution than the thinner, more smooth levee on its opposite side; iii) the levees at bend inflections inherit the characteristics of the upstream levees at channel bend apexes; iv) the downstream cross-section that cuts both sinuous channel beds and banks is characterized by thicker channel bed deposits intersected by arch-shaped thinner on-bank deposits; v) the levee deposit displays an asymmetric wave pattern in the downstream direction, and the flow direction is from the short rising side to the long dropping side. The flow and sedimentation patterns summarized in this study may promote the understanding of physical processes of submarine turbidity currents and their depositions and assist in interpreting seismic, outcrop and/or borehole data for reconstruction of original depositional environment.

Flow processes and sedimentation associated with erosion and filling of sinuous submarine channels

This article uses measurements from a three-dimensional exposure of a sinuous submarine channel and its fill to document, for the first time, how flow properties and sedimentation differ between erosional and filling stages. Two units, recording this sequential evolution, are documented. Unit 1 records gravity flows that deepened and laterally migrated the channel, resulting in the deposition of point bars, with coarsening-upward profiles, on the inner part of channel bends. These gravity flows were mud-rich with a wide grain-size distribution. Flow heights exceeded the depth of the channel resulting in the deposition of levees. Strong secondary flow is evident with a helical pattern reversed to their subaerial counterparts. Strata in point bars and levees are inclined and deposited primarily by tractive processes. Unit 2 records gravity flows that filled the channel. These gravity flows were sand-rich with a relatively narrow grain-size distribution. Flow heights scaled to the depth of the channel, and they contain no evidence for secondary flow. Associated strata are horizontal and deposited primarily from suspension.

Architecture analysis of a river flood-dominated delta during an overall sea-level rise (early Pliocene, SE Spain)

The sedimentary record of delta-front to prodelta transition that developed during the early Pliocene (Bajo Segura Basin, peri-Mediterranean Betic Cordillera Basin) represents an unusual infilling of an incised valley cutting downwards into late Messinian pelagic marls. The complete delta succession, less than 65 m thick, consists of four upwardly fining and thinning sequences several metres thick. Each sequence consists of basal conglomeratic and coarse-grained sandstone beds (delta-front deposits) evolving upwards to alternating sandstones and silty-marl beds (prodelta deposits). The retrogradational stratal succession pattern is interpreted as having been deposited during an overall sea-level rise. Facies association is characterised by quasi- to planar-laminated sandstones and pebbly sandstones alternating with clast-supported boulders to pebbles, sheet-like climbing cross-laminated sandstones with abundant plant remains, and epsilon cross-stratified and channelized gravels and sandstones. These are interpreted as having been deposited by highly concentrated flows interpreted as hyperpycnal turbidity flows from high (cohesionless debris flows and sinuous submarine channels) to moderate (sheet-like climbing cross-laminated sandstones) discharge river-flood events. Slabs of sediment from unstable banks, outsized clasts which roll onto thin beds from channel mouths, slumps and intraformational breccias in delta front are other signatures (including a lack of bored clasts) which indicate high-energy and rapid deposition at the river mouth, encouraging the delta-front gravitational instability failures. Major changes in the discharge of the river-fed delta could have been caused by brief catastrophic, heavy-precipitation events coupled with quasi-stationary convective orographic rainfall typical of the modern Mediterranean.

Gravity-driven flow in a submarine channel bend: Direct field evidence of helical flow reversal

Submarine meandering channels are conduits that transport gravity-driven flows and sediments into the deep sea. Such channel systems form distributive networks across submarine fans, ultimately forming the largest sedimentary deposits on Earth. Despite this, our understanding of flow processes and the sedimentary evolution of sinuous submarine channel systems remains poor, primarily due to a lack of, thus far elusive, direct field observation and measurements of flows within submarine channel bends. In the absence of direct field measurements, our understanding of these systems has necessarily been speculative, relying until very recently on bend flow theory derived from research in subaerial river channel bends. Although recent measurements and results from scaled laboratory experiments and numerical modeling of submarine channel bends have advanced our understanding of some of the relations between flows, forms, and the implications for sedimentary deposits, key results from this recent research have been contradictory. Notably, several studies have indicated that the helical flow structures within submarine bend flows closely match those found ubiquitously in subaerial river channels, while conflicting research has reported a reversal of this helical flow structure, with associated far-reaching implications for deep-sea sedimentology. This paper presents the first direct three-dimensional measurements of the flow field in a natural submarine channel bend. The results, from a submarine channel bend on the Black Sea shelf, demonstrate for the first time that a reversed helical flow structure can occur in seafloor channel bends. Such findings have major implications for process sedimentology in these environments, because the direction and strength of helical flow fields are known to impart a significant influence on cross-stream sediment sorting in bends and thus the stratigraphy of the deposits produced by these channel systems.

Facies and Architectural Asymmetry in a Conglomerate-Rich Submarine Channel Fill, Cerro Toro Formation, Sierra Del Toro, Magallanes Basin, Chile

Abstract Cross-sectional asymmetry is characteristic of sinuous channels in both fluvial and submarine settings. Less well documented are the facies distributions of asymmetric channels, particularly in submarine settings. Exposures of the axial submarine channel-belt in the Magallanes retro-arc foreland basin on Sierra del Toro represent the fill of a 3.5 km wide, 300 m thick channel complex, here termed the “Wildcat,” that displays an asymmetric cross section and facies distribution. Measured sections and mapping demonstrate that facies proportion, degree of amalgamation, and margin architecture vary laterally from east to west across the Wildcat channel complex. The eastern side is characterized by thick-bedded, amalgamated sandstone and clast- and matrix-supported conglomerate that onlap a steep, simple margin adjacent to sandy overbank deposits. The western side contains thin-bedded, sandy and muddy strata that onlap a shallow composite margin adjacent to mud-rich out-of-channel strata. The observed asymmetry is likely due to centrifugal flow forces and was caused by a low-sinuosity right-hand meander bend of the Cerro Toro axial channel-belt. The facies and architecture of the opposing margins indicate that the eastern and western sides constitute the outer and inner bends of the Wildcat channel complex, respectively. The modest cross-sectional asymmetry of the Wildcat complex is likely a product of the low channel-belt sinuosity. The absence of lateral accretion surfaces and deposits suggests that the channel did not migrate during filling. Flows depositing the uppermost channel fill were only weakly confined, resulting in flow divergence and overbank deposition. A depositional model that incorporates the asymmetric facies distributions and the contrasting outer-bend and inner-bend architecture of the Wildcat channel complex is also presented. Similar facies distributions exist in other low-sinuosity submarine channels, and even more extreme facies and cross-sectional asymmetry probably characterize more highly sinuous channels. Data on facies distributions presented here represents a useful resource for constraining numerical and experimental models of the evolution of sinuous submarine channels as well as reservoir models of sinuous submarine channels.

Research Spotlight: Coriolis forces affect flow in submarine channels

Coriolis forces due to the Earth's rotation deflect winds and ocean flows to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. In sinuous submarine channels, Coriolis forces can drive secondary circulation of turbidity currents and determine where erosion and sediment deposition occur.Cossu and Wells conducted laboratory experiments with a channel in a rotating tank to study the conditions under which Coriolis forces dominate the channel flow and investigate how these forces affect sediment deposition. They found that channels at high latitudes and those with large bends are dominated by Coriolis forces, while channels at lower latitudes and those with smaller bends are less influenced by Coriolis forces.

Sedimentary Architecture in Meanders of a Submarine Channel: Detailed Study of the Present Congo Turbidite Channel (Zaiango Project)

Abstract Sinuous deep-water channels are recognized in most large deep-sea fans in the world. They present a particular interest to oil companies, since they are significant hydrocarbon reservoirs in deep offshore environments. The understanding of their geometries and their internal sedimentary architecture is necessary to better characterize reservoir heterogeneity of sinuous submarine channels. Therefore, numerous studies have been undertaken recently to better understand the behavior and sedimentary architecture of deep-water channels. The aim of this paper is to present our results concerning the development of the meandering channel of the present Congo turbidite system (or Zaire turbidite system). The study is based on high-resolution data including multibeam bathymetry, seismic lines, echosounder profiles, high-resolution side-scan sonar images, and gravity cores, collected by IFREMER along the submarine Congo channel between 1994 and 2000, during Guiness and ZaiAngo surveys. The present Congo turbidite channel is a long incised turbidite channel. It is presently active. It has been built gradually by progradation of the distal depositional area. The most distal part of the channel is the youngest part and shows an immature morphology: the channel presents a low incision and a low sinuosity. In contrast, the upper part of the channel has undergone a long evolutionary history. Its pathway is mature and complex, with numerous abandoned meanders visible in the morphology. This paper presents evidence of progressive channel migration and meander development of the Congo channel. It describes and explains the presence of terraces inside the channel. The detailed characterization of channel morphology and migration geometry shows that the evolution of the channel path is very similar to fluvial meandering systems with (1) lateral meander extension or growing, (2) downstream translation of the thalweg, and (3) meander cutoff. Seismic and 3.5 kHz echosounder profiles show that the terraces, which are visible in the seafloor morphology, are not the imprints of incisional processes. Terraces are true depositional units infilling the channel. They are built during and after the lateral migration of the channel. They are composed of (1) point-bar deposits and (2) inner-levee deposits aggrading above the point bar deposits. Point-bar deposits are characterized by low-angle oblique reflectors forming deposits with a sigmoidal shape. They seem very similar to those observed in fluvial systems. The similarity between fluvial and turbidite point bars suggests that the basal part of the turbidity currents flowing in this channel can be considered as very similar to river flow. With the high-resolution dataset collected in a present Congo turbidite channel, we provide a new description of the channel morphology and evolution, at a “reservoir” scale, intermediate between outcrop observations and 2D and 3D seismic data. The detailed interpretation of intrachannel sedimentation, associated with lateral channel migration, also provides new data for interpretation of flow dynamics in submarine meandering channels.

Coriolis forces influence the secondary circulation of gravity currents flowing in large‐scale sinuous submarine channel systems

A combination of centrifugal and Coriolis forces drive the secondary circulation of turbidity currents in sinuous channels, and hence determine where erosion and deposition of sediment occur. Using laboratory experiments we show that when centrifugal forces dominate, the density interface shows a superelevation at the outside of a channel bend. However when Coriolis forces dominate, the interface is always deflected to the right (in the Northern Hemisphere) for both left and right turning bends. The relative importance of either centrifugal or Coriolis forces can be described in terms of a Rossby number defined as Ro = U/fR, where U is the mean downstream velocity, f the Coriolis parameter and R the radius of curvature of the channel bend. Channels with larger bends at high latitudes have ∣Ro∣ < 1 and are dominated by Coriolis forces, whereas smaller, tighter bends at low latitudes have ∣Ro∣ ≫ 1 and are dominated by centrifugal forces.

The influence of bend amplitude and planform morphology on flow and sedimentation in submarine channels

We present a series of experiments that investigate the morphology of sediment deposits within sinuous submarine channels of different sinuosity ( S = 1.14–1.94) and planform (symmetric and asymmetric bends), generated by bedload-dominated turbidity current flows. Flows were generated by releasing dense saline gravity currents over a mobile sediment bed through pre-formed sinuous channels. Flows had a basal-outwards helicity and produced a characteristic bed morphology with point bars downstream of the bend apex at the inside of bends and scour at the outside of bends. An increasing loss of fluid through overspill with increasing channel sinuosity results in a decreasing magnitude of cross-stream velocity downstream, a decreasing amount of erosion and deposition, and decreasing transverse slopes of in-channel deposits. Basal fluid from within the channel is transported over the outer-levee at bends, implying that proximal outer-bend levee deposits will have similar sediment composition to that within the channel. More deposition of coarse material might be expected on levees and in overbank regions close to higher amplitude bends. No simple relationship was observed between superelevation and sinuosity, probably due to changes in the relative influences of downstream velocity and bend curvature on centrifugal force and inertial run-up. In the channel with the tightest initial bend curvature, overspill fluid from Bend 1 re-entered the channel at Bend 2, dominating flow characteristics and disrupting the basal-outwards helicity observed in the other channels. Higher sinuosity channels and those with shallow regional and levee slopes are thus more likely to have a higher proportion of anomalous flow and sedimentation patterns due to the influence of overspill fluid re-entry into the channel. The results of this investigation are combined with published observations to enable the synthesis of a new model for sedimentation in sinuous submarine channels.

Outer-Bank Bars: A New Intra-Channel Architectural Element within Sinuous Submarine Slope Channels

Abstract This study describes for the first time “outer-bank bars,” an intra-channel architectural element that is unique to sinuous submarine channels. The study is based on interpretation of 3D seismic data of sinuous channels in the upper slope of the Amazon Fan, offshore Brazil. Outer-bank bars comprise channel-fill deposits found in tight meander loops at outer corners of bend apices or slightly upstream or downstream of apices. This architectural element can be up to 1.5 km wide and up to 150 m thick with continuous internal reflectors dipping 1–20° towards the inner bend of the meander loop, a direction of dip opposed to that of fluvial point bars or submarine lateral-accretion packages. The angles of outer-bank bar surfaces increased to a maximum at bend apices. Outer-bank bars occur in the final aggradational channel fill in the two separated channel–levee systems in the Amazon data set. They may have been formed as a result of enhanced deposition on the outside of the bend caused by a combination of flow-volume reduction and flow superelevation. Accretion of outer-bank bars may cause the thalweg position to shift towards the inner bends leading to sinuosity reduction. Since this architectural element is likely to be sand prone, its recognition may have significant implications for prediction of hydrocarbon reservoirs.

Interactions between turbidity currents and topography in aggrading sinuous submarine channels: A laboratory study

We present results from a laboratory experiment documenting the evolution of a sinuous channel form via sedimentation from 24 turbidity currents having constant initial conditions. The initial channel had a sinuosity of 1.32, a wavelength of 1.95, an amplitude of 0.39 m, and three bends. All currents had a densimetric Froude number of 0.53 and an initial height equal to the channel relief at the start of the experiment. Large superelevation of currents was observed at bend apexes. This superelevation was 85%–142% greater than the value predicted by a balance of centrifugal and pressure-gradient forces. An additional contribution to the superelevation was the runup of the current onto the outer banks of bends. This runup height is described by a balance between kinetic and potential energy. Runup resulted in deposition of coarse particles on levee crests that were indistinguishable from those deposited on the channel bottom. Deposit thickness and composition showed a strong cross-channel asymmetry. Thicker, coarser, steeper levees grew on the outer banks relative to the inner banks of bends. Zones of flow separation were observed downstream from bend apexes along inner banks and affected sedimentation patterns. Sedimentation from currents caused the channel to aggrade with almost no change in planform. However, channel relief decreased throughout the experiment because deposition on the channel bottom always exceeded deposition at levee crests. The first bend served as a filter for the properties of the channelized current, bringing discharge at the channel entrance into agreement with the channel cross-sectional area. Excess discharge exited the channel at this filtering bend and was lost to the overbank surface.