Sustained Turbidity Currents Research Articles

Flow Behavior of Ponded Turbidity Currents

Abstract Sea-floor topography can constrict, deflect, or reflect turbidity currents resulting in a range of distinctive deposits. Where flows rebound off slopes and a suspension cloud collects in an enclosed basin, ponded or contained turbidites are deposited. Ponded turbidites have been widely recognized in slope mini-basins and on small, structurally confined basin floors in strike-slip and foreland-basin settings. They can have a variable internal structure the significance of which remains poorly understood in terms of flow behavior. New experiments demonstrate that the ponding process can comprise up to four phases: 1) cloud establishment, 2) inflation, 3) steady-state maintenance, and 4) collapse. The experiments explored the behavior of sustained turbidity currents draining into small basins and show that the ponded suspensions that form are characterized by an important internal interface; this divides a lower outbound-moving layer from an upper return layer. The basal layer evolves to constant concentration and grain size, whereas the upper layer is graded (concentration and grain size decrease upward). During the cloud inflation stage, the concentration and velocity profiles of the ponded suspension evolve, and this phase can dominate the resulting deposit. Outbound internal waves can travel along the interface between the outbound and return layers and impinge against the confining slope, and their amplitude is highest when the density contrast between layers is greatest, e.g., when the input flows are thin and dense. The experiments show that flow reversals can arise in several ways (initial rebound, episodic collapse of the wedge of fluid above the counter slope, “grounding” of the internal velocity interface) and that despite steady input, velocities decay and the deposit grades upwards. Internal waves emanate from the input point, i.e., do not form as reflections off the counter slope. The internal grain-size interface within the suspension may dictate textural trends in sands onlapping the confining slopes. Where flows are partially ponded, internal waves can generate pulsing overspill to basins down dip.

Hydraulic and sediment transport properties of autogenic avulsion cycles on submarine fans with supercritical distributaries

AbstractSubmarine fans, like other distributive systems, are built by repeated avulsion cycles. However, relative to deltas and alluvial fans, much less is known about avulsions in subaqueous settings. In this study, we ran a set of subaqueous fan experiments to investigate the mechanics associated with autogenic avulsion cycles of self‐formed channels and lobe deposits on steep slopes. The experiments used saline density currents with crushed plastic to emulate sustained turbidity currents and bed load transport. We collected detailed hydraulic and bathymetric measurements and made use of a 1‐D laterally expanding density current model to better understand different aspects of the avulsion cycle. Our results reveal three major components of the avulsion cycles: (1) distributary channel incision, extension, and stagnation; (2) mouth bar aggradation and hydraulic jump initiation; and (3) hydraulic jump sedimentation and upstream retreat. Interestingly, in all but one experiment, the avulsion cycles led to fans that remained perched above the basin slope break. Experimental data and hydraulic theory were used to unravel actual mechanics associated with cycles. We found that channels stopped extending into the basin due to a decay in sediment transport capacity relative to sediment supply and that the reduction in capacity was primarily an outcome of expansion‐driven velocity reduction; dilution played a secondary role. Once channel extension ceased, mouth bar deposits aggraded to a thickness approximately equal to the critical step height needed to create a choked flow condition. The choke then initiated a hydraulic jump on the upstream side of the bar. Once formed, the jump detained a majority of the incoming sediment and forced the channel‐to‐lobe transition upstream, filling the channel with steep backset bedding and capping the entire channel with a mounded lobate deposit. These intrinsic processes repeated through multiple avulsion cycles to build the fan.

Contrasting slope channel styles on a prograding mud-prone margin

The Pleistocene slope clinoform succession of the Pescara Basin foredeep (Central Adriatic Basin, Italy) has been investigated combining the analysis of the rollover point trajectories with 2D seismic facies analysis and 3D seismic geomorphology approach and visualization techniques. The study documents two different types of slope clinoforms, each of which is characterized by a specific slope turbidite system with distinctive morphologies, internal geometries and architectural elements.Type 1 clinoforms are characterized by rectilinear, slope-confined gullies which develop during a phase of margin growth. Type 1 clinoforms are characterized by a rise of the trajectories of the rollover points, which can be indicative of intervals of relative sea level rise. In type 1 clinoforms most of the coarse-grained sediments are stored in the shelf. The slope-confined gullies barely indent the shelf break, are not connected to rivers, and develop thin (<30 m) frontal splays at the base of the slope. All these evidences indicate that the gullies are inefficient pathways to transfer sandy sediment basinward. The gully depressions are net-erosional and also the inter-gully sectors are sites of dominant sediment bypass. The infill deposit of the gullies consists of channel-wide high-amplitude reflectors (HARs) wedging-out downslope. The origin of the slope-confined gullies is interpreted to be linked to mass wasting processes affecting the upper slope and the shelf-edge during conditions of relative sea level rise. Longshore drifts and storm-driven turbidity currents rework the outer shelf, promoting the maintenance of gully systems, which become filled during the ensuing high-stand of the sea level.Type 2 clinoforms are characterized by a negative trajectory of the rollover points, and develop during a lowering of the sea level and during lowstand periods. Type 2 clinoforms develop sinuous, aggradational slope channels which are river-connected, with incised valleys that supply sediment directly to the head of the slope channel. The slope channels are fed through sustained turbidity currents possibly associated with hyperpycnal flows. Type 2 slope channels develop in a net-aggradational setting, forming erosive based channel-fill deposits about 150 m thick which are characterized by high-amplitude reflectors (HARs). Type 2 slope channels are able to transfer larger amount of sediment to the basin, comparing with type 1 gullies, as the presence of thick (∼150 m) channelized frontal–splay complexes seems to confirm. The combination of 2D and 3D data analysis has lead to the reconstruction of a sedimentological model which has the potential to be applied as analogue for seismic or outcrop-based studies of slope channels in supply-dominated, mud-prone margins.

Field-scale numerical modeling of breaching as a mechanism for generating continuous turbidity currents

The term “breaching” refers to the slow, retrogressive failure of a steep subaqueous slope, so forming a nearly vertical turbidity current directed down the face. This mechanism, first identified by the dredging industry, has remained largely unexplored, and yet evidence exists to link breaching to the formation of sustained turbidity currents in the deep sea. In this paper we model a breach-generated turbidity current using a three-equation, layer-averaged model that has as its basis the governing equations for the conservation of momentum, water, and suspended sediment of the turbidity current. In the model, the turbidity current is divided into two regions joined at a migrating boundary: the breach face, treated as vertical, and a quasi-horizontal region sloping downdip. In this downstream region, the bed slope is much lower (but still nonzero), and is constructed by deposition from a quasi-horizontal turbidity current. The model is applied to establish the feasibility of a breach-generated turbidity current in a field setting, using a generic example based on the Monterey Submarine Canyon, offshore California, USA.

Sustained turbidity currents and their interaction with debrite-related topography; Labuan Island, offshore NW Borneo, Malaysia

The Temburong Fm (Early Miocene), Labuan Island, offshore NW Borneo, was deposited in a lower-slope to proximal basin-floor setting, and provides an opportunity to study the deposits of sustained turbidity currents and their interaction with debrite-related topography. Two main gravity-flow facies are identified; (i) slump-derived debris-flow deposits (debrites) — characterised by ungraded silty mudstones in beds 1.5 to > 60 m thick which are rich in large (> 5 m) lithic clasts; and (ii) turbidity current deposits (turbidites) — characterised by medium-grained sandstone in beds up to 2 m thick, which contain structureless (T a) intervals alternating with planar-parallel (T b) and current-ripple (T c) laminated intervals. Laterally discontinuous, cobble-mantled scours are also locally developed within turbidite beds. Based on these characteristics, these sandstones are interpreted to have been deposited by sustained turbidity currents. The cobble-mantled scours indicate either periods of intense turbidity current waxing or individual flow events. The sustained turbidity currents are interpreted to have been derived from retrogressive collapse of sand-rich mouth bars (breaching) or directly from river effluent (hyperpycnal flow). Analysis of the stratal architecture of the two facies indicates that routing of the turbidity currents was influenced by topographic relief developed at the top of the underlying debrite. In addition, turbidite beds are locally eroded at the base of an overlying debrite, possibly due to clast-related substrate ‘ploughing’ during the latter flow event. This study highlights the difficulty in constraining the origin of sustained turbidity currents in ancient sedimentary sequences. In addition, this study documents the importance large debrites may have in generating topography on submarine slopes and influencing routing of subsequent turbidity currents and the geometry of their associated deposits.

Laboratory sustained turbidity currents form elongate ridges at channel mouths

The turbulent flow structure, suspended sediment dynamics and deposits of experimental sustained turbidity currents exiting a channel across a break in slope into a wide tank are documented. The data shed light on the flow evolution and deposit geometry of analogous natural channel-fed submarine fans. Flows generated in a 0·3 m wide, sloping (0°, 6°, 9° or 20°) channel crossed an angular slope break and spread onto a horizontal tank floor. Flow development comprised: (i) channelized phase (unsteady channel flow developing into steady channel flow); (ii) initial lateral expansion phase (unsteady-spreading wall jet phase); (iii) constant lateral expansion phase (steady-spreading wall jet phase); and (iv) rapid waning phase. Phases (i) and (iv) are similar to laterally constrained turbidity currents, but phases (ii) and (iii) are considerably different from such two-dimensional currents. Steeper channel slopes produced greater flow velocities and turbulence intensities, but these effects diminished markedly with distance from the channel mouth. Flow velocity vectors in the tank had similar patterns for all channel slopes, with a central core of faster velocity and narrow vector dispersion and slower flow with larger dispersion at the jet margins. Suspended sediment concentrations were higher within flow heads and dense basal layers in flow bodies. Time-averaged acoustic backscatter data showed vertical concentration gradients, confirmed by siphon samples. The deposits comprised a thick central ridge, of similar order width to the channel mouth, with abrupt margins and a surrounding, very thin, fan-like sheet. The ridge was coarser grained and better sorted than the original sediment, with grain-size fining downstream, particularly over the fan-like sheet. The formation of a central ridge suggests that, in the tank, vertical turbulent momentum exchange is more significant for sediment dynamics than spanwise momentum exchange due to lateral expansion. The streamwise elongate geometry of the ridge contrasts with conventional fan-like geometry found with surge-type turbidity flows, a result that has widespread stratigraphic and economic implications.

Hyperpycnites Deposited 700 km Away from River Mouths in the Central Japan Sea

Abstract Relatively little is known about the characteristics of deposits and potential runout distances of hyperpycnal currents. This study describes and discusses evidence of sediment deposition from sustained (long-duration), quasi-steady turbidity currents in the distal part of the Toyama deep-sea channel, which extends ca. 700 km from river mouths in the central Japan Sea. The study is based on gravity cores and airgun seismic reflection profiles obtained from the channel distal reaches. The silty turbidite beds of the channel's terminal fan show rhythmic layering that indicates sustained turbidity currents with distinct flow-strength fluctuations. Some of the rhythmite beds show a fining-upward internal trend (net flow-strength waning), whereas others show an upward coarsening (net flow-strength waxing) followed by fining. Seismic reflection profiles from the channel levees show large bedforms of climbing-dune type, attributed to the spillover of thick sustained turbidity currents. The deposition of the terminal fan rhythmites and accretionary levee bedforms is attributed to turbidity currents generated by hyperpycnal river effluent. A quantitative assessment of sediment concentrations in the coastal rivers indicates that their effluents could become hyperpycnal nearly every year or during every major seasonal flood. The density of a river-generated underflow would increase by the entrainment of saline water and seafloor sediment on the steep slope of the Toyama Bay, resulting in robust, long-runout turbidity currents. The estimated flow velocities of these currents were around 0.3 m/s, and their recurrence period for the last 1000 years was of the order of 70 years. The estimated duration of hyperpycnal flows required for the deposition of rhythmite beds 700 km away from the river mouth is of the order of several days to 3–4 weeks. The study provides new insights into the recognition and classification of hyperpycnites in distal zones of deep-marine turbiditic systems.

Trapping of sustained turbidity currents by intraslope minibasins

AbstractDepositional turbidity currents have filled many intraslope minibasins with sediment creating targets for petroleum exploration. The dynamics of sustained turbidity currents and their depositional characteristics are investigated in a scaled physical model of a minibasin. Each turbidity current deposited a downstream thinning wedge of sediment near the inlet. Farther downstream the turbidity current was ponded by a barrier. The ponded part of the turbidity current was separated from the sediment‐free water above by a relatively sharp, horizontal settling interface indicating highly Froude‐subcritical flow. The very slow moving flow within the ponded zone created conditions for the passive rainout of suspended sediment onto the bed. In the lower part of the ponded zone, the concentration and mean grain‐size of the sediment in suspension tended to be relatively uniform in both the vertical and streamwise directions. As a result, the deposit emplaced in the ponded zone showed only a weak tendency toward downstream fining and was passively draped over the bed in such a way that irregularities in the inerodible bed were accurately reflected. The discharge of suspended sediment overflowing the downstream end of the minibasin was significantly less than the inflow discharge, resulting in basin sediment trapping efficiencies &gt;95%. A simple model is developed to predict the trapping of sediment within the basin based on the relative magnitudes of the input discharge of turbid water and the detrainment discharge of water across the settling interface. This model shows a limiting case in which an intraslope basin captures 100% of the sediment from a ponded turbidity current, even through a succession of sustained flow events, until sediment deposition raises the settling interface above the downstream lip of the minibasin. This same process defines one of the mechanisms for minibasin filling in nature, and, when this mechanism is operative, the trap efficiency of sediment can be expected to be high until the minibasin is substantially filled with sediment.

Quantifying velocity and turbulence structure in depositing sustained turbidity currents across breaks in slope

AbstractTwo‐dimensional experiments investigating sediment transport and turbulence structure in sustained turbidity currents that cross breaks in slope are presented as analogue illustrations for natural flows. The results suggest that in natural flows, turbulence generation at slope breaks may account for increased sand transport into basins and that the formation of a hydraulic jump may not be necessary to explain features such as the occurrence of submarine plunge pools and the deposition of coarser‐grained beds in the bottomsets of Gilbert‐type fan deltas. Experimental flows were generated on 0°, 3°, 6° and 9° slopes of equal length which terminated abruptly on a horizontal bed. Two‐component velocities were measured on the slope, at the slope break and downstream of the slope break. Flows were depositional and non‐uniform, visibly slowing and thickening with distance downstream. One‐dimensional continuous wavelet transforms of velocity time series were used to produce time‐period variance maps. Peaks in variance were tested against a background red‐noise spectrum at the 95% level; a significant period banding occurs in the cross‐wavelet transform at the slope break, attributed to increased formation of coherent flow structures (Kelvin–Helmholtz billows). Variance becomes distributed at progressively longer periods and the shape of the bed‐normal‐velocity spectral energy distribution changes with distance downstream. This is attributed to a shift towards larger turbulent structures caused by wake stretching. Mean velocity, Reynolds shear stress and turbulent kinetic energy profiles illustrate the mean distribution of turbulence through the currents. A turbulent kinetic energy transfer balance shows that flow non‐uniformity arises through the transfer of mean streamwise slowing to mean bed‐normal motion through the action of Reynolds normal stresses. Net turbulence production through the action of normal stresses is achieved on steeper slopes as turbulence dissipation due to mean bed‐normal motion is limited. At the slope break, an imbalance between the production and dissipation of turbulence occurs because of the contrasting nature of the wall and free‐shear boundaries at the bottom and top of the flows, respectively. A rapid reduction in mean streamwise velocity predominately affects the base of the flows and steeper proximal slope flows have to slow more at the break in slope. The increased turbulent kinetic energy, limited bed‐normal motion and strong mixing imposed by steep proximal slopes means rapid slowing enhances turbulence production at the break in slope by focusing energy into coherent flow structures at a characteristic period. Thus, mean streamwise slowing is transferred into turbulence production at the slope break that causes increased transport of sediment and a decrease in deposit mass downstream of the slope break. The internal effects of flow non‐uniformity therefore can be separated from the external influence of the slope break.

Parallel Lamination Formed by High-Density Turbidity Currents

Experiments were conducted in order to investigate processes of sediment transport and deposition from sustained turbidity currents with initial volume-fraction sediment concentration of 0.20, 0.25, and 0.35. Parallel lamination was formed at bed-aggradation rates up to 4 mm s-1 and bedload-layer sediment concentration up to 0.36. This observation contradicts the generally held assumption that parallel lamination forms only from low-density flows and at low bed-aggradation rates. This misconception illustrates a poor understanding of threshold values of grain-support mechanisms in the bedload layer. The present results bring new insight into understanding sediment transport in the deep sea and interpreting the sedimentary record.

Breaching in fine sands and the generation of sustained turbidity currents in submarine canyons

Abstract Natural, moderately loosely packed sands can only erode from the surface of the bed after an increase in pore volume. Because of this shear dilatancy, negative pore pressures are generated in the bed. In cases of low permeability, these negative pressures are released relatively slowly, which retards the maximum rate of erosion. This effect is incorporated in a new, analytically derived, pick‐up function that can explain the observation of gradual retrogressive failure of very steep subaqueous slopes, sometimes more than 5 m high, in fine non‐cohesive sands. This process, termed ‘breaching’ in the field of sediment dredging, may produce large failures in sand bars or river banks. The analytical function that describes the breaching process in fine sand is incorporated in a one‐dimensional, steady‐state numerical model of turbidity currents describing the spatial development of flow. This model is applied to simulate a large ‘flushing’ event in Scripps Submarine Canyon, Pacific coast of California. Breach retrogradation and the successive evolution in time of the resulting turbidity current in the canyon are predicted in a sequence of discrete steps. Predicted velocities are compared with values measured during a flushing event. Implications for the interpretation of deep‐water massive sands are discussed.

Anatomy of shelf deltas at the edge of a prograding Eocene shelf margin, Spitsbergen

Abstract Although shelf‐edge deltas are well‐imaged seismic features of Holocene and Pleistocene shelf margins, documented outcrop analogues of these important sand‐prone reservoirs are rare. The facies and stratigraphic architecture of an outcropping shelf‐edge delta system in the Eocene Battfjellet Formation, Spitsbergen, is presented here, as well as the implications of this delta system for the generation of sand‐prone, shelf‐margin clinoforms. The shelf‐edge deltas of the Battfjellet Formation on Litledalsfjellet and Høgsnyta produced a 3–5 × 15 km, shelf edge‐attached, slope apron (70 m of sandstones proximally, tapering to zero on the lower slope). The slope apron consists of distributary channel and mouth‐bar deposits in its shelf‐edge reaches, passing downslope to slope channels/chutes that fed turbiditic lobes and spillover sheets. In the transgressive phase of the slope apron, estuaries developed at the shelf edge, and these also produced minor lobes on the slope. The short‐headed mountainous rivers that drained the adjacent orogenic belt and fed the narrow shelf, and the shelf‐edge position of the discharging deltas, made an appropriate setting for the generation of hyperpycnal turbidity currents on the slope of the shelf margin. The abundance of organic matter and of coal fragments in the slope turbidites is consistent with this notion. Evidence that many of the slope turbidites were generated by sustained turbidity currents that waxed then waned includes the presence of scour surfaces and thick intervals of plane‐parallel laminae within turbidite beds in the slope channels, and thick spillover lobes with repetitive alternations of massive and flat‐laminated intervals. The examined shelf‐edge to slope system, now preserved mainly below the shelf break and dominated by sediment gravity‐flow deposits, has a threefold stratigraphic architecture: a lower, progradational part, in which the clinoforms have a slight downward‐directed trajectory; a thin aggradational zone; and an upper part in which clinoforms backstep up onto the shelf edge. A greatly increased density of erosional channels and chutes marks the regressive‐to‐transgressive turnaround within the slope apron, and this zone becomes an angular unconformity up near the shelf edge. This unconformity, with both subaerial and subaqueous components, is interpreted as a sequence boundary and developed by vigorous sand delivery and bypass across the shelf edge during the time interval of falling relative sea level. The studied shelf‐margin clinoforms accreted mostly during falling stage (sea level below the shelf edge), but the outer shelf later became estuarine as sea level became re‐established above the shelf edge.