Supercritical and subcritical turbidity currents and their deposits--A synthesis

Postma, George; Cartigny, Matthieu

doi:10.1130/g35957.1

Cited by 140 publications

(122 citation statements)

References 44 publications

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“…The density flows triggered at delta fronts can be either stratified or non‐stratified. If they are stratified, the basal layer can then be either supercritical or subcritical, which will affect the type of bedform created (Postma & Cartigny, ). In general, density flows will be supercritical on slopes steeper than 0·6° (Komar, ; Hand, ; Sequeiros, ) or 0·07° (Fricke et al ., ), which is well below the mean slopes of 5° to 10° in the fjord‐lakes presented here.…”

Section: Discussioncontrasting

confidence: 55%

“…The possibility of a density flow becoming supercritical mostly depends on slope and flow stratification (Postma & Cartigny, ). The density flows triggered at delta fronts can be either stratified or non‐stratified.…”

Section: Discussionmentioning

confidence: 99%

“…Therefore, the slopes observed on all four fjord‐lake deltas are sufficiently steep to produce supercritical flows. However, the flows could be subcritical if the discharge is limited, which would reduce momentum to maintain a supercritical basal layer (Postma & Cartigny, ). These subcritical density flows are most common at the base of steep delta slopes or at channel‐lobe transitions (Postma & Cartigny, ), where there is a strong loss of confinement and where the slopes are gentle.…”

Section: Discussionmentioning

confidence: 99%

“…However, the flows could be subcritical if the discharge is limited, which would reduce momentum to maintain a supercritical basal layer (Postma & Cartigny, ). These subcritical density flows are most common at the base of steep delta slopes or at channel‐lobe transitions (Postma & Cartigny, ), where there is a strong loss of confinement and where the slopes are gentle. The bedforms studied here, which are located in the upper delta slope, are thus less likely to be formed by subcritical flows and suggest a supercritical origin.…”

Section: Discussionmentioning

confidence: 99%

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Morphological expression of bedforms formed by supercritical sediment density flows on four fjord‐lake deltas of the south‐eastern Canadian Shield (Eastern Canada)

et al. 2016

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High‐resolution swath bathymetry data collected in fjord‐lakes Pentecôte, Walker and Pasteur (eastern Québec, Canada) allowed imaging in great detail the deltas of four rivers in order to understand the factors controlling the formation and downslope evolution of bedforms present on their slopes. The morphometry and morphology of 199 bedforms reflect the behaviour of sediment density flows. The shape of the bedforms, mostly crescentic, and the relationships between their morphological properties indicate that they were formed by supercritical density flows and that they are cyclic steps. The crescentic shape suggests an upslope migration while the aspect ratios and increasing wavelengths with distance from the shore (and decreasing slopes) are compatible with a cyclic step origin. At the rollover point, the acceleration of the density flows on steep slopes produces tightly spaced hydraulic jumps and favours short wavelength and symmetrical bedforms. Further downslope, decreasing slopes and increasing specific discharge increase the wavelength and asymmetry of the bedforms. The wavelength and asymmetry are increased because density flows require longer distances to become supercritical again on lower slopes after each successive hydraulic jump. Bedform morphometry and morphology are used to reconstruct density flow behaviour downslope. Froude numbers are high near the rollover point and gradually decrease downslope as the slope becomes gentler. Conversely, the specific discharge and flow depth are low near the rollover point and gradually increase downslope as the flow either erodes sediments or becomes more dilute due to sediment deposition and water entrainment. The supercritical density flows are believed to be triggered mainly by hyperpycnal flows but some evidence of delta‐front slope failures is also observed. The differences in delta morphology and bedform development between the four deltas are linked to basin morphology and watershed hydrology, but also mainly to the fjord heritage of the lakes that allowed the focusing of sediment at the delta front.

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Section: Discussioncontrasting

confidence: 55%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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Morphological expression of bedforms formed by supercritical sediment density flows on four fjord‐lake deltas of the south‐eastern Canadian Shield (Eastern Canada)

et al. 2016

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“…(1) associated with an overbank erosion/ bypass surface (2). Backsets are thought to develop under flow conditions approaching, at, or above Froude unity, that is, supercritical flow, or by a flow passing through a hydraulic jump (Jopling and Richardson, 1966;Skipper, 1971;Schmincke et al, 1973;Skipper and Bhattacharjee, 1978;Postma and Cartigny, 2014). (B) The scours trapped and preserved some of the sediment of the dominantly bypassing flow.…”

Section: Interpretation Of Overbank Bedformsmentioning

confidence: 99%

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

McArthur

Kane²,

Bozetti

et al. 2019

The Depositional Record

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Overbank 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 coarsergrained 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. K E Y W O R D SBaja California, deep-marine, Rosario Formation, slope channels, turbidity currents, Upper Cretaceous |McARTHUR eT Al.

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Sedimentological regimes for turbidity currents: Depth‐averaged theory

2017

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Turbidity currents are one of the most significant means by which sediment is moved from the continents into the deep ocean; their properties are interesting both as elements of the global sediment cycle and due to their role in contributing to the formation of deep water oil and gas reservoirs. One of the simplest models of the dynamics of turbidity current flow was introduced three decades ago, and is based on depth‐averaging of the fluid mechanical equations governing the turbulent gravity‐driven flow of relatively dilute turbidity currents. We examine the sedimentological regimes of a simplified version of this model, focusing on the role of the Richardson number Ri [dimensionless inertia] and Rouse number Ro [dimensionless sedimentation velocity] in determining whether a current is net depositional or net erosional. We find that for large Rouse numbers, the currents are strongly net depositional due to the disappearance of local equilibria between erosion and deposition. At lower Rouse numbers, the Richardson number also plays a role in determining the degree of erosion versus deposition. The currents become more erosive at lower values of the product Ro × Ri, due to the effect of clear water entrainment. At higher values of this product, the turbulence becomes insufficient to maintain the sediment in suspension, as first pointed out by Knapp and Bagnold. We speculate on the potential for two‐layer solutions in this insufficiently turbulent regime, which would comprise substantial bedload flow with an overlying turbidity current.

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Supercritical and subcritical turbidity currents and their deposits--A synthesis

Cited by 140 publications

References 44 publications

Morphological expression of bedforms formed by supercritical sediment density flows on four fjord‐lake deltas of the south‐eastern Canadian Shield (Eastern Canada)

Morphological expression of bedforms formed by supercritical sediment density flows on four fjord‐lake deltas of the south‐eastern Canadian Shield (Eastern Canada)

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

Sedimentological regimes for turbidity currents: Depth‐averaged theory

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