Turbidity current

About: Turbidity current is a research topic. Over the lifetime, 3236 publications have been published within this topic receiving 125429 citations.

Sediment Gravity Flows: II Depositional Models with Special Reference to the Deposits of High-Density Turbidity Currents

Abstract: Four principal mechanisms of deposition are effective in the formation of sediment gravity flow deposits. Grains deposited by traction sedimentation and suspension sedimentation respond individually and accumulate directly from bed and suspended loads, respectively. Those deposited by frictional freezing and cohesive freezing interact through either frictional contact or cohesive forces, respectively, and are deposited collectively, usually by plug formation. Sediment deposition from individual sediment flows commonly involves more than one of these mechanisms acting either serially as the flow evolves or simultaneously on different grain populations. Deposition from turbidity currents is treated in terms of three dynamic grain populations: 1) clay- to medium-grained sand-sized particles that can be fully suspended as individual grains by flow turbulence, 2) coarse-grained sand to small-pebble-sized gravel that can be fully suspended in large amounts mainly in highly concentrated turbulent suspensions where grain fall velocity is substantially reduced by hindered settling, and 3) pebble- and cobble-sized clasts having concentrations greater than 10 percent to 15 percent that will be supported largely by dispersive pressure resulting from clast collisions and by buoyant lift provided by the interstitial mixture of water and finer-grained sediment. The effects of hindered settling, dispersive pressure, and matrix buoyant lift are con entration dependent, and grain populations 2 and 3 are likely to be transported in large amounts only within flows having high particle concentrations, probably in excess of 20 percent solids by volume. Low-density turbidity currents, made up largely of grains of population 1, typically show an initial period of traction sedimentation, forming Bouma (Tb) and Tc) divisions, followed by one of mixed traction and suspension sedimentation (Td), and a terminal period of fine-grained suspension sedimentation (Te). The sediment loads of high-density turbidity currents commonly include grains belonging to populations 1, 2, and 3. Consequently, deposition often occurs as a series of discrete sedimentation waves as flows decelerate and individual grain populations can no longer be maintained in transport. Each sedimentation wave tends to show increasing unsteadiness and accelerating sedimentation rate as it evolves, passing from an initial stage of traction sedimentation, to one of mixed frictional freezing and suspension sedimentation within traction carpets, to a final stage of direct suspension sedimentation. Sequences of sedimentary structure divisions representing this succession of depositional stages are here termed the ecoR1-3) sequence, representing population 3 grains, and the S1-3) sequence, representing population 2. Deposition of the high-density suspended load leaves behind a residual low-density turbidity current composed largely of population 1 grains. At their distal ends, high-density turbidity currents deposit mainly by suspension sedimentation, forming thin (S3) divisions. These (S3) divisions are the same as Bouma (Ta) and, if subsequently capped by (Tb-e) deposited by the residual low-density flows, become the basal divisions of normal turbidities. Liquefied flows deposit by direct high-density suspension sedimentation. Grain flows of sand are characterized by frictional freezing and their deposits are limited mainly to angle-of-repose slipface units. Density-modified grain flows, in which larger clasts are partially supported by matrix buoyancy, and traction carpets, in which a dense frictional grain dispersion is driven by an overlying turbulent flow, are important in the buildup of natural deposits on submarine slopes. Cohesive debris flows depost sediment mainly by cohesive freezing, commonly modified by suspension sedimentation of the largest clasts.

The physical character of subaqueous sedimentary density flows and their deposits

Abstract: The complexity of flow and wide variety of depositional processes operating in subaqueous density flows, combined with post-depositional consolidation and soft-sediment deformation, often make it difficult to interpret the characteristics of the original flow from the sedimentary record. This has led to considerable confusion of nomenclature in the literature. This paper attempts to clarify this situation by presenting a simple classification of sedimentary density flows, based on physical flow properties and grain-support mechanisms, and briefly discusses the likely characteristics of the deposited sediments. Cohesive flows are commonly referred to as debris flows and mud flows and defined on the basis of sediment characteristics. The boundary between cohesive and non-cohesive density flows (frictional flows) is poorly constrained, but dimensionless numbers may be of use to define flow thresholds. Frictional flows include a continuous series from sediment slides to turbidity currents. Subdivision of these flows is made on the basis of the dominant particle-support mechanisms, which include matrix strength (in cohesive flows), buoyancy, pore pressure, grain-to-grain interaction (causing dispersive pressure), Reynolds stresses (turbulence) and bed support (particles moved on the stationary bed). The dominant particle-support mechanism depends upon flow conditions, particle concentration, grain-size distribution and particle type. In hyperconcentrated density flows, very high sediment concentrations (>25 volume%) make particle interactions of major importance. The difference between hyperconcentrated density flows and cohesive flows is that the former are friction dominated. With decreasing sediment concentration, vertical particle sorting can result from differential settling, and flows in which this can occur are termed concentrated density flows. The boundary between hyperconcentrated and concentrated density flows is defined by a change in particle behaviour, such that denser or larger grains are no longer fully supported by grain interaction, thus allowing coarse-grain tail (or dense-grain tail) normal grading. The concentration at which this change occurs depends on particle size, sorting, composition and relative density, so that a single threshold concentration cannot be defined. Concentrated density flows may be highly erosive and subsequently deposit complete or incomplete Lowe and Bouma sequences. Conversely, hydroplaning at the base of debris flows, and possibly also in some hyperconcentrated flows, may reduce the fluid drag, thus allowing high flow velocities while preventing large-scale erosion. Flows with concentrations <9% by volume are true turbidity flows (sensuBagnold, 1962), in which fluid turbulence is the main particle-support mechanism. Turbidity flows and concentrated density flows can be subdivided on the basis of flow duration into instantaneous surges, longer duration surge-like flows and quasi-steady currents. Flow duration is shown to control the nature of the resulting deposits. Surge-like turbidity currents tend to produce classical Bouma sequences, whose nature at any one site depends on factors such as flow size, sediment type and proximity to source. In contrast, quasi-steady turbidity currents, generated by hyperpycnal river effluent, can deposit coarsening-up units capped by fining-up units (because of waxing and waning conditions respectively) and may also include thick units of uniform character (resulting from prolonged periods of near-steady conditions). Any flow type may progressively change character along the transport path, with transformation primarily resulting from reductions in sediment concentration through progressive entrainment of surrounding fluid and/or sediment deposition. The rate of fluid entrainment, and consequently flow transformation, is dependent on factors including slope gradient, lateral confinement, bed roughness, flow thickness and water depth. Flows with high and low sediment concentrations may co-exist in one transport event because of downflow transformations, flow stratification or shear layer development of the mixing interface with the overlying water (mixing cloud formation). Deposits of an individual flow event at one site may therefore form from a succession of different flow types, and this introduces considerable complexity into classifying the flow event or component flow types from the deposits.

Grain Size Distributions and Depositional Processes

Abstract: Extensive textural study of both modern and ancient sands has provided the basis for a genetic interpretation of sand texture. Analysis is based on recognizing sub-populations within individual log-normal grain size distributions. Each log-normal sub-population may be related to a different mode of sediment transport and deposition, thus providing a measure of their importance in the genesis of a sand unit. The three modes of transport reflected are: (1) suspension; (2) saltation; and (3) surface creep or rolling. Each of these is developed as a separate sub-population within a grain size distribution. The number, amount, size-range, mixing, and sorting of these populations vary systematically in relation to provenance, sedimentary process, and sedimentary dynamics. The analysis of th se parameters is the basis for determining the process-response characteristics of individual sand units. A number of processes are uniquely reflected in log-probability curves of grain size distributions of sands and sandstones. These include: (1) current; (2) swash and backwash; (3) wave; (4) tidal channel; (5) fallout from suspension; (6) turbidity current; and (7) aeolian dune. The combination of two or more of these processes also produce characteristic log-probability curve shapes. Ancient sands show some differences from their modern analogues, but these are usually minor. Log-probability plots of ancient sands are directly comparable to those from modern sands. The principal limitation of this study is in comparing sands formed under comparable conditions and obtaining an independent determination of the processes of formation of ancient sands.

Turbidity Currents Generated at River Mouths during Exceptional Discharges to the World Oceans

Abstract: A marine hyperpycnal plume is a particular kind of turbidity current occurring at a river mouth when the concentration of suspended sediment is so large that the density of the river water is greater than the density of sea water. The plume can then plunge and possibly erode the seafloor to become self-maintained for a particular period of time (hours to weeks). Frequency of hyperpycnal plumes emanating from river discharge can be predicted with knowledge of rating curve characteristics, particularly during flood conditions. Examples of these curves are shown for middle-sized North American rivers. Semi-empirical relationships among average discharge, average sediment concentration, and the discharge during flood are proposed and applied to 150 world rivers. Results show the importance of small and medium sized rivers in their ability to trigger underflow at their mouth. There are at least nine "dirty" rivers that may trigger underflows during one or more periods of the year. Most other rivers are cleaner...

Subaqueous sediment density flows: Depositional processes and deposit types

Abstract: Submarine sediment density flows are one of the most important processes for moving sediment across our planet, yet they are extremely difficult to monitor directly. The speed of long run-out submarine density flows has been measured directly in just five locations worldwide and their sediment concentration has never been measured directly. The only record of most density flows is their sediment deposit. This article summarizes the processes by which density flows deposit sediment and proposes a new single classification for the resulting types of deposit. Colloidal properties of fine cohesive mud ensure that mud deposition is complex, and large volumes of mud can sometimes pond or drain-back for long distances into basinal lows. Deposition of ungraded mud (TE-3) most probably finally results from en masse consolidation in relatively thin and dense flows, although initial size sorting of mud indicates earlier stages of dilute and expanded flow. Graded mud (TE-2) and finely laminated mud (TE-1) most probably result from floc settling at lower mud concentrations. Grain-size breaks beneath mud intervals are commonplace, and record bypass of intermediate grain sizes due to colloidal mud behaviour. Planar-laminated (TD) and ripple cross-laminated (TC) non-cohesive silt or fine sand is deposited by dilute flow, and the external deposit shape is consistent with previous models of spatial decelerating (dissipative) dilute flow. A grain-size break beneath the ripple cross-laminated (TC) interval is common, and records a period of sediment reworking (sometimes into dunes) or bypass. Finely planar-laminated sand can be deposited by low-amplitude bed waves in dilute flow (TB-1), but it is most likely to be deposited mainly by high-concentration near-bed layers beneath high-density flows (TB-2). More widely spaced planar lamination (TB-3) occurs beneath massive clean sand (TA), and is also formed by high-density turbidity currents. High-density turbidite deposits (TA, TB-2 and TB-3) have a tabular shape consistent with hindered settling, and are typically overlain by a more extensive drape of low-density turbidite (TD and TC,). This core and drape shape suggests that events sometimes comprise two distinct flow components. Massive clean sand is less commonly deposited en masse by liquefied debris flow (DCS), in which case the clean sand is ungraded or has a patchy grain-size texture. Clean-sand debrites can extend for several tens of kilometres before pinching out abruptly. Up-current transitions suggest that clean-sand debris flows sometimes form via transformation from high-density turbidity currents. Cohesive debris flows can deposit three types of ungraded muddy sand that may contain clasts. Thick cohesive debrites tend to occur in more proximal settings and extend from an initial slope failure. Thinner and highly mobile low-strength cohesive debris flows produce extensive deposits restricted to distal areas. These low-strength debris flows may contain clasts and travel long distances (DM-2), or result from more local flow transformation due to turbulence damping by cohesive mud (DM-1). Mapping of individual flow deposits (beds) emphasizes how a single event can contain several flow types, with transformations between flow types. Flow transformation may be from dilute to dense flow, as well as from dense to dilute flow. Flow state, deposit type and flow transformation are strongly dependent on the volume fraction of cohesive fine mud within a flow. Recent field observations show significant deviations from previous widely cited models, and many hypotheses linking flow type to deposit type are poorly tested. There is much still to learn about these remarkable flows.