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SEDIMENTARY PROCESSES Particle-Driven Subaqueous Gravity Processes M Felix and W McCaffrey, University of Leeds, Leeds, UK ? 2005, Elsevier Ltd. All Rights Reserved. Introduction Particulate subaqueous gravity flows are sediment-water mixtures that move as a result of gravity acting on the sediment-induced density excess compared with the ambient water. The mixtures can range from densely-packed sediment flows, essentially submarine landslides, to very dilute flows carrying only a few kg/m³ of sediment. Gravity flow can take place in lakes and oceans but some dense flows also occur in rivers. Sediment volumes transported by individual events can range up to thousands of cubic kilometres although most events are much smaller. Due to their infrequent occurrence and destructive nature, much information about subaqueous gravity processes comes from the study of their deposits and laboratory experiments. Flow initiation mechanisms, sediment transport mechanisms, and flow types are described separately to emphasize the sense of process continuum needed to appreciate the development of most natural subaqueous gravity flows. This is followed by a description of internal and external influences on flow behaviour. Finally, the influence of flow regime on individual deposits is outlined. River outflow is less than that of the ocean and turbid surface plumes are generated. Nevertheless, particulate gravity flows can also form from surface plumes if material settling out collects near the bed at high enough concentrations to begin moving. A similar effect results from flow generated by glacial plumes where sediment is slowly released into the water body. Where the interstitial fluid in a hyperpycnal plume is of lower density than that of the ambient fluid, as is the case when freshwater rivers flow into brackish or fully saline bodies of water, ongoing sedimentation may induce buoyancy reversal. Thus, the gravity current will loft, similar to some subaerial pyroclastic density flows and the flow will essentially cease to travel forwards resulting in the development of abrupt deposit margins. Sediment Resuspension Loose sediment on the seafloor can be resuspended if bed shear stress is high enough. This can occur during storms or passage of flows caused by density differences as a result of temperature or salinity. The resulting suspended sediment concentrations can be high enough to allow the mixtures to flow under gravity influence. As in case of river-derived flows, resuspension usually generates initially dilute currents. Slope Failure Flow Initiation Mechanisms A variety of processes can generate subaqueous gravity currents with varying initial concentrations. Direct Formation From Rivers Currents can be formed when turbid river water flows into bodies of standing water such as lakes or oceans. If the bulk density of the turbid river water (sediment plus interstitial fluid) is higher than that of the receiving body of water, the river outflow will plunge travelling along the bed as a hyperpycnal flow (or plume) beneath ambient water. Such sediment-laden underflows may mix with ambient water and transport sediment oceanward as particulate gravity currents. Although sometimes these river-derived flows are of high concentration e.g., Yellow River hyperpycnal plume, mostly they are dilute. Direct formation of subaqueous gravity currents in this way is however the exception rather than the rule. Flows of much higher concentration may form as a result of slope failure. Sediment on submarine slopes can become unstable due to oversteepening during ongoing sedimentation and during sealevel falls, high inherited pore fluid pressures and gas hydrate exsolution. Slope failure can alternatively be triggered by externally applied stresses due to earthquakes or loading induced by internal waves in the water column above which chiefly occur in oceans. Initially, the failing mass becomes unstable along a plane of instability and a whole segment of the slope starts moving. Retrogressive failure and breaching can continue adding material following initial loss of stability. The concentration of this mass is at packing density but can become more dilute as flow continues. Terrestrial Input Not all subaqueous gravity flows need originate underwater. Landslides, pyroclastic flows, and aeolian sediment transport originating on land can enter lakes or oceans and continue flowing underwater if rates of mass flux are sufficiently high. Grain Transport Mechanisms Matrix Strength and Particle-Particle Interactions Within dense flows grains can be prevented from settling as a result of matrix strength. This strength may arise if some or all particles are cohesive resulting in a cohesive matrix that prevents both cohesive and non-cohesive particles from settling out. In addition, particles can be supported by matrix strength within flows of non-cohesive grains if the particles are in semi-permanent contact as is the case for flows whose densities are close to static loose-packed sediment. For slightly lower concentrations inter-particle collisions will help keep particles in suspension. Hindered Settling and Buoyancy Settling of particles can be slowed down by water displaced upwards by other settling particles. Such hindered settling is especially effective in dense mixtures with a range of grain sizes so that smaller particles are slowed down by settling of larger particles. The presence of smaller particles also increases the effective density of the fluid that the particles are settling in and thus enhances buoyancy of suspended particles reducing settling rates. Turbulence The motion of sediment-laden flows can generate turbulence through shear at the bed, internally in the flow or at top of dense layer. Turbulent bursts generated at the bed tend to have an asymmetrical vertical velocity structure with slower downward sweeps and more rapid upward bursts. This turbulence pattern counteracts downwards settling of particles moving them higher up in the flow. However, turbulence generation is hindered and dissipation increased if particle concentration is high or if flow is very cohesive or highly stratified. Flow Types Broadly speaking flows can be divided into three main types depending on density: Dense Relatively Undeformed Flows Creeps Slides and Slumps Flows of this type essentially have the same density as pre-failure material. In each case sediment moves as one large coherent mass with varying amounts of internal deformation. Grains remain in contact during flow thus matrix strength is the main sediment transport mechanism. Such flows will stop moving or shear stress becomes too low to overcome friction at which point entire mass comes to rest. Flow thickness and deposit thickness are essentially same although flows may thicken via internal thrusting or ductile deformation as they decelerate prior to arrest. Slope creep caused by gravity moves beds slowly downslope with gentle internal deformation of original depositional structure. Slides undergo little or no pervasive internal deformation while slumps undergo partial deformation but original internal structure is still recognisable in separate blocks. Thicknesses of slides and slumps range from several tens of metres to 1–2 km travel distances can be up to about 100 km with displaced volumes of up to 10¹² m³ although most flows are considerably smaller. Dense Deformed Flows Rockfalls Grain Flows Debris Flows and Mudflows In flows of this type sediment still moves as one coherent mass but concentrations can be lower and mass is generally well mixed with little or no preservation of remnant structure from original failed material. Sediment support mechanisms are matrix strength buoyancy hindered settling and grain-grain collisions. Rheologically such flows are plastic i.e., they have a yield strength. Clast types generally range from purely cohesive in mudflows to cohesive and non-cohesive in debris flows (Figure 2) and purely non-cohesive for grain flows and rockfalls where movement is by freefall on very steep slopes. These types of flow are formed asa result of rapid internal deformation following slope failure high concentration river input or reconcentration of dilute flows described below. Flow and deposit thicknesses can be up to several tens of metres with travel distances of several hundreds of kilometres. Erosion can add material to the flow thus extend both travel distances and size of deposit neither which necessarily relate to initial flow mass. Motion will stop once friction is too high and flows will generally deposit en masse. Debris flows may develop a rigid plug of material at top of flow where applied stress falls below yield strength such flows move along basal zone of deformation and may progressively freeze from top downwards ultimately coming to rest when freezing interface reaches substrate. (Partially) Dilute Flows Turbidity Currents In flows of this type sediment does not move Ключевые слова: increase, refrigerator door, range, material, homogeneous flattening, mare crisium, beta regio, frozen profile, red clay, geophysical, surface, fluid, ameen, liou, press, vice versa, middleton, velocity, san diego, themis regiones, ridge, correlative conformity, early, scale, time, irish midlands, image, society, lambiase, water, bed, gillot, palgrave macmillan, global, plate, wa, layer, skinner, biotic events, carbonate, earth, environment, payton, hancock, university, ha, al-jabbari al-ansari, structure, norwegian caledonides, troodos ophiolite, high, phillips, impact, fault, rocks, lithospheric transferal, clausen, interstitial porewaters, millers tale, sequence, formed, von huene, rawson, size, tectonicsmid-ocean ridges, apectodinium acme, sediment, process, coates, model, mackenzie, accretionary wedges, geochemical thermometer, science, mare nubium, form, earthquake, maund, frostick, mare tranquillitatis, change, rift, major, ma, piper djw, volcanic, von damm, york, significant recrystallization, behaves mechanically, deposition, tectonicsrift valleys, record, wolfe, period, fracture, stow dav, crustal, hansen, depth, mare orientale, decker, deposit, geophysicalgeological transect, london, bolivian altiplano, seasonal frost, wold cottage, solar, fossil, mid-ocean ridges, journal, moon, rate, schopf, le, small, trace, trends rhythm, north, field, region, boundary, table, temperature, summerhayes, clark-lowes, nriagu, los angeles, continental, sea, data, thetis regiones, naar, year, formation, macedon, dune leeside, eocene-oligocene climatic, crane, jones, hydaspis chaos, zone, boudreau, pattrick rad, british isles, space, weathering, area, coinciding, result, occur, von stackelberg, stress, mineral, crust, costa rica, international commission, tectonic, tectonics, activity, kun lun, level, geological, ocean, seismic, polya, mulder efj, rock, alpe arami, saxonian erzgebirge, hydrothermal, iron, flow, sedimentary, mn ni, updated regularly, san francisco, basin, plain, marine, geology, pressure, event, first-second-, deep, unit, type, atterberg limits, feature, zuber, caue itabirite, lawton, condition, age, term, challenger reports, nature, clay, refractor, particle, dolomite, thirlwall, sun, south, stratigraphic, indian oceans, wave-dominated shoreline, discoaster pentaradiatus, soil, international union, large, walker, hughes, diagenesis, shear