Turbidity Current

A turbidity current is a kind of density current that flows downslope along the bottom of an ocean or lake because of density contrasts with the surrounding (ambient) water arising from sediment that becomes suspended in the water owing to turbulence. ^[1] The suspended sediment makes the turbidity current denser than the clear water above and around it - gravity then pulls this denser mixture downslope, and the resulting flow can pick up more sediment as it goes, further enhancing its density and velocity.

Turbidity currents can be generated experimentally in the laboratory by the sudden release of muddy, dense water into the end of a sloping flume filled with less dense, clear water. ^[1] They have been observed to occur under natural conditions in lakes where muddy river water enters the lakes, and they are believed to have occurred throughout geologic time in the marine environment on continental margins. ^[1] In this marine setting, turbidity currents originate particularly in or near the heads of submarine canyons. ^[1]

Generation Mechanisms

Turbidity currents can be generated by a variety of mechanisms, including sediment failure, storm-triggered flow of sand and mud into canyon heads, bedload inflow from rivers and glacial meltwater, and flows during eruption of airfall ash. ^[1] They may move as surges or as steady, uniform flows. ^[1]

Surge-Type Turbidity Currents

Surges, or spasmodic turbidity currents, are initiated by some short-lived catastrophic event, such as earthquake-triggered massive sediment slumping or storm waves acting on a continental shelf. ^[1] Such an event creates intense turbulence in the water overlying the seafloor, resulting in extensive erosion and entrainment of sediment that is rapidly thrown into suspension. ^[1] The sediment then remains suspended, supported in the water column by turbulence, generating a dense, turbid cloud that moves downslope, eroding and picking up more sediment as it increases in speed. ^[1]

Surge flows develop into three main parts as they move away from the source: the head, body, and tail. ^[1]

Head

The head of the surge is about twice as thick as the rest of the flow and is characterised by intense turbulence. ^[1] The head is overhanging and is divided transversely into lobes and clefts. ^[1] The velocity U_head at which the head advances into still water is given by: ^[1]

U_head = 0.7√(Δρ/ρ · gh)

where Δρ is the density contrast between the turbidity current and the ambient water, ρ is the density of the ambient water, g is gravitational acceleration, and h is the height of the head. ^[1]

Body

Flow within the body of a surge-type turbidity current is nearly steady and uniform, and the flow is almost uniform in thickness. ^[1] The body moves at a velocity U_body that is faster in deep water than that of the head. ^[1] This difference in velocity causes the forward part of the body to consume itself within the head in the process of mixing with the ambient water. ^[1]

The tail of the flow thins abruptly away from the body and becomes more dilute. ^[1]

Steady, Uniform Turbidity Currents

Some turbidity currents are steady, uniform flows that lack a turbulent head. ^[1] These flows move at velocities similar to those of the body of surge-type flows; although the velocity is sensitive to the slope over which flow takes place, flow may occur on slopes as low as 1 degree. ^[1] Steady, uniform flows have been observed along the sloping bottom of lakes where sediment-laden rivers run into the lakes. ^[1]

Deposition and Autosuspension

Once sediment is suspended in a turbidity current, the current continues to flow for some time under the action of gravity and inertia. ^[1] Flow will stop when the sediment-water mixture that produces the density contrast with the ambient water is exhausted by settling of the suspended load. ^[1]

Theoretically, sediment remaining in suspension after initial deposition of coarse material in the proximal area can, during further transport, be maintained in suspension for a very long time in a state of dynamic equilibrium called autosuspension. ^[1] A condition of autosuspension is presumably maintained because turbulence continues to be generated in the bottom of the flow owing to gravity-generated downslope flow of the turbidity current over the bed - loss of energy by friction of the flow with the bottom is compensated for by gravitational energy. ^[1]

A turbidity current triggered by the 1929 Grand Banks earthquake off Nova Scotia appears to have traveled south across the floor of the Atlantic for a distance of more than 300 km at velocities up to 67 km/hr (19 m/s), as timed by breaks in submarine telegraph cables. ^[1] Transport of sediment over this distance suggests that autosuspension may actually work; nonetheless, some geologists remain sceptical of the autosuspension process. ^[1]

Deceleration and Deposit Formation

The velocity of a turbidity current eventually diminishes owing to flattening of the canyon slope, overbank flow of the current along a submarine channel, or spreading of the flow over the flat ocean floor at the base of the slope. ^[1] As the flow slows, turbulence generated along the sole of the flow also diminishes, and the current gradually becomes more dilute owing to mixing with ambient water around the head and along the upper interface. ^[1] The remaining sediment carried in the head eventually settles out, causing the head to sink and dissipate. ^[1]

Rapid deposition of coarser particles from suspension appears to occur in regions near the source owing to early decay of extremely intense turbulence generated by the initial event. ^[1] This proximal-to-distal grading of grain size in turbidites - coarse sand and gravel close to source, fine sand and mud far from source - is the fundamental basis for interpreting turbidite successions in the rock record.

Deposition Mechanics Within the Flow

The precise process by which material falls out from different parts of a turbidity current is still not thoroughly understood, although experimental evidence makes clear that deposition does not occur simultaneously across the whole flow. ^[1] This means the head, body, and tail are not simply different zones of the same process happening in parallel - they interact and influence one another in ways that produce very different sedimentary outcomes depending on position.

At any given moment, the head may actually be eroding the seafloor while the body directly behind it is simultaneously laying down sediment. ^[1] Material that drops out rapidly from energetic zones such as the head is buried so quickly that it has little or no chance to be reworked by traction forces before the overlying sediment seals it in place. ^[1] In contrast, in regions farther from the source - or wherever the head spills out over the channel banks and spreads thin - a different sequence unfolds: the passing head first scours the bottom, and then the slower-moving body and tail deposit material gradually, during which time that sediment can be reworked by traction transport before the tail finally shuts the process down. ^[1] The final pulse of deposition, from the tail, occurs only after the current has slowed to the point where it can no longer drag sediment along the bed at all. ^[1]

Sediment Concentration: Low-Density and High-Density Flows

Not all turbidity currents carry the same load. The sediment concentration within a flow varies with both the position within the flow and the volume of material initially thrown into suspension, and these differences in concentration profoundly affect how the flow behaves and what it deposits. ^[1] Two broad categories are recognised on this basis: low-density flows, which carry less than roughly 20-30% grains by volume, and high-density flows, which carry greater concentrations. ^[1]

Low-density flows consist predominantly of clay, silt, and fine- to medium-grained sand, all kept aloft entirely by turbulence within the water. ^[1] Once turbulence wanes, nothing else prevents these grains from settling, which is why low-density flows tend to produce well-graded, finer grained turbidite beds. High-density flows are more complex: alongside fine sediment they may carry coarse-grained sands and clasts as large as pebbles or cobbles. ^[1] Keeping those coarse particles in motion requires more than turbulence alone - support also comes from hindered settling (the coarse grains impede each other’s descent at high concentrations) and from the buoyancy provided by the dense mixture of water and fine sediment that fills the spaces between them. ^[1]

High-density turbidity flows are distinct from debris flows, which are non-turbulent and far less fluid in behaviour. ^[1] Within a single turbidity current these two concentration types can coexist: the head - the densest, most energetic part - may qualify as a high-density flow, while the trailing tail thins and dilutes into a low-density flow. ^[1] This internal variation within a single event helps explain why turbidite beds often show a systematic transition from massive coarse basal units upward into progressively finer, better-structured upper layers.

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