Encyclopedia of Geology Volume 4.pdf

SEDIMENTARY PROCESSES/Particle-Driven Subaqueous Gravity Processes 1

Particle-Driven Subaqueous Gravity Processes

M Felix and W McCaffrey, University of Leeds,

Leeds, UK

river outflow is less than that of the ocean, and turbid

surface plumes are generated. Nevertheless, particu-

late 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 the 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 cur-

rent will loft, in a manner similar to some subaerial

pyroclastic density flows, and the flow will essentially

cease to travel forwards, resulting in the development

of abrupt deposit margins.

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, that are essen-

tially submarine landslides, to very dilute flows carry-

ing only a few kg m 3 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 of much smaller

magnitude. Due to their infrequent occurrence and

destructive nature, much information about subaque-

ous gravity processes comes from the study of their

deposits and from laboratory experiments. Flow ini-

tiation mechanisms, sediment transport mechanisms,

and flow types are described here separately, to em-

phasise the sense of process continuum needed to

appreciate the development of most natural subaque-

ous 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.

Sediment Resuspension

Loose sediment on the seafloor can be resuspended if

bed shear stress is high enough. This can occur during

storms or during 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 the

influence of gravity. As in the 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.

Flows of much higher concentration may form as a

result of slope failure. Sediment on submarine slopes

can become unstable as a result of slope oversteepen-

ing during ongoing sedimentation, and during sea-

level falls, as a result of high inherited pore fluid

pressures and gas hydrate exsolution. Slope failure

can alternatively be triggered by externally applied

stresses, due to earthquakes, or as a result of 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/or breaching can continue,

adding material following the initial loss of stability.

The concentration of this mass is at packing density

but can become more dilute as flow continues.

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 receiv-

ing body of water, the river outflow will plunge, trav-

elling along the bed as a hyperpycnal flow (or plume)

beneath the ambient water. Such sediment-laden

underflows may mix with the ambient water and

transport sediment oceanward as particulate gravity

currents. Although sometimes these river-derived

flows are of high concentration (e.g., the 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. More commonly, the bulk density of the turbid

Terrestrial Input

Not all subaqueous gravity flows need originate

under water. Landslides, pyroclastic flows, and aeo-

lian sediment transport originating on land can enter

2 SEDIMENTARY PROCESSES/Particle-Driven Subaqueous Gravity Processes

lakes or oceans and continue flowing underwater if

the rates of mass flux are sufficiently high.

Flow Types

Broadly speaking, flows can be divided into three

main types, depending on density:

Grain Transport Mechanisms

Matrix Strength and Particle-Particle Interactions

Dense, Relatively Undeformed Flows, Creeps,

Slides and Slumps

Within dense flows, grains can be prevented from

settling as a result of matrix strength ( Figure 1 ).

This strength may arise if some or all of the particles

are cohesive. The resulting cohesive matrix prevents

both cohesive and non-cohesive particles from set-

tling 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 that of

static, loose-packed sediment. For slightly lower con-

centrations, inter-particle collisions will help keep

particles in suspension.

Flows of this type essentially have the same density as

the pre-failure material. In each case the sediment

moves as one large coherent mass, but with varying

amounts of internal deformation. Grains remain in

contact during flow and thus matrix strength is the

main sediment transport mechanism. Such flows will

stop moving or shear stress becomes too low to over-

come friction, at which point the entire mass comes to

rest. Flow thickness and deposit thickness are essen-

tially the same, although flows may thicken via in-

ternal thrusting or ductile deformation as they

decelerate prior to arrest. Slope creep caused by grav-

ity moves beds slowly downslope with gentle internal

deformation of the original depositional structure.

Slides undergo little or no pervasive internal deform-

ation, while slumps undergo partial deformation but

the original internal structure is still recognisable in

separate blocks. Thicknesses of slides and slumps

range from several tens of metres to 1–2 km and

travel distances can be up to about 100 km, with

displaced volumes of up to 10 12 m 3 , although most

flows are considerably smaller.

Hindered Settling and Buoyancy

Settling of particles can be slowed down by

water displaced upwards by other settling particles

( Figure 1 ). Such hindered settling is especially effect-

ive in dense mixtures with a range of grain sizes so

that the smaller particles are slowed down by settling

of the larger particles. The presence of smaller par-

ticles also increases the effective density of the fluid

that the particles are settling in and thus enhances

the buoyancy of the suspended particles and reduces

settling rates.

Dense, Deformed Flows: Rockfalls, Grain flows,

Debris Flows and Mudflows

Turbulence

In flows of this type, sediment still moves as one

coherent mass, but concentrations can be lower and

the mass is generally well mixed, with little or no

preservation of remnant structure from the 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 mud-

flows, to cohesive and/or 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 as a

The motion of sediment-laden flows can generate

turbulence through shear at the bed, internally in

the flow or at the top of a dense layer. The turbulent

bursts generated at the bed tend to have an asym-

metrical vertical velocity structure, with slower

downward sweeps and more rapid upward bursts.

This turbulence pattern counteracts the downwards

settling of particles, moving them higher up in the

flow ( Figure 1 ). Turbulence generation is hindered

and dissipation increased, however, if the particle

concentration is high, or if the flow is very cohesive

or highly stratified.

Figure 1 Schematic illustration of the principal grain transport mechanisms, shown in decreasing order of concentration from left to

right.

SEDIMENTARY PROCESSES/Particle-Driven Subaqueous Gravity Processes 3

Figure 2 A laboratory debris flow from right to left. Note: a

dilute turbidity current has been generated on the upper surface

of the debris flow due to erosion of material by fluid shear. (After

Mohrig et al . (1998) GSA Bulletin 110: 387–394.)

Figure 3 A laboratory turbidity current flow from right to left.

Field of view is 55cm wide. (After McCaffrey et al . (2003) Marine and

Petroleum Geology 20: 851–860, with permission from Elsevier.)

to turbulent entrainment of ambient water. Velocities

can be up to tens of m s, but more commonly are

around 1 m s or less. Larger flows, such as the well-

documented Grand Banks event of 1929, may travel

distances of a few thousand kilometres, even on

nearly flat slopes, although distances of tens to hun-

dreds of kilometres are more common. Sediment

eroded during flow can add to the driving force and

will increase flow duration and travel distance. Flows

will gradually slow down as sediment settles out, with

coarse material being deposited proximally and fine

material distally. Deposit thicknesses generally are

significantly smaller than flow thickness and are on

the order of cm to dm, but can be up to multi-metre

scale for large flows. However, ongoing sedimenta-

tion from flows of long duration can result in deposits

whose thickness relates principally to flow longevity

rather than flow thickness. Consequently, it is ge-

nerally more difficult to interpret flow properties

from analysis of turbidity current deposits (turbidites)

than it is for the denser flow types.

result of rapid internal deformation following slope

failure, from high concentration river input or from

reconcentration of dilute flows (described below).

Flow and deposit thicknesses can be up to several

tens of metres with travel distances of several hun-

dreds of kilometres. Erosion can add material to the

flow and thus extend both travel distances and size of

deposit – neither of which, therefore, necessarily

relate to the 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 the top of the flow, where the applied

stress falls below the yield strength. Such flows

move along a basal zone of deformation, and may

progressively ‘freeze’ from the top downwards, ultim-

ately coming to rest when the freezing interface

reaches the substrate.

(Partly) Dilute Flows: Turbidity Currents

In flows of this type, the sediment does not move as

one coherent mass ( Figure 3 ). These flows are gener-

ally dilute although parts of these flows can be of high

concentration, especially near the bed. In the dilute

parts of these flows, sediment is transported in either

laminar or turbulent suspension. In higher concen-

tration areas additional sediment transport mechan-

isms, such as grain-grain interactions, hindered

settling, and buoyancy effects may also play a role.

Rheologically, the dense parts of such flows can

behave plastically, but the dilute parts are Newton-

ian. Concentrations in turbidity currents range from

only a few kg m 3 to concentrations approaching those

of static, loose-packed sediment. The dilute parts of

these flows are commonly strongly vertically density-

stratified. Turbidity currents can be formed via dilu-

tion of debris flows (see below), directly from river

input or from resuspension of sediment.

Turbidity current thicknesses can be up to several

hundreds of metres and can increase during flow due

Flow Transformations

Transformations of one flow type into another are

common. Initially-dense slide masses may be dis-

rupted due to internal shear, liquefaction, and disag-

gregation on various scales. If this deformation is

sufficiently vigorous all the original structure of the

failed material will be lost and the slides transformed

into debris flows. In turn, these can transform into

turbidity currents by erosion of sediment from the

front and top of the dense mass due to ambient fluid

shear ( Figure 2 ), by disaggregation and dilution, and

by deposition of sediment, diluting the flow. Turbidity

currents can be transformed into debris flows if they

reconcentrate, for example when mud-rich flows

slow down. Further transformation into slides is

not possible once the original internal structure is

broken up.

The extent of transformation depends on flow size,

velocity, and sediment content. Variable degrees of

4 SEDIMENTARY PROCESSES/Particle-Driven Subaqueous Gravity Processes

transformation can lead to the development of differ-

ent flow types within one current, both vertically and

from front to back. This co-occurrence of different

flow types is especially common in flows with a dense

basal layer and more dilute upper part. Thus, classifi-

cation schemes which subdivide flows on the basis of

discrete flow types do not recognise the diversity of

natural flows, in which different types of flow may

occur simultaneously and vary in relative importance

in time and space as the flows evolve.

flow to keep sediment in suspension, known as

the flow ‘efficiency’, directly affects flow run-out

lengths. Flow efficiency depends on flow magnitude,

with larger flows being more efficient, and on grain

size, as finer grains settle out more slowly than

coarser grains. The presence of fine sediment in the

flow also increases the ability to carry coarse sedi-

ment so both types of sediment will be carried further

and both flow duration and run-out length will be

increased.

Internal and External Influences

on Flow Behaviour

Flow behaviour is influenced both by internal factors

such as concentration and grain size distribution

and external factors such as input conditions and

topography.

Spatial and Temporal Changes to Flow

Flows are influenced both by the input conditions and

by the terrain over which flow takes place. Flow

behaviour therefore varies both temporally and

spatially, causing local areas of erosion and depos-

ition that lead to a deviation from a simple deceler-

ating depositing flow and complicate the depositional

pattern. Both spatial and temporal changes in flow

behaviour can be caused by changes in sediment con-

tent of the flow: erosion adds driving force to the flow

and increases velocity, while deposition slows flows

down. Temporal changes to flow can also be caused

by changing input conditions. River input from floods

leads to flows that initially have a progressive in-

crease in velocity followed by a long period of de-

creasing velocity. In retrogressive failure ongoing

detachment of discrete sediment masses will result in

pulsed sediment input; the rate of input generally

tends to peak rapidly, and then diminish as successive

slope failures reduce in size.

Local spatial changes in flow are caused by changes

in the topography ( Figure 4 ). The angle of the slope

on which flow takes place is obviously important for

gravity driven flows; when slope angle increases, the

flow will go faster although the velocity increase will

be diminished by the increase of friction with the

ambient water. Nevertheless, small changes in slope

angle can change flow behaviour. If the slope angle

decreases, very dense flows can be stopped as the

basal friction becomes too high. More dilute flows

may undergo hydraulic jumps, in which they abruptly

thicken and decelerate. This deceleration can cause

coarser sediment to be deposited. Local changes to

flow can also be caused by changes in the constriction

of the flow path. When a flow goes into a constric-

tion, velocity will increase. Where a flow can expand,

as at the end of submarine canyons, velocity will

decrease.

Flow Velocity

The driving force, and hence velocity of subaqueous

gravity currents increases with both concentration

and flow size. However, resistance to internal shear

will increase with increasing viscosity due to increas-

ing particle concentrations, and with increasing yield

strength caused by cohesive particles. This will inhibit

the increase of flow velocities. However, because con-

centration-induced resistance to shear does not scale

with flow size, it can more readily be overcome by the

higher gravitational driving forces of larger flows,

which are, therefore, faster than smaller flows.

Flow Duration and Run-Out Length

Slope failure-induced slumps and slides that do not

transform into debris flows and/or turbidity currents

will generally be of short duration and have run-out

lengths on the order of the initial failure size. If the

failed sediment mass does transform into a debris

flow, the duration and run-out length depend on the

mobility as described above, with larger flows travel-

ling further. However, because debris flows stretch

out as they are flowing and because they may incorp-

orate material by erosion, their run-out length may

not be directly related to the initial failure size.

The duration and run-out length of turbidity cur-

rents depend on their size and sediment content, and

hence also on their formation mechanism. Sustained

input from rivers or glacial plumes can result in long

duration flows, even if the input concentration is low.

Turbidity currents that are generated from slope fail-

ures can have a short duration input, but tend to

stretch considerably due to turbulent mixing and

will thus increase in flow duration provided the trans-

ported sediment is kept in suspension. The ability of a

Momentum Loss

The evolution of flow behaviour can be different

along flow-parallel and flow-transverse directions.

Momentum will be greater in the direction of flow

SEDIMENTARY PROCESSES/Particle-Driven Subaqueous Gravity Processes 5

than in the transverse direction. For coarse sediment

in dilute flows, this means transport is principally in

the main flow direction as rapid transverse momen-

tum loss results in rapid deposition. This is less the

case for fine-grained sediment, which will stay in

suspension more easily and will thus generate mo-

mentum for flow in the transverse direction. These

differences are not so important in restricted parts of

the flow path, such as in canyons, but are important

in less confined settings.

Channelised flow

Figure 4 Schematic illustration of the interaction of turbidity

currents with (A) high amplitude and (B) low amplitude bathym-

etry. Flows are uniform if the velocity does not change with

distance and are non-uniform if the velocity does change. Accu-

mulative flows have spatially increasing velocity while depletive

flows have decreasing velocity. (After Kneller and McCaffrey

(1995) SEPM, Gulf Coast Section, 137–145.) Published with the

permission of the GCSSEPM Foundation; Further copying re-

quires permission of the GCSSEPM Foundation.

If flows are erosive they can create conduits (inci-

sional channels) both for themselves and for later

flows. In aggradational systems, dense flows such as

debris flows will start to form levees at their edges

where flow becomes too thin to overcome the matrix

strength. Sideway expansion of coarser-grained

turbidity currents may lead to loss of momentum in

the transverse direction, and thus greater rates of off-

axis than on-axis deposition. This incipient levee for-

mation may lead to the development of aggradational

channels ( Figure 5 ). These channels, which are gener-

ally sinuous, and often meandering, partly confine

flow and can carry sediment downstream for long

distances. Dilute parts of the flow can overtop the

levee crests resulting in overspill and deposition of

thin sheets of relatively fine-grained sediment that

decrease in thickness away from the channel. This

winnowing process causes the flows progressively

to become relatively depleted in fine grained

material, resulting in the development of sandy lobe

deposits at the end of relatively muddy channel-levee

systems. Levee height decreases downstream and

flows become less confined. Like subaerial channels,

Figure 5 GLORIA image of sinuous submarine channels on the Indus fan. (From Kenyon et al . (1995). In: Pickering et al . Atlas of

Deepwater Environments: architectural style in turbidite systems: London: Chapman and Hall, 89–93.)

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