Gary Nichols. Sedimentology and Stratigraphy(Second Edition)

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from river and delta systems and bypasses the shelf.

There is also intrabasinal material available in ocean

basins, comprising mainly the hard part of plants

and animals that live in the open oceans, and air-

borne dust that is blown into the oceans. These

sources of sediment all contribute to oceanic deposits

(Douglas 2003). The large clastic depositional sys-

tems are mainly found near the margins of the ocean

basin, although large systems may extend a thou-

sand kilometres or more out onto the basin plain,

and the ocean basin plains provide the large st

depositional environments on Earth.

The problem with these deep-water depositional

systems, however, is the difficulty of observing and

measuring processes and products in the present day.

The deep seas are profoundly inaccessible places. Our

knowledge is largely limited to evidence from remote

sensing: detailed bathymetric surveys, side-scan sonar

images of the sea floor and seismic reflection surveys

(22.2) of the sediments. There are also extremely

localised samples from boreholes, shallow cores and

dredge samples. Our database of the modern ocean

floors is comparable to that of the surface of the Moon

and understanding the sea floor is rather like trying to

interpret all processes on land from satellite images

and a limited number of hand specimens of rocks

collected over a large area. However, our knowledge

of deep-water systems is rapidly growing, partly

through technical advances, but also because hydro-

carbon exploration has been gradually moving into

deeper water and looking for reserves in deep-water

deposits.

16.1.1 Morphology of ocean basins

Continental slopes typically have slope angles of

between 28 and 108 and the continental rise is even

less (11.1). Nevertheless, they are physiographically

significant, as they contrast with the very low gradi-

ents of continental shelves and the flat ocean floor.

Continental slopes extend from the shelf edge, about

200 m below sea level, to the basin floor at 4000 or

5000 m depth and may be up to a hundred kilometres

across in a downslope direction. Continental slopes

are commonly cut by submarine canyons, which,

like their counterparts on land, are steep-sided ero-

sional features. Submarine canyons are deeply

incised, sometimes into the bedrock of the shelf, and

may stretch all the way back from the shelf edge to

the shoreline. They act as conduits for the transfer of

water and sediment from the shelf, sometimes feeding

material directly from a river mouth. The presence of

canyons controls the formation and position of sub-

marine fans.

The generally flat surface of the ocean floor is

interrupted in places by seamounts, underwater

volcanoes located over isolated hotspots. Seamounts

may be wholly submarine or may build up above

water as volcanic islands, such as the Hawaiian

island chain in the central Pacific. As subaerial volca-

noes they can be important sources of volcaniclastic

sediment to ocean basins. The flanks of the volcanoes

are commonly unstable and give rise to very large-

scale submarine slides and slumps that can involve

several cubic kilometres of material. Bathymetric

mapping and sonar images of the ocean floor around

volcanic islands such as Hawaii in the Pacific and the

Canary Islands in the Atlantic have revealed the

existence of very large-scale slump features. Mass

movements on this scale would generate tsunami

(11.3.2) around the edges of the ocean, inundating

coastal areas.

The deepest parts of the oceans are the trenches

formed in regions where subduction of an oceanic

plate is occurring. Trenches can be up to 10,000 m

shelf

mid-ocean ridge

surface of ocean crust

deepens away from

mid-ocean ridge

young, buoyant crust

old, denser crust

Fig. 16.1 Deep water environments are floored by ocean crust and are the most widespread areas of deposition worldwide.

248 Deep Marine Environments

deep. Where they occur adjacent to continental

margins (e.g. the Peru–Chile Trench west of South

America) they are filled with sediment supplied from

the continent, but mid-ocean trenches, such as the

Mariana Trench in the west Pacific, are far from

any source of material and are unfilled, starved of

sediment.

16.1.2 Depositional processes in deep seas

Deposition of most clastic material in the deep seas is

by mass-flow processes (4.5). The most common are

debris flows and turbidity currents, and these form

part of a spectrum within which there can be flows

with intermediate characteristics.

Debris-flow deposits

Remobilisation of a mass of poorly sorted, sediment-

rich mixture from the edge of the shelf or the top of

the slope results in a debris flow, which travels down

the slope and out onto the basin plain. Unlike a debris

flow on land an underwater flow has the opportunity

to mix with water and in doing so it becomes more

dilute and this can lead to a change in the flow

mechanism and a transition to a turbidity current.

The top surface of a submarine debris flow deposit will

typically grade up into finer deposits due to dilution of

the upper part of the flow. Large debris flows of mate-

rial are known from the Atlantic off northwest Africa

(Masson et al. 1992) and examples of thick, extensive

debris-flow deposits are also known from the strati-

graphic record (Johns et al. 1981; Pauley 1995).

Debris-flow deposits tens of metres thick and extend-

ing for tens of kilometres are often referred to as

megabeds.

Turbidites

Dilute mixtures of sediment and water moving as

mass flows under gravity are the most important

mechanism for moving coarse clastic material in

deep marine environments. These turbidity currents

(4.5.2) carry variable amounts of mud, sand and

gravel tens, hundreds and even over a thousand kilo-

metres out onto the basin plain. The turbidites depos-

ited can range in thickness from a few millimetres to

tens of metres and are carried by flows with sediment

concentrations of a few parts per thousand to 10%.

Denser mixtures result in high-density turbidites

that have different characteristics to the ‘Bouma

Sequences’ seen in low- and medium-density turbi-

dites. Direct observation of turbidity currents on the

ocean floor is very difficult but their effects have been

monitored on a small number of occasions. In Novem-

ber 1929 an earthquake in the Grand Banks area off

the coast of Newfoundland initiated a turbidity cur-

rent. The passage of the current was recorded by the

severing of telegraph cables on the sea floor, which

were cut at different times as the flow advanced.

Interpretation of the data indicates that the turbidity

current travelled at speeds of between 60 and

100 km h

1

(Fine et al. 2005). Also, the deposits of

recent turbidity flows have been mapped out, for

example, in the east Atlantic off the Canary Islands

a single turbidite deposit has been shown to have a

volume of 125 km

(Masson 1994).

High- and low-efficiency systems

A deep marine depositional system is considered

to be a low-efficiency system if sandy sediment is

carried only short distances (tens of kilometres) out

onto the basin plain and a high-efficiency system if

the transport distances for sandy material are hun-

dreds of kilometres (Mutti 1992). High-volume flows

are more efficient than small-volume flows and the

efficiency is also increased by the presence of fines

that tend to increase the density of the flow and

hence the density contrast with the seawater. The

deposits of low-efficiency systems are therefore con-

centrated near the edge of the basin, whereas mud-

dier, more efficient flows carry sediment out on to the

basin plain. The high-efficiency systems will tend to

have an area near the basin margin called a

bypass zone where sediment is not deposited, and

there may be scouring of the underlying surface,

with all the deposition concentrated further out in

the basin.

Initiation of mass flows

Turbidity currents and mass flows require some form

of trigger to start the mixture of sediment and water

moving under gravity. This may be provided by an

earthquake as the shaking generated by a seismic

shock can temporarily liquefy sediment and cause it

to move. The impact of large storm waves on shelf

sediments may also act as a trigger. Accumulation of

Ocean Basins 249

sediment on the edge of the shelf may reach the point

where it becomes unstable, for example where a delta

front approaches the edge of a continental shelf. High

river discharge that results in increased sediment

supply can result in prolonged turbidity current flow

as sediment-laden water from the river mouth flows

as a hyperpycnal flow across the shelf and down onto

the basin plain. Such quasi-steady flows may last

for much longer periods than the instantaneous trig-

gers that result in flows lasting just a few hours. A fall

in sea level exposes shelf sediments to erosion, more

storm effects and sediment instability that result in

increased frequency of turbidity currents.

16.1.3 Composition of deep marine deposits

The detrital material in deep-water deposits is highly

variable and directly reflects the sediment source area.

Sand, mud and gravel from a terrigenous source are

most common, occurring offshore continental mar-

gins that have a high supply from fluvial sources.

Material that has had a short residence time on the

shelf will be similar to the composition of the river but

extensive reworking by wave and tide processes can

modify both the texture and the composition of the

sediment before it is redeposited as a turbidite. A

sandstone deposited by a turbidity current can there-

fore be anything from a very immature, lithic wacke

to a very mature quartz arenite. Turbidites composed

wholly or partly of volcaniclastic material occur in

seas offshore of volcanic provinces. The deep seas near

to carbonate shelves may receive large amounts of

reworked shallow-marine carbonate sediment, rede-

posited by turbidity currents and debris flows into

deeper water: recognition of the redeposition process

is particularly important in these cases because the

sediment will contain bioclastic material that is char-

acteristic of shallow water environments. Because

there is this broad spectrum of sandstone composi-

tions in deep-water sediments, the use of the term

‘greywacke’ to describe the character of a deposit is

best avoided: it has been used historically as a descrip-

tion of lithic wackes (2.3.3 ) that were deposited as

turbidites and the distinction between composition

and process became confused as the terms turbidite

and greywacke came to be used almost as synonyms.

‘Greywacke’ is not part of the Pettijohn classification

of sandstones and it no longer has any widely

accepted meaning in sedimentology.

16.2 SUBMARINE FANS

A submarine fan is a body of sediment on the sea

floor deposited by mass-flow processes that may be

fan-shaped, but more elongate, lobate geometries are

also common (Fig. 16.2). They vary in size from a few

kilometres radius to depositional systems covering

over a million square kilometres and forming some

of the largest geomorphological features on Earth.

The morphology and depositional character of sub-

marine fan systems are strongly controlled by the

composition of the material supplied, particularly the

proportions of gravel, sand and mud present. In this

sense submarine fans are very much like other deposi-

tional systems such as deltas (Chapter 12), which also

show considerable variability depending on the grain-

size distribution in the material supplied. Note that

although coarse-grained deltas are sometimes referred

to as fan deltas and are largely submarine, the term

submarine fan is restricted to fan-shaped bodies that

sediment supply

shelf canyon

shelf

medial fan lobe deposits

10s-1000s kilometres

basin

plain

proximal fan channel and levee deposits

distal fan

deposits

Fig. 16.2 Depositional environments on

a submarine fan.

250 Deep Marine Environments

are deposited by mass-flow, mainly turbidity current,

processes.

A submarine fan could form of any clastic mate-

rial, but the larger fans are all composed of terri-

genous c lastic material supplied by large river

systems. Carbonate shelves can be impor tant

sources of sediment redeposited in the ocean basins

by turbidites, but the supply of carbonate sediment

is rarely focused at discrete points along the con-

tinental slope: submarine fans composed of carbo-

nate ma terial are therefore rarely formed, and most

carbonate turbidites are a ssociated with slope-apron

systems (16.3).

16.2.1 Architectural elements of submarine

fan systems

A submarine fan can be divided into a number of

‘architectural elements’, components of the deposi-

tional system that are the products of different pro-

cesses and subenvironments of deposition (Fig. 16.3).

Submarine fan channels form distinct elements on

the fan surface and may have levees associated with

them: these channels may incise into, or pass distally

into, depositional lobes, which are broad, slightly

convex bodies of sediment.

Submarine fan channels and levees

The canyons that incise into the shelf edge funnel

sediment and water to discrete points at the edge of

the ocean basin where turbidity currents flowing

down the canyons pass into channels. Unlike the

canyons, the channels are not incised into bedrock,

but may scour into underlying submarine fan deposits

(Fig. 16.4). Submarine fan channels are variable in

size: some of the larger modern examples are several

tens of kilometres wide and over a thousand metres

deep, and in the stratigraphic record there are sub-

marine fan channels with thicknesses of up to 170 m

and 20 km across (Macdonald & Butterworth 1990).

The deposits in the channel are typically coarse sands

and gravel that form thick, structureless or crudely

graded beds characterised by T

of the Bouma

sequence and S

1–3

of the ‘Lowe-type’ high-density

turbidite model (4.5.2). The lateral extent of these

turbidite beds is limited by the width of the channel,

which, when it is filled, forms a lenticular body made

up of stacked coarse-grained turbidites.

Fig. 16.3 The proportions of different

architectural elements on submarine fans

are determined by the dominant grain size

deposited on the fan.

poorly channelised

braided channels

channel and levee

complexes

channel and levee

complexes

channelised lobes

depositional lobes

sheets

Muddy

systems

Mixed sand

and mud

systems

Sand-rich

systems

Gravel-rich

systems

Channels Lobes

Sheets

Submarine Fans 251

Most of an individual turbidity flow is confined to

the channel but the upper, more dilute part of the

flow may spill out of the channel l aterally. This is

analogous to the channel and ov erbank setting

familiar from fluvial environments (Chapter 9).

The overbank flow from the channel contains fine

sand, silt and mud and this spreads out as a fine-

grained turbidity current away from the channel to

form a submarine channel levee. The levee turbi-

dites consist of the upper parts of Bouma sequences

c–e

and T

) and they thin away from the channel

margin with a low-angle, wedge-shaped geometry.

Levee successions can build up to form units

hundreds of metres thick, especially if the channel is

aggrading, that is, filling up with sediment and

building up its banks at the same time. Channel and

levee complexes are also preserved when the channel

migrates laterally or avulses, to leave its former

position abandoned.

Depositional lobes

At the distal ends of channels the turbidity currents

spread out to form a lobe of turbidite deposits that

occupies a portion of the fan surface. An individual

lobe is constructed by a succession of turbidity cur-

rents that tend to deposit further and further out on

the lobe through time. A simple progradational geom-

etry results if fan deposition is very ordered, with each

turbidity current event of approximately the same

magnitude and each depositing progressively further

from the mouth of the channel. However, turbidity

currents are of varying magnitude and so the pattern

tends to be more complex. As the lobe builds out the

flow in the more proximal part tends to become chan-

nelised. Lobe progradation continues until the chan-

nel avulses to another part of the fan. Avulsion occurs

because an individual lobe will start to build up above

the surrounding fan surface and eventually flows

start to follow the slightly steeper gradient on to a

lower area of the fan.

The succession built up by depositional lobe pro-

gradation is ideally a coarsening-up succession

capped by a channelised unit (Fig. 16.5). Individual

turbidites will show normal grading but as the lobe

progrades currents will carry coarser sediment

further out on the fan surface. Successive deposits

therefore should contain coarser sediment and

hence generate an overall coarsening-up pattern. A

thickening-up of the beds should accompany the

coarsening-up pattern (Fig. 16.5). Commonly this

overall coarsening-up and thickening-up is not seen

because of the complex, often random pattern of

deposition on depositional lobes (Anderton 1995).

Therefore there may not be any consistent vertical

pattern of beds deposited on a submarine fan lobe.

Depositional lobe deposits often contain the most

complete Bouma sequences (T

a–e

and T

b–e

). The

whole lobe succession may be tens to hundreds of

metres thick and an individual lobe may be kilometres

or tens of kilometres across. Lobes will be stacked both

vertically and laterally against each other, although

the lateral limits of an individual lobe may be difficult

to identify.

Fig. 16.4 Thick sandstone beds

deposited in a channel in the proximal

part of a submarine fan complex.

252 Deep Marine Environments

Turbidite sheets

Turbidite sheets are deposits of turbidity currents

that are not restricted to deposition on a lobe but

have spread out over a larger area of the fan. They

are thin, fine-grained turbidites characterised by

Bouma divisions T

c–e

and T

with little or no organi-

sation into patterns or trends in grain size and bed

thickness (Fig. 16.6). Interbedding with hemipelagic

mudstones (16.5.3) is common.

16.2.2 Submarine fan systems

The architectural elements described are found in

various proportions and are made up of different

grain sizes of material depending on the characteris-

tics and volume of the sediment supplied to the sub-

marine fan. Any combination is possible, but it is

convenient to consider four representative models:

gravel-dominated, sandy, mixed sand and mud, and

muddy, with the usual caveat that any intermediate

form can exist. The examples shown in Figs 16.7–

16.10 are for systems that have a single entry point

supplying a fan-shaped body of sediment, but for each

case there are also scenarios of multiple supply points,

which form coalescing bodies of sediment that do not

form an overall fan-shape (Reading & Richards 1994;

Stow et al. 1996). Submarine fan systems are com-

monly divided into upper fan (inner fan), mid-fan and

lower fan (outer fan) areas: in these schemes the

upper fan is dominated by channel and levee com-

plexes, the mid-fan by depositional lobes and the

lower fan by sheets. Although this works well for

some examples (e.g. sandy and mixed systems) the

divisions are not so appropriate for gravelly and

muddy systems (see below).

Gravel-rich systems

Coarse sediment may be deposited at the edge of a

basin in coarse-grained deltas supplied by braided

river or alluvial fans. The deeper parts of these deltas

can merge into small submarine fans of material

forming wedge-shaped bodies at the base of the slope

(Fig. 16.7). The gravel is mainly deposited by debris

flows and sands are rapidly deposited by high-density

turbidity currents. These fan bodies tend to pass

abruptly into thin-bedded distal turbidites and hemi-

pelagic mudstones.

Distal fan. Thin,

fine-grained

turbidites

Mid-fan.

Coarsening-up

succession of sandy

turbdites

100s

metres

Mid-fan. Channel

on lobe

Inner Fan.

Thin-bedded levee

deposits

Inner fan.

Submarine fan

channel filled with

thick conglomerate

and sandstone

turbidites

MUD

clay

silt

SAND

GRAVEL

gran

pebb

cobb

boul

Submarine fan

Scale

Lithology

Structures etc

Notes

Fig. 16.5 Schematic graphic sedimentary logs through

submarine fan deposits: proximal, mid-fan lobe deposits and

lower fan deposits.

Submarine Fans 253

Sand-rich systems

A submarine fan system is considered to be sand-rich

if at least 70% of the deposits in the whole system are

sandy material (Fig. 16.8). They are usually sourced

from sand-rich shelves where waves, storms and tidal

currents have sorted the material, removing most of

the mud and leaving a sand-rich deposit that is

reworked by turbidity currents. Sand-rich turbidity

currents have a low efficiency and do not travel very

far, so the fan body is likely to be relatively small, less

than 50 km in radius (Reading & Richards 1994).

Deposition is largely by high-density turbidity cur-

rents and the fan is characterised by sandy channels

and lobes. The inner fan area is dominated by chan-

nels with some lobes, while the mid-fan area is mainly

coalesced lobes, often channelised. Due to the low

transport efficiency the transition to finer grained

sheet deposits of the lower fan is abrupt. Inactive

areas of the fan (abandoned lobes) become blanketed

by mud. Strata formed by these systems consist of

thick, moderately extensive packages of sandy high-

density turbidites separated by mud layers that repre-

sent the periods of lobe abandonment.

Mixed sand–mud systems

Where a river/delta system provides large quantities of

both sandy and muddy material, a mixed sand–mud

Fig. 16.6 A succession of sandy and

muddy turbidite beds deposited on the

distal part of a submarine fan complex.

steep hinterland relief

coarse alluvial supply

narrow shelf

coarse mass flow deposits

kilometres

deep sea

floor

Fig. 16.7 Facies model for a gravel-rich submarine fan: typically found in front of coarse fan deltas, the fan is small and

consists mainly of debris flows.

254 Deep Marine Environments

depositional system results; these systems are defined

as consisting of between 30% and 70% sand

(Fig. 16.9). These higher efficiency systems are tens

to hundreds of kilometres in diameter and consist of

well-developed channel levee systems and deposi-

tional lobes. Deposits in the channels in the inner

and mid-fan areas include lags of coarse sandstone,

sandy, high-density turbidite beds and channel aban-

donment facies that are muddy turbidites (Reading &

Richards 1994). They form lenticular units flanked by

levee deposits of thin, fine-grained turbidites and

muds. The depositional lobes of the mid-fan are very

variable in composition, including both high- and

lower density turbidites, becoming muddier in the

lower fan area. In a sedimentary succession, the lobe

deposits form very broad lenses encased in thin sheets

of the lower fan and muds of the basin plain.

Muddy systems

The largest submarine fan systems in modern oceans

are mud-rich (Stow et al. 1996), and are fed by very

large rivers. These large mud-rich fans include the

Bengal Fan fed by the Ganges and Brahmaputra rivers

and the large submarine fan beyond the mouth of the

Mississippi. These submarine fan systems are over

1000 km in radius and consist of less than 30%

sand (Fig. 16.10). Channels are the dominant archi-

tectural element of these systems and when some

modern submarine fan channels are viewed in plan

they are seen to follow a strongly sinuous course that

looks like a meandering river pattern (Reading &

Richards 1994). The channels deposits are sandy

while some sand and more mud are deposited on

the channel margins as well-developed levees.

mainly sandy supply

(e.g. from sandy delta)

shelf canyon

shelf

sandy lobe deposits

10s kilometres

deep sea

floor

Fig. 16.8 Facies model for a sand-rich submarine fan: sand-rich turbidites form lobes of sediment that build out on the basin

floor, with switching of the locus of deposition occurring through time.

mixed sandy and

muddy sediment

supply

shelf canyon

shelf

sandy and muddy

lobe deposits

10s-100s kilometres

deep sea

floor

Fig. 16.9 Facies model for a mixed sand–mud submarine fan: the lobes are a mixture of sand and mud and build further out as

the turbidites travel longer distances.

Submarine Fans 255

Depositional lobes are rather poorly developed and

thin: the outer fan area consists mainly of thin sand-

stone sheets interbedded with mudrocks of the basin

plain. In a succession of mud-rich fan deposits the

sandstone occurrences are limited to lenticular chan-

nel units and isolated, thin lobes and sheets in the

lower fan.

16.2.3 Ancient submarine fan systems

Successions of turbidites are found in places where

deep-sea deposits have been uplifted by tectonic forces

and are now exposed on land: this occurs at ocean

margins where accretionary prisms form (24.3.2) and

around mountain belts where foreland basin deposits

are thrust to the surface (24.4.1). The beds are com-

monly quite deformed, having been folded and faulted

during the process of uplift, so interpretation of the

successions, which may be many thousands of metres

thick, is not always easy. The type of depositional

system can be assessed by considering the ranges of

the grain sizes of the material and the distribution of

channel, levee, lobe and sheet facies. Because of the

size of most submarine fan systems, the beds exposed

will often represent only a very small part of a whole

system, even if the outcrop extends for tens of kilo-

metres or more.

The palaeogeography of the system can be estab-

lished by using the distribution of the different facies,

and by using indicators of palaeoflow. Scouring during

the flow of a turbidity current leaves marks on the

underlying surface that are filled in as casts when

deposition subsequently occurs. These scour and tool

marks (4.7) can be very abundant on the bases of

turbidite sandstone beds and measurement of their

orientation can be used to determine the direction of

flow of the turbidity current: flute marks indicate the

flow direction while groove marks show the orienta-

tion of the axis of the flow. Cross-lamination in the

Bouma ‘c’ division can also be measured and used as

a palaeocurrent indicator. Palaeoflow indicators in

turbidites provide reliable information about where

the source area was (back-tracking along the flow

directions), except where a turbidity current encoun-

ters an obstacle and is diverted or in small basins

where they may flow all the way across to the oppo-

site margin and rebound back again.

Through time a deep-water basin may be wholly or

partially filled up with the deposits of a submarine fan.

During this process the fan system will prograde as it

builds out into the basin. This means that the deposits

of the lower fan will be overlain by mid-fan deposits

and capped by upper fan facies (Fig. 16.5), but the

succession is unlikely to be as simple as presented in

this diagram.

16.3 SLOPE APRONS

Slope aprons are depositional systems on continental

slopes and adjacent parts of the basin floor that are

not fed by discrete point sources but instead have a

linear supply from a stretch of the shelf. Deposition is

by mass flow processes ranging from submarine slides

and slumps to debris flows and turbidity currents

(Fig. 16.11). Coarser debris tends to move as ava-

lanches of detritus, including blocks of rock metres

muddy sediment supply

shelf canyon

shelf

muddy lobe deposits

100s kilometres

deep sea

floor

Fig. 16.10 Facies model for a muddy submarine fan: lobes are very elongate and most of the sand is deposited close to the

channels.

256 Deep Marine Environments

to tens of metres across known as olistoliths, and as

debris flows of material that accumulate on the slope.

Sand reworked from the edge of the shelf is trans-

ported as high-density turbidity currents down the

slope to form spill-over sand deposits (Stow 1986).

Mud and mixtures of sand and mud are redistributed

further down the slope and onto the adjacent plain by

turbidity currents. Finer deposits on the slope are

reworked by debris flows and as more coherent slides

and slumps of material.

The proportions of gravel, sand and mud will be

determined by the nature of the sediment supply from

the shelf edge and there is additionally hemipelagic

deposition on the slope. The mass-flow deposits are

therefore often interbedded with hemipelagic muds,

although these are sometimes remobilised and

deformed in slump units. The character of slope

basin deposits therefore tends to be heterogeneous

and generally chaotic. At the edges of carbonate plat-

forms carbonate slopes may develop at angles ran-

ging from a few degrees to slopes in excess of 308

(Wright & Burchette 1996). The steeper slopes are

sites of slumping and redeposition of material by deb-

ris flows, while at the base of the slope an apron of

carbonate turbidites is deposited.

16.4 CONTOURITES

Ocean currents that are geostrophic and/or thermo-

haline in origin (11.4) are mainly currents that flow

along the sea floor parallel to, or nearly parallel to, the

bathymetric contours of the continental margin. The

deposits of bottom currents are hence called contour-

ites. The contour currents are significant in the dee-

per marine environments and therefore contourites

are not considered to accumulate on shelf areas. Con-

tourites may be sheets of fine muddy and silty sedi-

ment, known as ‘drifts’, that cover hundreds of

thousands to millions of square kilometres on the

abyssal plain and may be tens to hundreds of metres

thick (Stow et al. 1998). Sometimes they form more

elongate bodies parallel to the basin margin, formed

by bottom (contour) currents that may be strong

enough to transport sand. Their composition depends

on the material available to the current and may be

terrigenous, calcareous, or volcaniclastic: the grain

size and sedimentary structures formed depend on

the flow velocity.

In contrast to turbidites, which are single event

deposits, contourites are the products of continuous

flow, and their characteristics are determined by var-

iations in the current velocity, the amount and type of

sediment supply and the degree of modification by

processes such as bioturbation. A cycle of increase

followed by decrease in velocity would produce a

reverse and then normally graded pattern

(Fig. 16.12), but other patterns are possible. Con-

tourites can be difficult to recognise in the strati-

graphic record, and a fine-grained deposit can often

only be confidently interpreted as the product of a

bottom current if it can be shown that it was not

Spill-over sands

Sea level

Slumps

Debris

flows

Pelagic

muds

Fig. 16.11 Slope apron deposits include pelagic sediment, slumps, debris flows and sands from the shelf edge.

(From Stow 1986.)

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