thermohaline and wind-induced circulation systems
(Faugères and Stow, 2008). In addition, bottom currents
can operate as part of upwelling and downwelling currents, up- and down-canyon currents, internal tides and
waves, and seafloor polishing and spillover (see Faugères
and Stow, 2008; Shanmugam, 2008; and references
therein). These processes produce rather isolated
contourite facies commonly interbedded with other deepsea sediments.
Bottom currents are predominantly unidirectional subsurface currents controlled by the ocean-basin configuration. When their flow direction largely parallels the
submarine topography, they are called contour currents.
Although bottom currents show a quasi-steady flow, they
are highly variable in location, direction, and velocity over
relatively short time scales (hours to months). Deepwater
tidal effects, seasonal changes in water mass properties,
and laterally migrating eddies modulate the velocity of
the main bottom-current flow and even flow reversals
may occur (Shanmugam, 2008).
The mean velocity of a bottom current and its ability to
redistribute sediment are controlled by the Coriolis force
and by the topography of the ocean basin (Stow et al.,
2002a; Faugères and Mulder, 2011). Stow et al. (2008)
differentiate between (i) low-velocity flows (mean velocity <10 cm s
À1
), which typically move as a broad sluggish
water body over low-gradient slopes and abyssal plains;
(ii) intermediate-velocity flows (10–30 cm s
À1
), which
become intensified over steeper slopes and around topographic obstacles; and (iii) high-velocity flows (>30 cm
s
À1
), which are constricted and accelerated in narrow passages and over shallow sills. Flow velocities may exceed
100 cm s
À1 where the flow is particularly constricted, such
as through gateways on the deep-sea floor and within
shallower straits, where flow velocities in excess of
200 cm s
À1 have been recorded (Stow et al., 2002a).
Episodic downslope flows, which are not in equilibrium conditions with the oceanic water masses, do not represent bottom currents; for example, turbidity currents are
short-lived (in the range of hours to months) and dense
(more heavily loaded with suspended sedimentary particles) currents driven by gravity.
Contourite drifts
Bottom current-induced erosion and redeposition produce
large-scale accumulations of sediment termed contourite
drifts, which may be part of an even larger contourite
depositional system, comprising several related drifts
and associated erosional elements (Faugères et al., 1993;
Stow et al., 2002b; Rebesco, 2005; Faugères and Stow,
2008). Contourite drifts were first described in the North
Atlantic Ocean, where they were called “outer ridges” or
“sediment drifts” (e.g., McCave and Tucholke, 1986). Present at most of modern continental margins, they are as
frequent as deep-sea turbidite systems, occurring anywhere from the abyssal seafloor to slope settings. Sediment bodies deposited on the uppermost part of the
continental slope and across the outer shelf edge are
named “shallow-water contourite drifts.”
Contourite drifts range in size from about 100 km
2
(small patch drifts) to larger than 100,000 km
2 (giant elongated drifts) and occasionally even approaching
1,000,000 km
2 (abyssal sheet drifts). They may form a
positive relief of up to 2,000 m height. Larger contourite
drifts may record long-term continuity of deposition of
over several millions of years, resulting in the accumulation of several hundreds of meters of contourite sediments.
Contourite drifts are mainly composed of the sediments
deposited by bottom currents, but they may also enclose
associated deepwater sediments, particularly pelagites,
hemipelagites, and glaciomarine sediments, as well as
being locally intercalated with turbidites and other
density-flow deposits. They are often closely associated
with erosional areas that display coeval current-induced
winnowing of the seafloor, sediment bypassing, and the
formation of coarser-grained (gravel-lag) contourites
(Stow et al., 2008).
The principal types of contourite drifts that have been
identified include sheeted, elongate-mounded, channelrelated, confined, infill, and mixed systems (McCave
and Tucholke, 1986; Faugères et al., 1999; Stow et al.,
2002a; Rebesco, 2005). Case studies can be found in Stow
and Faugères (1993, 1998), Stow et al. (2002c), Viana and
Rebesco (2007), and Rebesco and Camerlenghi (2008).
The classification summarized below (Figure 1) is based
on a combination of criteria, such as drift distribution
(i.e., location) and morphology, and furthermore illustrates the drift development in the context of a particular
hydrological background (Faugères and Stow, 2008;
Faugères and Mulder, 2011):
(1) Contourite-sheeted drifts accumulate over a broad
area with very low relief and form a sediment body
of more or less constant thickness. They represent relatively slow rates of deposition of fine-grained
contourites (Stow et al., 2008). Extensive, impressive
fields of sediment waves may cover their surface.
Abyssal-sheeted drifts can extend across large basin
plains with deposits of up to a few hundreds of meters
thick, while slope-sheeted drifts are plastered against
the continental margin and cover smaller areas.
(2) Mounded drifts show an elevated relief with thicker
accumulation over a narrower, elongate region. They
represent relatively enhanced rates of deposition of
fine- to medium-grained contourites, commonly
focused into slope-parallel, elongate sediment bodies
over moderate to large areas of the deep seafloor
(Stow et al., 2008). Along one or both lateral margins
of mounded drifts, the flow tends to be markedly
intensified, in many cases causing distinct zones that
experience intermitted erosion and bypassing
(contourite channels, moats, and marginal valleys).
(3) Elongate-mounded drifts are the archetypical
contourite accumulations because of their distinctly
mounded and elongated geometry. Giant elongated
128
CONTOURITES
(Faugères and Stow, 2008). In addition, bottom currents
can operate as part of upwelling and downwelling currents, up- and down-canyon currents, internal tides and
waves, and seafloor polishing and spillover (see Faugères
and Stow, 2008; Shanmugam, 2008; and references
therein). These processes produce rather isolated
contourite facies commonly interbedded with other deepsea sediments.
Bottom currents are predominantly unidirectional subsurface currents controlled by the ocean-basin configuration. When their flow direction largely parallels the
submarine topography, they are called contour currents.
Although bottom currents show a quasi-steady flow, they
are highly variable in location, direction, and velocity over
relatively short time scales (hours to months). Deepwater
tidal effects, seasonal changes in water mass properties,
and laterally migrating eddies modulate the velocity of
the main bottom-current flow and even flow reversals
may occur (Shanmugam, 2008).
The mean velocity of a bottom current and its ability to
redistribute sediment are controlled by the Coriolis force
and by the topography of the ocean basin (Stow et al.,
2002a; Faugères and Mulder, 2011). Stow et al. (2008)
differentiate between (i) low-velocity flows (mean velocity <10 cm s
À1
), which typically move as a broad sluggish
water body over low-gradient slopes and abyssal plains;
(ii) intermediate-velocity flows (10–30 cm s
À1
), which
become intensified over steeper slopes and around topographic obstacles; and (iii) high-velocity flows (>30 cm
s
À1
), which are constricted and accelerated in narrow passages and over shallow sills. Flow velocities may exceed
100 cm s
À1 where the flow is particularly constricted, such
as through gateways on the deep-sea floor and within
shallower straits, where flow velocities in excess of
200 cm s
À1 have been recorded (Stow et al., 2002a).
Episodic downslope flows, which are not in equilibrium conditions with the oceanic water masses, do not represent bottom currents; for example, turbidity currents are
short-lived (in the range of hours to months) and dense
(more heavily loaded with suspended sedimentary particles) currents driven by gravity.
Contourite drifts
Bottom current-induced erosion and redeposition produce
large-scale accumulations of sediment termed contourite
drifts, which may be part of an even larger contourite
depositional system, comprising several related drifts
and associated erosional elements (Faugères et al., 1993;
Stow et al., 2002b; Rebesco, 2005; Faugères and Stow,
2008). Contourite drifts were first described in the North
Atlantic Ocean, where they were called “outer ridges” or
“sediment drifts” (e.g., McCave and Tucholke, 1986). Present at most of modern continental margins, they are as
frequent as deep-sea turbidite systems, occurring anywhere from the abyssal seafloor to slope settings. Sediment bodies deposited on the uppermost part of the
continental slope and across the outer shelf edge are
named “shallow-water contourite drifts.”
Contourite drifts range in size from about 100 km
2
(small patch drifts) to larger than 100,000 km
2 (giant elongated drifts) and occasionally even approaching
1,000,000 km
2 (abyssal sheet drifts). They may form a
positive relief of up to 2,000 m height. Larger contourite
drifts may record long-term continuity of deposition of
over several millions of years, resulting in the accumulation of several hundreds of meters of contourite sediments.
Contourite drifts are mainly composed of the sediments
deposited by bottom currents, but they may also enclose
associated deepwater sediments, particularly pelagites,
hemipelagites, and glaciomarine sediments, as well as
being locally intercalated with turbidites and other
density-flow deposits. They are often closely associated
with erosional areas that display coeval current-induced
winnowing of the seafloor, sediment bypassing, and the
formation of coarser-grained (gravel-lag) contourites
(Stow et al., 2008).
The principal types of contourite drifts that have been
identified include sheeted, elongate-mounded, channelrelated, confined, infill, and mixed systems (McCave
and Tucholke, 1986; Faugères et al., 1999; Stow et al.,
2002a; Rebesco, 2005). Case studies can be found in Stow
and Faugères (1993, 1998), Stow et al. (2002c), Viana and
Rebesco (2007), and Rebesco and Camerlenghi (2008).
The classification summarized below (Figure 1) is based
on a combination of criteria, such as drift distribution
(i.e., location) and morphology, and furthermore illustrates the drift development in the context of a particular
hydrological background (Faugères and Stow, 2008;
Faugères and Mulder, 2011):
(1) Contourite-sheeted drifts accumulate over a broad
area with very low relief and form a sediment body
of more or less constant thickness. They represent relatively slow rates of deposition of fine-grained
contourites (Stow et al., 2008). Extensive, impressive
fields of sediment waves may cover their surface.
Abyssal-sheeted drifts can extend across large basin
plains with deposits of up to a few hundreds of meters
thick, while slope-sheeted drifts are plastered against
the continental margin and cover smaller areas.
(2) Mounded drifts show an elevated relief with thicker
accumulation over a narrower, elongate region. They
represent relatively enhanced rates of deposition of
fine- to medium-grained contourites, commonly
focused into slope-parallel, elongate sediment bodies
over moderate to large areas of the deep seafloor
(Stow et al., 2008). Along one or both lateral margins
of mounded drifts, the flow tends to be markedly
intensified, in many cases causing distinct zones that
experience intermitted erosion and bypassing
(contourite channels, moats, and marginal valleys).
(3) Elongate-mounded drifts are the archetypical
contourite accumulations because of their distinctly
mounded and elongated geometry. Giant elongated
128
CONTOURITES