the abyssal plains. McCave and Tucholke (1986)
identified plastered, separated, and detached
elongate-mounded drifts.
(4) Drifts related to deep channels or other oceanic passages (gateways, seaways, gaps, and straits), where
the bottom currents are constricted and flow velocities
are substantially increased, occur as axial and lateral
patch drifts (either mounded or sheet form). Within
the channel, the localized deposition of finer-grained
contourite facies is closely related to significant erosion of the passage seafloor, widespread sediment
bypassing, and the formation of coarse-grained lag
deposits. Near the channel exit region, where the flow
is expanded and decelerated, sheetlike contourite fans
form, typically with a downflow decrease in
contourite grain size. The channel-related patch drifts
are typically small, a few kilometers in diameter, and
up to 150 m thick. The contourite fans are much larger
cone-shaped accumulations, up to 100 km or more in
radius and 300 m in thickness.
(5) Confined contourite drifts are characterized by an
elongate-mounded geometry aligned parallel to the
axis of a relatively small confining basin.
A significant feature is the presence of distinct moats
along both sides of the drift, suggesting that the fairly
slow contour current is confined on both margins or
that it develops into some kind of circulatory pattern
within the basin (Faugères and Stow, 2008).
(6) Mixed drift systems are those that involve significant
interaction of along-slope bottom currents with other
depositional processes in the buildup of the drift body.
These drifts include substantial deepwater facies
deposited either by episodic downslope processes
(e.g., turbidites, debrites, and hyperpycnites) or by
continuous vertical settling (e.g., pelagites,
hemipelagites, periplatform carbonates, ice-rafted
debris). The regular contourite-drift morphologies
may be markedly modified by significant downslope
sediment transport, and vice versa, bottom currents
crossing deep-sea turbidite systems may induce a lateral shift of the channel-levee complexes. Mulder
et al. (2008) outline the variable aspects of these alternating and interacting processes and the resulting drift
geometries.
Contourite sediment facies
Depending on the flow characteristics and on local sediment sources, a large variety of contourite facies occurs
in modern oceans (Stow et al., 1996, 2002a; Stow and
Faugères, 2008). The grain size of contourites varies from
fine muds, through silts and sands, to sand and gravel-lag
components. The composition of the sediment can be terrigenous siliciclastic, biogenic (calcareous and siliceous),
volcaniclastic, and chemogenic (manganiferous, phosphatic). Biogenic components typically comprise whole
tests or shell fragments of foraminiferas, radiolarians, diatoms, spicules, and coccoliths. The different grain-size
fractions and diverse materials are often poorly sorted
mixtures. The rare contourite deposits identified in the fossil record are mainly carbonate-rich contourites (Hüneke
and Stow, 2008; e.g. Hüneke 2013).
It is the bottom-current velocity that mainly governs the
depositional processes and resulting sediment facies
(Faugères and Mulder, 2011). Low-velocity flows allow
vertical setting of fine-grained suspended particles from
the nepheloid layer, whereas intermediate-velocity flows
induce more bed load transport and deposition of silt and
sand. High-velocity flows can affect still larger grain sizes
and generate large-scale winnowing and erosion, resulting
in coarse-grained lag deposits. Dissolution and
authigenesis can accompany the current-induced processes, generating very low sedimentation rates and sediment bypassing.
The large variety of contourite facies typically displays
(a) abundant bioturbational sedimentary structures,
(b) bad preservation of hydrodynamic sedimentary structures, and (c) an irregular vertical variation in grain size.
Burrowing animals intensely colonize contourite sediments due to their low accumulation rates and to the generally well-oxygenated (cold) water masses prevailing
during bottom-current-induced deposition. Several types
of ichnofabrics can be distinguished, depending on the
prevailing current intensity (Wetzel et al., 2008). Hydrodynamic (small-scale) sedimentary structures are scarce
or completely lacking, being obliterated by the permanent
bioturbation, except when the sedimentation rate exceeds
the bioturbation rate (Martin-Chivelet et al., 2008). Consequently, the vertical variation in grain size and sediment
facies, which typically displays a repeated superposition
of coarsening-upward and fining-upward units, may be
difficult to recognize.
The facies model for contourites typifies the vertical
grain-size variation of a standard mud-silt-sand contourite
sequence, which results from variation in contour-current
velocity (Faugères et al., 1984; Stow et al., 2002a; Stow
and Faugères, 2008). A complete sequence of any particle
composition consists of five principal divisions:
(C1) lower muddy contourite division, (C2) lower mottled
silty contourite division, (C3) middle sandy contourite
division, (C4) upper mottled silty contourite division,
and (C5) upper muddy contourite division. Traction structures, including rare cross-lamination, are more evident in
the coarse silts and sands than in finer-grained facies.
There may be an indistinct and discontinuous parallel lamination, and lenses of coarser materials may occur in all
intervals.
The centimeter- to decimeter-thick successions of the
standard sequence may be truncated. In that case, some
divisions are missing, and erosional-sharp contacts or
nondeposition surfaces are present between the facies
units (typically at division C3). In fact, partial sequences
130
CONTOURITES
identified plastered, separated, and detached
elongate-mounded drifts.
(4) Drifts related to deep channels or other oceanic passages (gateways, seaways, gaps, and straits), where
the bottom currents are constricted and flow velocities
are substantially increased, occur as axial and lateral
patch drifts (either mounded or sheet form). Within
the channel, the localized deposition of finer-grained
contourite facies is closely related to significant erosion of the passage seafloor, widespread sediment
bypassing, and the formation of coarse-grained lag
deposits. Near the channel exit region, where the flow
is expanded and decelerated, sheetlike contourite fans
form, typically with a downflow decrease in
contourite grain size. The channel-related patch drifts
are typically small, a few kilometers in diameter, and
up to 150 m thick. The contourite fans are much larger
cone-shaped accumulations, up to 100 km or more in
radius and 300 m in thickness.
(5) Confined contourite drifts are characterized by an
elongate-mounded geometry aligned parallel to the
axis of a relatively small confining basin.
A significant feature is the presence of distinct moats
along both sides of the drift, suggesting that the fairly
slow contour current is confined on both margins or
that it develops into some kind of circulatory pattern
within the basin (Faugères and Stow, 2008).
(6) Mixed drift systems are those that involve significant
interaction of along-slope bottom currents with other
depositional processes in the buildup of the drift body.
These drifts include substantial deepwater facies
deposited either by episodic downslope processes
(e.g., turbidites, debrites, and hyperpycnites) or by
continuous vertical settling (e.g., pelagites,
hemipelagites, periplatform carbonates, ice-rafted
debris). The regular contourite-drift morphologies
may be markedly modified by significant downslope
sediment transport, and vice versa, bottom currents
crossing deep-sea turbidite systems may induce a lateral shift of the channel-levee complexes. Mulder
et al. (2008) outline the variable aspects of these alternating and interacting processes and the resulting drift
geometries.
Contourite sediment facies
Depending on the flow characteristics and on local sediment sources, a large variety of contourite facies occurs
in modern oceans (Stow et al., 1996, 2002a; Stow and
Faugères, 2008). The grain size of contourites varies from
fine muds, through silts and sands, to sand and gravel-lag
components. The composition of the sediment can be terrigenous siliciclastic, biogenic (calcareous and siliceous),
volcaniclastic, and chemogenic (manganiferous, phosphatic). Biogenic components typically comprise whole
tests or shell fragments of foraminiferas, radiolarians, diatoms, spicules, and coccoliths. The different grain-size
fractions and diverse materials are often poorly sorted
mixtures. The rare contourite deposits identified in the fossil record are mainly carbonate-rich contourites (Hüneke
and Stow, 2008; e.g. Hüneke 2013).
It is the bottom-current velocity that mainly governs the
depositional processes and resulting sediment facies
(Faugères and Mulder, 2011). Low-velocity flows allow
vertical setting of fine-grained suspended particles from
the nepheloid layer, whereas intermediate-velocity flows
induce more bed load transport and deposition of silt and
sand. High-velocity flows can affect still larger grain sizes
and generate large-scale winnowing and erosion, resulting
in coarse-grained lag deposits. Dissolution and
authigenesis can accompany the current-induced processes, generating very low sedimentation rates and sediment bypassing.
The large variety of contourite facies typically displays
(a) abundant bioturbational sedimentary structures,
(b) bad preservation of hydrodynamic sedimentary structures, and (c) an irregular vertical variation in grain size.
Burrowing animals intensely colonize contourite sediments due to their low accumulation rates and to the generally well-oxygenated (cold) water masses prevailing
during bottom-current-induced deposition. Several types
of ichnofabrics can be distinguished, depending on the
prevailing current intensity (Wetzel et al., 2008). Hydrodynamic (small-scale) sedimentary structures are scarce
or completely lacking, being obliterated by the permanent
bioturbation, except when the sedimentation rate exceeds
the bioturbation rate (Martin-Chivelet et al., 2008). Consequently, the vertical variation in grain size and sediment
facies, which typically displays a repeated superposition
of coarsening-upward and fining-upward units, may be
difficult to recognize.
The facies model for contourites typifies the vertical
grain-size variation of a standard mud-silt-sand contourite
sequence, which results from variation in contour-current
velocity (Faugères et al., 1984; Stow et al., 2002a; Stow
and Faugères, 2008). A complete sequence of any particle
composition consists of five principal divisions:
(C1) lower muddy contourite division, (C2) lower mottled
silty contourite division, (C3) middle sandy contourite
division, (C4) upper mottled silty contourite division,
and (C5) upper muddy contourite division. Traction structures, including rare cross-lamination, are more evident in
the coarse silts and sands than in finer-grained facies.
There may be an indistinct and discontinuous parallel lamination, and lenses of coarser materials may occur in all
intervals.
The centimeter- to decimeter-thick successions of the
standard sequence may be truncated. In that case, some
divisions are missing, and erosional-sharp contacts or
nondeposition surfaces are present between the facies
units (typically at division C3). In fact, partial sequences
130
CONTOURITES