occur as circular and oval depressions on the seafloor, mostly as isolated features,
and they vary in size from 50 to more than 500 m in diameter, with a few tens of
metres of relief (Fig. 6.9b). The presence of these pockmarks has been attributed to
fluid seepage and interstitial fluids from the underlying sediments that in the Ceuta
Drift have been detected near the surface sediments (Mathys et al. 2005).
Other seepage fluid flow features, such as hydrocarbon-derived authigenic carbonate crusts and chimneys have been identified and sampled in the pockmark field
of the Ceuta Drift (Blinova et al. 2011), also pointing to seepage flow emissions as
these are necessary for their formation.
6.5.2.4 Contourite Features
The term contourites refers to those deposits formed by the action of alongslope
bottom currents, including wind-driven, thermohaline, geostrophic, contour, boundary, and abyssal currents (Rebesco et al. 2014). A wide spectrum of depositional
(drifts and sediment waves), erosional (moats, channels, and scarps), and mixed
(terraces) contourite features have been characterised from the shelf-break (>90 m
water depth) to the basin (Ercilla et al. 2002, 2016; Palomino et al. 2011; Juan et al.
2016) (Fig. 6.6). The drifts differ in magnitude: the largest appear along the slopes,
the base of slope, and basin domains. Plastered and sheeted drifts are the dominant
largest (a few hundreds of km long, a few to tens of km wide, and tens to a few
hundreds of m of relief) type of drifts. The Iberian and Moroccan slopes are mainly
characterised by plastered drifts. Sheeted drifts contribute to shaping the base of the
slope and infill the basin irregularities generating a roughly flat and smooth seafloor.
With respect to the small-scale drifts (a few to tens of km long and wide and a few
hundreds of metres high) they are dispersed throughout the Alboran Sea, particularly
around the different seamounts of the northern margin, the Alboran Ridge and highs,
in the narrow passages formed by the steep structural walls, within the Alboran
Trough and in the Al-Hoceima Valley. In addition to small-scale plastered and
sheeted drifts, another type of minor-scale drift includes channel-related drifts,
mounded confined drifts, and mounded, elongate and separated drifts. All drifts
show similar seismic facies and are defined by stacked layered deposits with regional
erosive unconformities. In cross-sections their stacking defines aggradational
mounds with upslope-and-downslope prograding clinoforms for the plastered, elongated, separated, an onlap-infilling pattern with subparallel, parallel and deformed
strata for the sheeted drifts, and aggrading mounds with subparallel strata for the
confined drifts and channel-related drifts (Fig. 6.6b). With respect to the sediment
waves, they have been locally mapped at the main entrance of the Strait of Gibraltar.
They comprise 3D sediment waves mostly characterized as sand waves (metric in
relief, and few to tens of metres in length) (Heezen and Johnson 1969; Kelling and
Stanley 1972).
Erosive contourite features are also dispersed along the margins and basins
(Fig. 6.6). The moats, which have U-shaped cross-sections of tens of m of relief,
are mainly mapped associated to the separated and confined drifts. Their dimensions
6 Seafloor Morphology and Processes in the Alboran Sea
175
and they vary in size from 50 to more than 500 m in diameter, with a few tens of
metres of relief (Fig. 6.9b). The presence of these pockmarks has been attributed to
fluid seepage and interstitial fluids from the underlying sediments that in the Ceuta
Drift have been detected near the surface sediments (Mathys et al. 2005).
Other seepage fluid flow features, such as hydrocarbon-derived authigenic carbonate crusts and chimneys have been identified and sampled in the pockmark field
of the Ceuta Drift (Blinova et al. 2011), also pointing to seepage flow emissions as
these are necessary for their formation.
6.5.2.4 Contourite Features
The term contourites refers to those deposits formed by the action of alongslope
bottom currents, including wind-driven, thermohaline, geostrophic, contour, boundary, and abyssal currents (Rebesco et al. 2014). A wide spectrum of depositional
(drifts and sediment waves), erosional (moats, channels, and scarps), and mixed
(terraces) contourite features have been characterised from the shelf-break (>90 m
water depth) to the basin (Ercilla et al. 2002, 2016; Palomino et al. 2011; Juan et al.
2016) (Fig. 6.6). The drifts differ in magnitude: the largest appear along the slopes,
the base of slope, and basin domains. Plastered and sheeted drifts are the dominant
largest (a few hundreds of km long, a few to tens of km wide, and tens to a few
hundreds of m of relief) type of drifts. The Iberian and Moroccan slopes are mainly
characterised by plastered drifts. Sheeted drifts contribute to shaping the base of the
slope and infill the basin irregularities generating a roughly flat and smooth seafloor.
With respect to the small-scale drifts (a few to tens of km long and wide and a few
hundreds of metres high) they are dispersed throughout the Alboran Sea, particularly
around the different seamounts of the northern margin, the Alboran Ridge and highs,
in the narrow passages formed by the steep structural walls, within the Alboran
Trough and in the Al-Hoceima Valley. In addition to small-scale plastered and
sheeted drifts, another type of minor-scale drift includes channel-related drifts,
mounded confined drifts, and mounded, elongate and separated drifts. All drifts
show similar seismic facies and are defined by stacked layered deposits with regional
erosive unconformities. In cross-sections their stacking defines aggradational
mounds with upslope-and-downslope prograding clinoforms for the plastered, elongated, separated, an onlap-infilling pattern with subparallel, parallel and deformed
strata for the sheeted drifts, and aggrading mounds with subparallel strata for the
confined drifts and channel-related drifts (Fig. 6.6b). With respect to the sediment
waves, they have been locally mapped at the main entrance of the Strait of Gibraltar.
They comprise 3D sediment waves mostly characterized as sand waves (metric in
relief, and few to tens of metres in length) (Heezen and Johnson 1969; Kelling and
Stanley 1972).
Erosive contourite features are also dispersed along the margins and basins
(Fig. 6.6). The moats, which have U-shaped cross-sections of tens of m of relief,
are mainly mapped associated to the separated and confined drifts. Their dimensions
6 Seafloor Morphology and Processes in the Alboran Sea
175