(Fig. 6.14, profiles 6, 7). These facies occur locally on the inner-middle shelf, off the
river mouths in the Iberian (Ercilla et al. 1994; Hernández Molina et al. 1993;
Bárcenas 2012) and African (El Moumni 1994) shelf, down to a depth of 70 m,
and have a lobate shape in plan-view (Bárcenas 2012). The age obtained for the
prodelta sediments analysed is about 1500 years BP at 25 cm core depth.
The continental slope, base of slope, and basin facies model consists of
contourites, turbidites, and mass-flow deposits. The turbidite facies (3) form vertical
succession of three textural types (sand, silt, and clay) displaying fining-up
sequences (Fig. 6.14, profile 6). These correspond to divisions of the Bouma
sequence (Ta to Te). The cores show turbidite sand and silt layers (Ta-d) alternating
with mud layers (Te) in the turbidite systems of the Iberian margin (Bozzano et al.
2009; Ercilla et al. 2019). They comprise terrigenous components and, in a minor
proportion, the sand fraction contains bivalves and gastropods, similar to those
described on the continental shelf (Ercilla et al. 1994). The coarsest-grain size occurs
in channel environments, while the finest-grain sizes are in the inter-canyon and
levee areas. The mass-flow facies (4) form vertical successions of sands and gravels,
displaying homogenous sequences with lithic clasts. These are found on the Ceuta
and Guadiaro Canyon floors (Ercilla et al. 2019). Coarse and fine contourite facies
(5) have been characterised. The coarse contourite facies have been sampled from
the shelf-break down to about 360 m water depth, on the erosive surface defining the
contouritic terrace, on the uppermost Iberian continental slope (Fig. 6.15, profiles
1, 5). There, these facies comprise successions of poorly sorted sand up to 73 cm
thick with sparse gravel-size biogenic clasts (shell fragments). The sand fraction of
this sediment is terrigenous, mainly comprising subrounded and rounded grains of
glauconite (44%) mixed with bioclastic debris (30%). This debris is mainly composed of fragments of molluscs that look old due to their greyish colour and because
they show mechanical and chemical abrasion with loss of ornamentation. The fine
contourite facies form successions of different textures from sandy muds, muds, and
silty clays with terrigenous and mixed components (Fig. 6.15 profiles 1 to 7). The
vertical succession produces a bi-gradational sequence, with coarsening-up followed
by fining-up, and corresponds to the five sediment divisions (C1 to C5) of Stow and
Faugères (2008). In addition, there are truncated sequences formed by C1, C3–C4–
C5, and C4–C5 (Fig. 6.14). The thickness and vertical/spatial distribution of the
contourites indicate that muds (C1 or C5) are predominant, interrupted by levels of
sandy muds (C2 or C4). The coarse-grained intervals occur more frequently in the
southern and western sectors of the Ceuta Drift (Ercilla et al. 2002). On the other
hand, the summits of the topographic highs of Tofiño, Xauen, and the Alboran
Ridge are surficially covered by nearshore sediments composed mostly of gravels
and sands of bryozoans, coralline algae, mollusc fragments, echinoid spines, and
foraminifera (benthic and planktonic) with abundant maerl (also known as rhodolith;
Bárcenas et al. 2001). The ages obtained for these sediments are around 13,430 to
3085 years (Fig. 6.14, profile 6).
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G. Ercilla et al.
river mouths in the Iberian (Ercilla et al. 1994; Hernández Molina et al. 1993;
Bárcenas 2012) and African (El Moumni 1994) shelf, down to a depth of 70 m,
and have a lobate shape in plan-view (Bárcenas 2012). The age obtained for the
prodelta sediments analysed is about 1500 years BP at 25 cm core depth.
The continental slope, base of slope, and basin facies model consists of
contourites, turbidites, and mass-flow deposits. The turbidite facies (3) form vertical
succession of three textural types (sand, silt, and clay) displaying fining-up
sequences (Fig. 6.14, profile 6). These correspond to divisions of the Bouma
sequence (Ta to Te). The cores show turbidite sand and silt layers (Ta-d) alternating
with mud layers (Te) in the turbidite systems of the Iberian margin (Bozzano et al.
2009; Ercilla et al. 2019). They comprise terrigenous components and, in a minor
proportion, the sand fraction contains bivalves and gastropods, similar to those
described on the continental shelf (Ercilla et al. 1994). The coarsest-grain size occurs
in channel environments, while the finest-grain sizes are in the inter-canyon and
levee areas. The mass-flow facies (4) form vertical successions of sands and gravels,
displaying homogenous sequences with lithic clasts. These are found on the Ceuta
and Guadiaro Canyon floors (Ercilla et al. 2019). Coarse and fine contourite facies
(5) have been characterised. The coarse contourite facies have been sampled from
the shelf-break down to about 360 m water depth, on the erosive surface defining the
contouritic terrace, on the uppermost Iberian continental slope (Fig. 6.15, profiles
1, 5). There, these facies comprise successions of poorly sorted sand up to 73 cm
thick with sparse gravel-size biogenic clasts (shell fragments). The sand fraction of
this sediment is terrigenous, mainly comprising subrounded and rounded grains of
glauconite (44%) mixed with bioclastic debris (30%). This debris is mainly composed of fragments of molluscs that look old due to their greyish colour and because
they show mechanical and chemical abrasion with loss of ornamentation. The fine
contourite facies form successions of different textures from sandy muds, muds, and
silty clays with terrigenous and mixed components (Fig. 6.15 profiles 1 to 7). The
vertical succession produces a bi-gradational sequence, with coarsening-up followed
by fining-up, and corresponds to the five sediment divisions (C1 to C5) of Stow and
Faugères (2008). In addition, there are truncated sequences formed by C1, C3–C4–
C5, and C4–C5 (Fig. 6.14). The thickness and vertical/spatial distribution of the
contourites indicate that muds (C1 or C5) are predominant, interrupted by levels of
sandy muds (C2 or C4). The coarse-grained intervals occur more frequently in the
southern and western sectors of the Ceuta Drift (Ercilla et al. 2002). On the other
hand, the summits of the topographic highs of Tofiño, Xauen, and the Alboran
Ridge are surficially covered by nearshore sediments composed mostly of gravels
and sands of bryozoans, coralline algae, mollusc fragments, echinoid spines, and
foraminifera (benthic and planktonic) with abundant maerl (also known as rhodolith;
Bárcenas et al. 2001). The ages obtained for these sediments are around 13,430 to
3085 years (Fig. 6.14, profile 6).
184
G. Ercilla et al.