to or below the shelf-break. The occurrence of turbidity and mass flows decreased
during interglacial highstand stages (e.g. Alonso and Maldonado 1992; Ercilla et al.
1992, 1994; Chiocci et al. 1997; Estrada et al. 1997; Alonso and Ercilla 2003). The
U-shape of many canyons and the presence of sinuous talwegs on the seafloor
(Fig. 6.10) suggest alternating deposition and erosion in recent times. The energy
of the turbidity and mass flows decreases when they reach the gentle gradients at the
base of the slope and in the adjacent basin, and their sediment charge deposits
forming leveed channels and lobes, although erosion may continue along the main
and distributary channel floors.
Recent studies have indicated that turbidity flow characteristics seem to be locally
affected by the action of bottom currents running across the turbidite systems in the
westernmost Alboran Sea (Ercilla et al. 2016, 2019). AW, LMW, and DMW
contribute to the piracy of the finer sediment travelling in the upper part of the
turbidity flows, making the turbidite systems sandier (La Linea and Guadiaro) closer
to the Strait of Gibraltar. The action of the DMW also seems to have been more
intense on the African margin, probably inhibiting the formation of leveed channels
and lobes at the mouth of the Nekor and Ceuta canyons.
Respect to landslides shaping the seafloor of the Alboran Sea (Figs. 6.6, 6.11 and
6.15) the literature reveals that most of them formed during the Quaternary (Casas
et al. 2011; Martínez-García et al. 2011; Alonso et al. 2014; Rodriguez et al. 2017;
Galindo-Zaldívar et al. 2018). Their fresh morphologies indicate that their activity
has continued into recent times. Slope sediment failure occurs when their metastable
equilibrium is disturbed, causing the sediment to move downslope (e.g. Casas et al.
2011; Ercilla and Casas 2012). Although the causes of landslides are still not fully
understood, several triggering factors, acting either individually or jointly, have been
tentatively suggested for the Alboran Sea: over-steepening, seismicity, under consolidation due to overpressure by interstitial fluids, stratigraphy, and high sedimentation rates (e.g. Casas et al. 2011; Alonso et al. 2014; Rodriguez et al. 2017).
After failure, the sediment runs downslope, disintegrating, and remoulding as a
consequence of the stress and incorporation of water during the movement
(e.g. Locat and Lee 2000). Based on this process and the runout distance, the
sediment remains as a coherent mass attached to or detached from the slide scar
(Fig. 6.11). Detailed studies of the Baraza slide reveal that the type of movement
may change through time. This landslide has been affected by two types of movement since its formation during the Upper Quaternary, firstly mass flow-type,
followed by a more recent slump-type movement (Casas et al. 2011).
6.6.3 Geohazards in the Alboran Sea
The seafloor morphology offers interesting clues to help assess the main potential
geological hazards. These hazards can be grouped into two major different categories, endogenic and exogenic. The endogenic are related to tectonic processes, and
the exogenic are related to bottom current and mass-movement processes.
192
G. Ercilla et al.
during interglacial highstand stages (e.g. Alonso and Maldonado 1992; Ercilla et al.
1992, 1994; Chiocci et al. 1997; Estrada et al. 1997; Alonso and Ercilla 2003). The
U-shape of many canyons and the presence of sinuous talwegs on the seafloor
(Fig. 6.10) suggest alternating deposition and erosion in recent times. The energy
of the turbidity and mass flows decreases when they reach the gentle gradients at the
base of the slope and in the adjacent basin, and their sediment charge deposits
forming leveed channels and lobes, although erosion may continue along the main
and distributary channel floors.
Recent studies have indicated that turbidity flow characteristics seem to be locally
affected by the action of bottom currents running across the turbidite systems in the
westernmost Alboran Sea (Ercilla et al. 2016, 2019). AW, LMW, and DMW
contribute to the piracy of the finer sediment travelling in the upper part of the
turbidity flows, making the turbidite systems sandier (La Linea and Guadiaro) closer
to the Strait of Gibraltar. The action of the DMW also seems to have been more
intense on the African margin, probably inhibiting the formation of leveed channels
and lobes at the mouth of the Nekor and Ceuta canyons.
Respect to landslides shaping the seafloor of the Alboran Sea (Figs. 6.6, 6.11 and
6.15) the literature reveals that most of them formed during the Quaternary (Casas
et al. 2011; Martínez-García et al. 2011; Alonso et al. 2014; Rodriguez et al. 2017;
Galindo-Zaldívar et al. 2018). Their fresh morphologies indicate that their activity
has continued into recent times. Slope sediment failure occurs when their metastable
equilibrium is disturbed, causing the sediment to move downslope (e.g. Casas et al.
2011; Ercilla and Casas 2012). Although the causes of landslides are still not fully
understood, several triggering factors, acting either individually or jointly, have been
tentatively suggested for the Alboran Sea: over-steepening, seismicity, under consolidation due to overpressure by interstitial fluids, stratigraphy, and high sedimentation rates (e.g. Casas et al. 2011; Alonso et al. 2014; Rodriguez et al. 2017).
After failure, the sediment runs downslope, disintegrating, and remoulding as a
consequence of the stress and incorporation of water during the movement
(e.g. Locat and Lee 2000). Based on this process and the runout distance, the
sediment remains as a coherent mass attached to or detached from the slide scar
(Fig. 6.11). Detailed studies of the Baraza slide reveal that the type of movement
may change through time. This landslide has been affected by two types of movement since its formation during the Upper Quaternary, firstly mass flow-type,
followed by a more recent slump-type movement (Casas et al. 2011).
6.6.3 Geohazards in the Alboran Sea
The seafloor morphology offers interesting clues to help assess the main potential
geological hazards. These hazards can be grouped into two major different categories, endogenic and exogenic. The endogenic are related to tectonic processes, and
the exogenic are related to bottom current and mass-movement processes.
192
G. Ercilla et al.