SUBMARINE MASS MOVEMENT ON THE EBRO SLOPE
D. CASAS, G. ERCILLA, B. ALONSO, J. BARAZA
CSIC, Institut de Ciencies del Mar. Passeig Marítim de la Barceloneta, 37-49 08003
BARCELONA, SPAIN
H. J. LEE
U.S. Geological Survey (USGS), Mail Stop 999, 345 Middlefield Road, Menlo Park, CA
94025, USA
A. MALDONADO
Instituto Andaluz de Ciencias de la Tierra (CSIC), Facultad de Ciencias, 18002
GRANADA, SPAIN
Abstract
Mass movement is an important process controlling the Quaternary sedimentary
structure of the Ebro continental slope. Seismic indicates that about 37% of the slope
surface is affected by mass movement features, which are variable in distribution, type,
size and morphology. Physical and geotechnical properties define two areas: upper slope
and the lower slope. The geotechnical modelling only explains certain aspects of massmovement features, but it is insufficient to explain their variability. In order to have a
good knowledge of geotechnical and sedimentary characteristics of the area and a good
approach to a future mapping of instability hazard it is required study individually each
depositional environment and failure event.
Keywords: Ebro, Continental slope, Submarine mass movements, geotechnical
modelling.
1. Introduction
The Ebro continental margin, in the Spanish NW Mediterranean Sea, is a NE-SW
trending passive and prograding margin initiated during the late Oligocene (Dañobeitia,
Alonso, & Maldonado, 1990). The Quaternary stratigraphic architecture of the shelf is
mainly made up of regressive deltas that extends to the continental slope (Farran &
Maldonado, 1990). The continental slope is characterised by the formation of submarine
canyons due to the action of mass movement processes (Farran & Maldonado, 1990;
Alonso, Díaz, Farran, Giró, Maldonado & Vázquez, 1984), and the base-of-slope by the
formation of turbidite channel-levee complexes (Alonso & Maldonado, 1990). These
sedimentary systems were mainly developed during the sea-level falls and lowstand
stages (Alonso & Maldonado, 1990). Low energy, hemipelagic sedimentation prevailed
over the entire margin during the high sea-level periods like the present (Alonso &
Maldonado, 1990; Baraza et al., 1990; Nelson & Maldonado, 1990). Today, the Ebro
continental slope is not an active depositional environment because most of the
sediment supplied by the river is trapped on the dams upstream and in the deltaic
environment (Palanques, Plana & Maldonado, 1989).
393
394
Casas et al.
Figure 1. Map of location of the study area and bathymetry of Ebro margin (grey lines) showing the three
geographic sectors (northern, central and southern) defined on the slope, and the different mass-movement
features seismically identified on the upper (200 to 500 m water depth) and lower (500 to 1300 m water depth)
slope. Black plots represent the position of sediment cores used for the geotechnical study.
Morphologically, the Ebro continental slope is a narrow (< 25 km) band with an average
gradient of 4.5º and extends from the shelf break at 160±20 m down to 1100 ± 200 m
water depth (Fig.1). It is characterised by the presence of numerous morphological
irregularities mostly in the form of gullies, short and straight submarine canyons and
intervalley banks (Field & Gardner, 1990). The Ebro continental slope has been divided
into three sectors, northern, central an southern, which are characterised by a relative
decreasing width and increasing slope gradients from north to south, and a more
important development of submarine canyons in the central sector (Fig.1).
Submarine Mass Movement on the Ebro Slope
395
2. Types of mass movement features
The existence of recent mass movement features on the Ebro continental slope can be
recognised, from a seismic point of view, based on elements indicative of the
disappearance of sediment packages and by depositional bodies formed by remobilised
sediment. The disappearance of sediment packages comprises erosive surfaces
associated to submarine canyons, and slide scars. The remobilised sediment comprise
different types of mass-movements, from slide to mass-flow deposits. The elements
indicative of mass-movement features are grouped into two groups based on their
environmental locations: (1) erosive surfaces and slides associated to submarine
canyons, and (2) slides and mass-flow deposits on the open continental slope.
2.1. EROSIVE SURFACES AND SLIDES ASSOCIATED TO SUBMARINE
CANYONS
Submarine canyons are the main morphologic features shaping the Ebro continental
slope and their erosive character shows the disappearance of sediment packages. The
seismic signatures that reveal this disappearance are: a) truncation of reflectors against
the canyon walls; b) several phases of scour and fill features on the canyon floor; and c)
presence of gullies (160 to 400 m wide, and 25 to 75 m relief) on the canyon heads and
walls, mostly on their upper courses (225 to 440 m water depth).
Slides are identified along the courses of the canyons, on their walls and axes. They
have dimensions varying between 20 and 80 ms in thickness, and from hundreds of
meters to a few kilometres in length. Most of the slided sediments lie adjacent to their
respective scars. The presence of the slides originates an asymmetric image of the
canyons in cross sections.
2.2. SLIDES AND MASS-FLOW DEPOSITS ON THE OPEN CONTINENTAL
SLOPE
They occur on the entire Ebro continental slope (Fig. 1). The slides are mostly observed
as morphologic relieves on the seafloor, although they are also identified in the
subbottom reflectors. They have a large scale range, varying from a few km to tens of
km in length.
The slides with the smaller dimensions occur mostly along the upper slope. They consist
of sigmoidal and lenticular packages, up to 100 ms thick that extend downslope for < 4
km distances. Some of these slides lie adjacent to steep scars (>100 ms vertical offset)
against which the upper reflections terminate sharply as result of truncation by failure.
Locally, isolated slide scars are also identified.
Two large-scale submarine slides also affect the Ebro continental slope: the
Columbretes and the Torreblanca slides located on the southern sector and southernmost
part of the central sector respectively (Fig.1). The Columbretes slide is rooted on the
upper slope at 170 m water depth and ends near the base-of-slope, at approximately
1100 m water depth (Martinez del Olmo, 1984). The slide scar extends from 170 to 454
m water depth and it is characterised by a steep (> 4º) slope surface that truncates the
prograding deposits of the shelf break. The slide extends 10 km on a downslope
direction and around 20 km across the slope and it is about 150 ms (≈ 113 m) thick of
average, which gives a total volume of 23 km3. The thickness of the main body of the
396
Casas et al.
slide displays the tendency to decrease toward the northeast in sections along slope
(from 150 to 80 ms). In spite of this slides appears as structureless and acoustically
chaotic body, the recognisable scar and failure plane points out to consider it as a slump
mass (irregular, chaotic) and nor as a mass flow deposits.
The Torreblanca slide has been identified on the southernmost part of the central sector
(Fig.1) and extends down to the base-of-slope (at least 1350 m water depth). The slide
scar occurs at about 360 m water depth and has a vertical offset of approximately 300
m. Between the scarp and the main body of the slide at 655 m depth, there is a small vshaped morphologic feature. This feature corresponds to a tensional depression,
characteristic of many submarine landslides. The main body of the Torreblanca slide has
a length at least up to 40 km, it displays a longitudinal lens-shape section, and it is
seismically defined by discontinuous stratified reflections locally affected by chaotic
and hyperbolic facies. The slide involves sediment up to 300 ms thick, affecting a
channel-levee complex, and displays a rough upper surface. The upper surface of the
sliding mass is rough with features of different genesis. On the most proximal areas,
tensile features are developed, and they correspond to normal faults or antithetic faults,
and tilted blocks. On the more distal areas, compressional features are developed, such
as folds or thrusts, giving to the slided sediment a hummocky morphology. The failure
plane of the Torreblanca slide is observed at depths between 90 and 260 m below the
top surface of the slide. It appears as a plane quite continuous, concordant with the
stratification, highlighted by parallel reflections but with high roughness.
Depositional bodies formed by mass-flow deposits have been also recognised and they
also display variability in their scale. Canals et al. (2000) have recognised a big mass
flow deposits (named BIG’95) that extends from the slope to base of slope covering an
area of 2000 km2. It has a recent age, between 10430 and 10250 ybp (Willmot et al,
2001). The small-scale mass flow deposits are characterised by chaotic facies with a
lenticular shape up to 0.26 s thick and 8 km long. They usually show a concave-upward
base of high reflectivity, a very irregular upper surface.
3. Geotechnical modelling
Geotechnical studies were done by Baraza et al. (1990) based on surficial cores (<3 m
long) on the central and southern sectors (Figs.1 and 2). Those studies define two
different areas on the basis of physical and geotechnical properties: the upper slope
(water depth les than 500m) and the lower slope (water depth greater than 500m). On
the upper slope, prodeltaic mud with a high silt content and a low to moderate sand
content are dominant (Fig. 2). The average water content is 33% dry weight, slightly
below the liquid limit, which is about 34%. The plasticity index is about 15% and the
sediment is highly to moderately overconsolidated (OCR as high as 8).On the lower
slope predominates hemipelagic sediment that has a lower sand and silt content than that
of the upper slope (Fig. 2). In addition, the water content is higher (approaching 90%)
and is above the liquid limit (ranging from 55 to 75%), the plasticity index is higher and
the degree of overconsolidation is lower (OCR 2-3). Vane shear strength and the
normalized strength parameter S (ratio of strength to consolidation stress) for normal
consolidation are both lower, whereas the cyclic strength degradation factor, Ar, is
higher than that of the upper slope.
Submarine Mass Movement on the Ebro Slope
397
According to the geotechnical results, the prodeltaic and winnowed deposits of the
upper slope are slightly more stable under undrained static loading conditions relative to
the hemipelagic deposits of the lower slope. The lower slope is more stable under
drained or very long term static conditions. Maximum slopes in our study area appear to
be stable under static (gravitational) loading. Nevertheless, localized instability might be
produced by a combination of oversteepening and infrequent, intense seismic loading.
Comparing the Ebro slope with others seismic areas like California or Alaska, where
we know the critic seismic acceleration, by a comparable situation the sediment of the
Ebro slope could be unstable between 200 and 700 m depth if the slope gradient was
between 5º and 10º.
4. Discussion
4.1. DISTRIBUTION AND VARIABILITY OF MASS MOVEMENT FEATURES
Mass-movement is a common processes in the Ebro slope at recent times, both in
submarine canyons and open slope environments. In fact, about 37 % of the Ebro slope
is affected by these features, which show variability in their distribution, type, size and
morphology. The canyons are mostly located on the central sector. The small-scale
mass-movements occur in the form of slides on the upper slope and in the form of massflow deposits on the lower slope in the three sectors. The large-scale slides and massflow deposits extends from the upper to lower slope mainly in the southernmost part of
the central sector and southern sector.
This variability in distribution, type, scale and morphology seems to be controlled by
different factors: sedimentary, physiographic/morphologic and tectonics. We can
tentatively consider that the relative higher sediment supply received by the central
slope sector (Farran & Maldonado, 1990) plus its oversteeping favoured a relative major
occurrence of failures on that sector, where the canyons are bigger and numerous. Those
factors also seem to have conditioned the occurrence of small scale mass-flow deposits;
they formed from mass movements on the thick regressive prograding deltas. The long
travelled distances and their remoulded stage could be explained by the relative higher
gradients of the open slope plus their open conditions without the presence of gullies or
canyons interrupting their travelling pathway. Likewise, the sedimentary factor has
conditioned the location of the small-scale and slide scars on the upper slope; in fact,
their occurrence is associated to the formation of regressive deltas and they can be
considered as delta front failures. On the other hand, tectonics and morphology have
governed the formation and location of the large-scale slides and mass-flow deposits.
The earthquake activity related to the emplacement of volcanic edificies (Field &
Gardner, 1990) and the lack of canyons favoured the Columbretes, Torreblanca and BIG
formation. In the case of the BIG mass flow deposits, its age also point to a sedimentary
controlling factor related to a lowstand position of sea-level in that time.
4.2. COMPARISON BETWEEN GEOTECHNICAL AND SEISMIC MODELLINGS
The geotechnical modelling proposed by Baraza et al. (1990) could explain certain
aspects of the mass-movement features on the Ebro slope. It is a good proxy to start
398
Casas et al.
understanding what are the general physical and geotechnical properties of the sediment
present in that area and its probable behaviour. In fact, the proposed geotechnical
zonation could explain why most of debris flow deposits are concentrated in the lower
slope, since there the sediment has a higher plasticity than the sediment present in the
upper slope.
Figure 2. Sedimentary facies defined in the upper and lower slope. Core logs show lithology and idealised
representation of sedimentary structures observed in X-radiographs.
Likewise, the geotechnical zonation could help to understand why the large scale slides
have their scars on the upper slope; this is because between 200 and 700 m water depth
the slope would the more susceptible area to failure triggered by a seismic loading.
Submarine Mass Movement on the Ebro Slope
399
But this model is certainly insufficient to explain the variability of settings, types,
scales, and geometries of the mass movement features defined on the seismic profiles.
The seismic analysis suggests a greater diversity of depositional environments (lower
and upper slope, northern-central-and southern sectors, canyon and open slope
environments) than the reflected by the geotechnical modelling (upper and lower slope).
This fact suggests that the distribution and variability of mass movement features and
their probable triggering mechanisms should be studied individually from a geotechnical
point of view in order to know local conditions of stability or failure. This would be a
good way to know why one region of seafloor remains intact whereas the neighbouring
sector fails and why it fails in the way that it does.
Besides the above mentioned, recent new data collected from piston cores on the
BIG’95 mass flow deposits (Willmot et al, 2001) show differences in the physical
property referring to water content with respect to that obtained in the geotechnical
modelling. The water content measured in the cores distributed along the BIG'95 shows
a general reduction about 43 % respect to the mean value obtained by the geotechnical
modelling in the surrounding area. In consequence, if index properties of sediment are
conditioned by water content in upper and lower zonation (Baraza, 1989), the behaviour
of this part of slope could be significantly different, enhancing the necessity of studying
individually the different seismically observed mass-movement features.
Another point unresolved by the geotechnical modelling is the behaviour during the
failure and post-failure stages of different mass movement features, even the possible
re-activation of the failed masses. The two large slides, Columbretes and Torreblanca,
are in an equivalent position from a point of view of the geotechnical zonation, but they
display a different seismic and morphologic features. The internal structure of
Torreblanca slide indicates that sediment is slightly disturbed and that its downslope
transport only caused rumpling and folding. The slab slide seems to exist as a geometric
entity, whereas at the Columbretes slide a progressive fragmentation seems to occur
during the downslope movement. The lack of bedding in the Columbretes slide indicates
that deposits have been displaced, distorted and mixed as the slide moved downslope.
The variable depth of the failure plane (shallower northeastern) beneath the seafloor and
the different stratigraphic levels that it affects, both suggest that failure is not rooted in a
single incompetent layer. The geotechnical information available can not explain at
present this different behaviour during the failure or post-failure stage for sediments a
priori equivalents.
5. Conclusions
The seismic analysis has offered indirect observations of the tectono-sedimentary
framework where the mass-movement features occur and how we see them being able
to define slide plane, internal pattern, scale of failure, slide geometry, run-out distances
etc. The geotechnical analysis has offered direct observation about index properties,
shear strength and consolidation of the slope sediments. But the obtained geotechnical
modelling has been insufficient to explain the variability of failures and depositional
environments where they occur. These results suggest that the different failure events
should be studied individually in order to know the failure dynamics and their impact on
slope instability. Likewise, it is required measurement of geotechnical parameters that
400
Casas et al.
are not available at present, such as in-situ geotechnical properties (i.e. shear strength
and pore pressures measurements), and a new collection of long cores, representative of
all described depositional environments. These data could provide a good knowledge of
geotechnical and sedimentary characteristics of the area and a good approach to a future
mapping of instability hazard and risk assessment.
6. Acknowledgements
This work was founded by the European Commission ANAXIMANDER project
(EVK3-2001-00123), and by the “Ministerio de Ciencia y Tecnología” MARCONI
(REN2000-0336-C03) project, MARSIBAL (REN2001-3868-C03) project and
ANT1999-1462-E. The support of these people and institutions is gratefully
acknowledged. David Casas thanks the Generalitat de Catalunya for the PhD grant
(1999FI 00002CSIC PG).
7. References
Alonso, B. (1986). El sistema del Abanico profundo del Ebro. Ph.D. Thesis. Universidad de Barcelona, 384.
Alonso, B., & Maldonado, A. (1990). Late Quaternary sedimentation patterns of the Ebro turbidite systems
(northwestern Mediterranean): Two styles of deep-sea deposition. In C.H. Nelson and A. Maldonado
(Eds.), The Ebro margin. Marine Geology, 95: 353-377.
Alonso, B., Díaz, J.I., Farran, M., Giró., S., Maldonado, A., & Vázquez, A. (1984). Cañones submarinos del
margen Catalán meridional: morfología y evolución. I Congreso Español de Geología, Segovia, 301311.
Baraza, J., Lee, J., Kayen, R., & Hampton, M.A. (1990). Geotechnical characteristics and slope stability on the
Ebro margin, western Mediterranean. In C.H. Nelson and A. Maldonado (Eds.), The Ebro margin.
Marine Geology, 95: 379-393.
Baraza, J. (1989). Procesos de Edificación y Características Geotécnicas del Talud Continental del Ebro. PhD
Thesis.
Canals, M., Casamor, J.L., Urgeles, R., Lastras, G., Calafat, A.M., Masson, D.G., Berné, S., & Alonso, B.
(2000). The Ebro Continental Margin, Western Mediterranean Sea: Interplay between CanyonChannel System and Mass Wasting Processes. GCSSEPM Foundation 20th Annual Research
Conference Deep-Water Reservoirs of the World. December 3-6. 152-174.
Dañobeitia, J.J., Alonso, B., & Maldonado, A. (1990). Geological framework of the Ebro continental margin
and surrounding areas. In C.H. Nelson and A. Maldonado (Eds.), The Ebro margin. Marine Geology,
95: 265-287.
Farran, M., & Maldonado, A. (1990). The Ebro continental shelf: Quaternary seismic stratigraphy and growth
patterns. In C.H. Nelson and A. Maldonado (Eds.), The Ebro margin. Marine Geology, 95: 289-312.
Field, E., & Gardener, V. (1990). Pliocene-Pleistocene growth of the Rio Ebro margin, northeast Spain: A
prograding-slope model. The geological Society of America bulletin, 102 (6): 721-733.
Martinez del Olmo, W. (1984). Un ejemplo actual y reciente de abanico turbidítico profundo: Columbretes
Fan. I Congreso Español de Geología, Segovia, 5: 53-75.
Nelson, C.H., & Maldonado, A. (1990). Factors controlling late Cenozoic continental margin growth from the
Ebro Delta to the western Mediterranean deep sea. In C.H. Nelson and A. Maldonado (Eds.), The
Ebro margin. Marine Geology, 95: 419-440.
Palanques, A., Plana, F., & Maldonado, A. (1990). Recent influence of man on the Ebro margin
sedimentation system (Northwestern Mediterranean Sea). In C.H. Nelson and A. Maldonado (Eds.),
The Ebro margin. Marine Geology, 95: 419-440.
Willmott, V., Canals, M., Lastras, G., & Casas, D. (2001). Caracterización sedimentológica de un debris flow
reciente en el margen continental del Ebro. Geotemas, 3(2): 117-120.