LESSONS FROM SEA-FLOOR AND SUBSEA-FLOOR IMAGERY OF THE
BIG’95 DEBRIS FLOW SCAR AND DEPOSIT
G. LASTRAS, M. CANALS, R. URGELES
GRC Geociències Marines, Dept. d’Estratigrafia, Paleontologia i Geociències Marines,
Universitat de Barcelona, E-08028, Barcelona, Catalonia, Spain
Abstract
Sea-floor (multibeam bathymetry, multibeam-derived backscattering, and side-scan sonar
data) and subsea-floor (very-high-resolution seismic reflection profiles) imagery has been
obtained in a series of surveys from the BIG’95 debris flow source area and deposit in the
Ebro continental slope and rise, Northwestern Mediterranean sea. Side-scan sonar imagery
provides valuable information on the morphology of the headwall scar, bathymetry and
backscattering are useful for imaging the deposit, while seismics show the internal structure
and relationship with the underlying sediments. Together with cores obtained in the area,
the existing data set for the BIG’95 is a powerful tool for the study of the morphology and
dynamics of debris flows and for their further modelisation.
Keywords: Debris flow, swath bathymetry, seismics, sidescan,
Northwestern Mediterranean
1. Introduction
A 2,000 km2 sedimentary deposit of mostly transparent seismic facies covers part of the
southern Ebro continental slope and base-of-slope at depths ranging from 600 to almost
2,000 m (Lastras et al., 2002). Named BIG’95 after the survey in which it was discovered,
it was attributed to a debris flow deposit following Mulder and Cochonat’s (1996) criteria.
It represents the largest mass-wasting deposit in the Ebro margin, and one of the largest in
the Western Mediterranean, together with the western Gulf of Lions debris flow (Canals,
1985; Berné et al., 1999), the Rhone deep-sea fan debris flow (Droz, 1983) and the
Balearic abyssal plain mega-turbidite (Rothwell et al., 1998). It is overlying the PlioceneQuaternary sequence, thus representing the youngest sedimentary event of the margin. It is
located offshore the city of Castelló and off the volcanic Columbretes Islets, between
39º30’N and 40º10’N, and 0º55’E and 1º55’E, amounting up 2,200 km2 of affected seafloor (Fig. 1). The volume of the deposit has been estimated to be at least 26 km3.
Accelerator mass spectrometer 14C dating of the debris flow deposit yields a consistent
minimum age of ca. 11,500 cal. yr. B.P. (Lastras et al., 2002).
This paper presents part of the study of the data set obtained from the BIG’95 debris flow
source area and deposit, focused on side-scan sonar imagery, along with swath bathymetry
and seismic records, which provide important insights on the dynamics of debris flows in
river-fed margins such as the Ebro continental slope.
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1.1 DATA SET
Five surveys have been carried out up to date to study the BIG’95 debris flow area: BIG-95
(R/V Hesperides, 1995), CALMAR (R/V L’Atalante, 1997), MATER-2 (R/V Hesperides,
1999), TTR-11 BIGIMAGES (R/V Professor Logachev, 2001) and GMO-2 (R/V Le Suroit,
2002).
Data used in this study includes swath bathymetry, obtained using EM-12S in 1995, EM12Dual in 1997, EM-1002 in 1999 and EM-300 in 2002, and covering more than 13,000 km2
of the seafloor. Side-scan sonar data were obtained using 30 kHz TOBI in 1995 and 30
kHz MAK-1M in 2001, for a total coverage of 4000 km2. Very-high resolution seismic data
were obtained in 1995 and 1999 (3.5 kHz TOPAS), 1997 (3.5 kHz mud penetrator), and
also by means of integrated deep-towed chirp systems (7 kHz and 5 kHz records for TOBI
and MAK-1M respectively), totalising more than 5,000 km of profiles.
1.2 GEOLOGICAL SETTING
The Valencia Trough extensional basin developed in the Late Oligocene-Early Miocene
and was almost completely opened at 10 Ma (Fernandez et al., 1995; Gueguen et al.,
1998). It is bounded by the Balearic Islands to the southeast, the Ibiza promontory to the
south and the Catalan and Ebro continental margin to the northwest; and it opens to the
northeast to the Provençal Basin (Fig. 1). At present, the Valencia Trough is incised in its
axis by the Valencia Channel (Canals et al., 2000).
The siliciclastic, passive, Ebro continental margin displays a 70 km wide shelf and a 10 km
narrow slope with a mean gradient of 4º, its shelf break being located at a mean depth of
130 m (Dañobeitia et al., 1990). Its growth patterns during Late Pleistocene-Quaternary
were controlled by glacioeustatic sea-level oscillations, subsidence and changes in
sediment supply (Farran and Maldonado, 1990). The Ebro slope is cut by several
submarine canyons often slightly incised into the shelf (Fig. 1). The base of slope is
occupied by channel-levee complexes, inter-channel areas, and debris flow and apron
deposits, forming the Ebro Turbidite System (Nelson and Maldonado, 1988; Field and
Gardner, 1990). There are several volcanic structures in the region (i.e., Ryan et al., 1973),
including the Columbretes Islets (Fig. 1), a volcanic archipelago in the Ebro outer shelf,
which is the topographic expression of a large, mostly buried volcanic field (Maillard and
Mauffret, 1993).
2. Results
Based on swath bathymetry data and TOPAS profiles, Lastras et al. (2002) distinguished
four regions within the debris flow: the source area, the proximal depositional area, the
intermediate blocky depositional area and the distal depositional area. TOBI side-scan
sonographs cover most of the source area and parts of the other three regions (Fig. 1),
while MAK-1M sonographs image the headwall scar almost completely (Fig. 2).
Lessons from BIG’95 debris flow imagery
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Figure 1. Bathymetric map (contours every 50 m) showing location of BIG’95 debris flow in the Nortwestern
Mediterranean (inset) and coverage of 1995 TOBI survey. Dashed line limits debris flow deposit. Location of
Columbretes Islets, Balearic slope and Valencia Channel is also labelled. Note the morphology of the margin, with
submarine canyons incised into the outer shelf, channel-levee complexes and inter-channel areas.
2.1 SOURCE AREA
The source area, at water depths from 600 to 1,200 m, is located between two prominent
canyon-channel systems in a segment of the Ebro slope apparently rejuvenated compared to
the rest of the margin. Within the source area, the BIG’95 debris flow headwall scar and
other secondary scars can be identified, each of them displaying different characteristics.
The headwall scar has a straight total length of about 20 km, it is up to 200 m high and has
a mean slope angle of 17º. It displays a SE-opened horseshoe shape, with its south-western
end being less prominent than the north-eastern one. Both TOBI and MAK-1M sonographs
image in detail the morphology of the headwall scar, and how it changes with height and
average slope of each section of the scar (Figs. 2 and 3).
In its easternmost section, the headwall scar displays a staircase morphology, where the
height jump is resolved by several up to 10 m high single steps, usually between 6 and 12
of them, their number decreasing to the east. In its central section it is made of a single
smooth and regular slope. This section includes an abrupt change in the geometry of the
scar, with an up to 100 m high promontory at about 1º00’E, also displaying a regular slope.
The linking between the middle regular and the eastern staircase section is a mixed section
where the upper part of the scar is made of 3 to 10 high single steps and the lower one is
made of a regular slope (Fig. 3). Finally, the westernmost section, where total height
decreases, consists of two to three single steps <50 m high.
The changes in scar morphology probably reflect the amount of failure, related to the
relative orientation of the scar compared to the direction of the flow. In the western section,
flow direction was almost perpendicular to the scar and thus sediment was easily detached
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Figure 2. Mosaic of TOBI side-scan sonographs in the headwall scar area. Note the staircase morphology of the
scar east of 1º30’E. Northeast trending stripes are acquisition artifacts. Arrows show ensonification direction, and
dashed lines show where this direction changes. Light areas represent reflections whereas dark areas are shadows.
from it, while in the eastern section, where the flow direction was oblique, some failed
blocks remained attached to it. In the central section, where the scar is steeper and higher,
further processes of instability (i.e., Fig. 2 at 1º00’E) have reworked the scar, smoothing it
to its present morphology. These processes have also affected partially the eastern section.
High resolution 5 kHz profiles clearly show the debris flow deposit as a surficial
transparent unit with a variable thickness of 6 to 20 m with irregular surface relief
overlapping a unit made of a series of parallel continuous reflectors with high amplitude,
frequency and lateral continuity (Fig. 4). The debris flow deposit partially ponds the
depressions originated by the scar.
2.2 DEPOSITIONAL AREAS
MAK 1-M side-scan sonar mosaics of the intermediate depositional area of the BIG’95
display a pattern of linear depressed areas surrounding topographically elevated blocks
(Fig. 5), as shown also in swath bathymetry backscatter maps (Lastras et al., 2002). Blocks
have an irregular morphology, whereas depressed areas are flat-bottomed. Blocks appear as
piles or lumps of sediment partially remoulded but still keeping part of their internal
coherence compared to the material in the depressions, which was totally remoulded and
probably liquefied during downslope transport. This is also shown in TOBI sonographs,
where the block and depression pattern can be observed in the southeastern line (Fig. 1),
although the fact that only one line was obtained in this area makes its interpretation
difficult.
Lessons from BIG’95 debris flow imagery
Figure 3 Mosaic of MAK-1M side-scan
sonographs from the headwall scar.
Different morphologies explained in the text
are labelled. East trending stripes are
acquisition artifacts. Black arrows show
ensonification direction, dotted lines show
where the direction changes. Light areas
Figure 4. 5 kHz sub-bottom profile across the
BIG’95 debris flow headwall scar. Note that, due
to the shape of the scar, the profile crosses it
three times (bold arrows). Transparent unit at the
top is labelled T. Location in Fig. 3.
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Two kinds of material (sediment forming the blocks and sediment forming the depressions)
are then present in the depositional area of the BIG’95 debris flow, apparently of different
mechanical properties and also with different granulometries (Lastras et al., 2002). The
presence of these two materials is determined by the existence of two different particular
source areas, and the pattern that these materials show was determined by the dynamics of
the debris flow and their specific sedimentological and geotechnical properties (Urgeles et
al., 2002).
TOBI records in the upper Valencia Channel show how it becomes less incised and almost
vanishes (Fig. 1), while 7 kHz profiles image a transparent unit covering it where affected
by the distal depositional area of the BIG’95 debris flow deposit.
Figure 5. Mosaic of MAK-1M side-scan sonographs in the intermediate blocky depositional area. Blocks and
depressions are clearly imaged, as explained in the text. East trending stripes are acquisition artifacts. Arrows show
ensonification direction, dotted line show where the direction changes. Light areas represent reflections whereas
dark areas are shadows. Location in Fig. 1.
3. Conclusions
TOBI and MAK-1M side-scan sonars and integrated deep-towed chirp systems are useful
tools to image the BIG’95 debris flow scar and deposit at different scales and resolutions.
Interpretation of the images yield important insights on the slided mass from its release to
its final deposition.
In particular, sonograph mosaics from the headwall scar area allow to distinguish different
sections showing a variety of morphologies in the scar. Information provided by these
systems, along with sedimentological and geotechnical data are to be integrated and used to
verify numerical models on debris flow dynamics.
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4. Acknowledgements
This study was supported by projects COSTA (EVK3-1999-00028) and GRANDES
(MAR98-0347), Generalitat de Catalunya grant 2001 SGR-00076, and by a Spanish
MECD fellowship (Galderic Lastras). The manuscript benefited from revisions by M. de
Batist and J. Locat.
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