THE VAR TURBIDITIC SYSTEM: SEDIMENT SUPPLIES, SLOPE INSTABILITIES AND MASS WASTING
Sébastien Migeon
1
*, Nabil Sultan
2
, Olivier Sardou
1
, Bruno Savoye
2
1
Géosciences Azur, Observatoire de Villefranche, Villefranche/Mer, France
2
Lab. Env. Sédim., IFREMER, Plouzané, France
Abstract
The Var turbidite system is a small sandy system located on the Ligurian margin and basin. Still active during the present sea-level
highstand, the system has been fed through time by the Var and Paillon canyons. Three major processes allow the transport and distribution
of particles from the shelf to the deep basin: (a) large-sized failure-induced turbidity currents, (b) small-sized turbidity currents, (c) and
hyperpycnal currents triggered at the Var-river mouth during high-magnitude ?oods. The continental slope of the Baie des Anges is also
affected by numerous surficial failures, such as the 1979 event initiated during the extension of Nice airport. Small-sized failures are
abundant at relatively shallow water depth, in areas where sediment supply from the Var and Paillon rivers has been high through time.
Larger-sized failures occur at greater water depth, probably triggered by seismic activity.
Keywords: Ligurian Basin, Var system, turbidity current, 1979 event, failures
Rapp. Comm. int. Mer Médit., 37,2004
58
The Var turbidite system: geological setting and morphology
The Var turbidite system is a small sandy system, extending
seaward of Nice and the Var prodelta to the base of the continental
slope nearby Corsica. The Var system has been built during the
Pliocene-Quaternary in a flat-floored basin formed during the
Messinian salinity crisis (1).
The Var system was fed through time by the Var and Paillon
canyons that connect directly to the Var- and Paillon-river mouth.
Only the Var Canyon is thought to be still active nowadays. The two
canyons deeply incise the slope and coalesce at about 1500 m water
depth to form the Upper Valley. At 2000 m water depth, the Upper
Valley abruptly passes into the east-trending Middle Valley which
extends 50 km eastward till a water depth of 2500 m where it reaches
a continuous trend of salt diapirs and then bends to the southeast. The
Lower Valley extends 100 km southwestward, feeding a sandy distal
lobe at a water depth of 2700 m water depth. Along the whole system,
gradient slope decreases from 11% in the canyon to 0.3% in the
Lower Valley. The main morphological feature of the system is the
well-developed right-hand levee called the Var Sedimentary Ridge (1,
2). Ridge height above the channel ?oor decreases from 400 m in the
west to less than 30 m to the east. The morphology of the system
suggests that significant deposition has been produced by both sandy
and muddy turbidity currents (3). Evidence of recent erosive currents
has been observed on deep-tow side-scan sonar images collected in
the distal part of the Ridge (4).
Processes of sediment supply
During the present highstand three major sediment transfert
processes are active in the Var system:
- Lar
ge f
ailure-induced turbidity currents
, such as the “1979 event”
when part of the Nice airport collapsed. Failures are generally induced
by the conjunction of earthquakes and of an under-consolidated state
of slope-sediment, although the “1979 event”, had anthropic causes.
The resulting current is a short-duration (a few hours), catastrophic
and fast surge. On the canyon steep slopes inferred velocities are
estimated at 30 m/s and are still in the order of 6 m/s 150 km away
from the canyon head (4). As a consequence, cobbles and boulders
can be transported near the bed over hundred of kilometres. Fine to
medium sand are transported up to the distal lobe, more than 200 km
from the continent.
-
Small-sized turbidity currents
; those are generated by retro-
gressive shallow failures on the slope or reconcentration process of
particles near the shelf break during storms. Several day-long currents
are common, as recorded by Gennesseaux et al.(5) in the Var canyon.
These low-velocity currents are able to transport and deposit fine-
grained particles on the slope and on the Var Ridge.
- Hyperp
ycnal turbidity currents
; triggered at the Var-river mouth
during high-magnitude ?oods, when critical discharge is close to 1250
m
3
/s and sediment concentration about 42 kg/m
3
. The Var river
discharge curve is typically bimodal and ?ash ?oods can occur both
in spring and autumn, owing to snow melt and convective rainfalls.
Using the rating curve of the Var river, return period of ?oods
triggering hyperpycnal ?ow is about 4 years (using instantaneous
discharge values) and 21 years (using daily discharge values). Such
currents transport significant amount of both fine-grained and coarse-
grained particles. Typical hyperpycnite-deposits are found in the distal
part of the Var Ridge where they exhibit thickness of about 5 to 20 cm.
The detailled analysis of one core collected on a terrace near the
base of the Upper Valley indicates that during the last 100 years, 70%
of the deposits result from hyperpycnal-?ow activity, 5% result from
failure-induced turbidity currents and 25% are the hemipelagic
background. During the Holocene-Pleistocene time, failure-induced
turbidity currents were as common as hyperpycnal ?ows (4).
Slope failures
On october 16 1979, a large failure, involving at least 8 x 10
6
m
3
of
material, occurred at shallow water depth during infilling operations
related to the enlargement of Nice airport. In situobservations and
modelling results have indicated that the slide transformed into a
debris ?ow then in a surge that reached the Var Ridge and probably
the distal lobe. The surge broke two submarine cables located at about
80 km and 107 km from the failure area. On the upper slope, the
failure generated a tsunami 2 m in height that caused damage and
people death in the Antibes region.
Recent high-resolution multibeam survey of the Nice upper slope
allowed detailed observation of the 1979 event and also revealed the
degree of destabilisation of the area. Failures are more abundant near
the Var- and Paillon-river mouth (about thirties of events) than in the
central slope domain (about tens of events), but they are globally
smaller (about one km
3
against several km
3
in the central zone). The
1979 failure appears as a medium-size slide. Failures are generated at
shallow water depth, near the shelf break, in areas close to the Var-
and Paillon-river mouth, and at greater water depth in the slope central
zone. Failures are easily triggered in areas where sediment supply is
important through time. This results in thick under-consolidated
accumulations, deposited near steep slope, that can be destabilised
under the action of gravity, or during episodes of ?ash ?oods
reworking deposits at river mouth. In the central zone, where sediment
supply is lower, external mechanismes such as horizontal or vertical
acceleration during an earthquake seem to be necessary to trigger
failures. In that case, volume of remobilised sliding sediment is more
important.
References
1-Savoye, B., D.J.W. Piper, L. Droz, 1993. Plio-Pleistocene evolution of
the Var deep-sea fan off the French Riviera. Marine and Petroleum
Geology, 10: 550-571.
2-Migeon, S., B. Savoye, J.-C. Faugères, 2000. Quaternary development
of migrating sediment waves in the Var deep-sea fan: distribution, growth
pattern and implication for levee evolution. Sedimentary Geology, 133:
265-293.
3-Piper, D.J.W., B. Savoye, 1993. Processes of late Quaternary turbidity
current flow and deposition on the Var deep-sea fan, north-west
Mediterranean Sea. Sedimentology, 40: 557-582.
4-Migeon, S., B. Savoye, E. Zanella, T. Mulder, J.-C. Faugères, O.
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5-Gennesseaux, M., Guibout, M., Lacombe, H., 1971. Enregistrement de
courants de turbidité dans la vallée sous-marine du Var (Alpes Maritimes).
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