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Seismic evidence for Messinian detrital deposits at the western
Sardinia margin, northwestern Mediterranean
Francoise Sagea,*, Gudela Von Gronefelda, Jacques Deverchereb, Virginie Gaullierc,
Agnes Maillardd, Christian Gorinie
aUMR 6526 GeoSciences Azur, Observatoire Oceanologique, 06235 Villefranche-sur-Mer, FrancebUMR 6538 Domaines Oceaniques, CNRS/UBO, Institut Universitaire Europeen de la Mer, 29280 Plouzane, France
cLEGEM, EA 3678, Universite de Perpignan, 66860 Perpignan, FrancedCICT, Universite de Toulouse, 31000 Toulouse, France
eSedimentologie et Geodynamique, Universite de Lille 1-SN5, 59650 Villeneuve d’Ascq, France
Received 7 January 2003; accepted 9 March 2005
Abstract
Based on new (‘SESAME 01’ cruise) and previous seismic reflection data, this paper evidences the effects of the Messinian Salinity Crisis
on the sedimentation at the western Sardinian margin. In the lower part of the margin, Messinian detrital deposits are organized into two
successive, w300 m thick units. According to their shape, facies and extent, they are similar to alluvial fans, presumably fed by subaerial
erosion of the upper margin. Whereas the earliest body is observed continuously all along the margin and the adjacent part of the deep basin,
the youngest one is restricted to the vicinity of the canyons. This pattern could therefore sign a late Messinian sea-level rise, as suggested by
detrital fan conglomerates sampled in a similar position along the northern Ligurian margin. We observe salt-related deformation of the
Messinian detrital units, and the top of the salt layer gradually turn into a detrital delta seaward, implying that at the deep margin, most of the
salt deposited before the low-stand Messinian delta emplaced, probably during the main Messinian sea-level drop or at the beginning of the
low-stand sea-level.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Messinian Salinity Crisis; Erosion; Salt tectonics
1. Introduction
The Messinian Salinity Crisis (hereafter referred as
MSC) is known to have deeply reshaped the margins of the
Mediterranean Sea and to have led, in the deep basin, to the
deposition of a thick sequence of evaporates, which are
found in three distinct units, the ‘Lower Evaporites’
(carbonates and evaporitic sediments), the ‘Salt’ (halite
and anhydrite, i.e.; the mobile layer) and the ‘Upper
Evaporites’ (marls and gypsum) (Ryan, 1973; Ryan et al.,
1973; Clauzon, 1982; Rehault et al., 1984). Owing to this
outstanding sea-level change, the ‘Messinian
0264-8172/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.marpetgeo.2005.03.007
* Corresponding author. Tel.: C33 04 9376 3746; fax: C33 04 9376
3766.
E-mail address: sage@obs-vlfr.fr (F. Sage).
Mediterranean’ is recognized to represent an excellent
target to understand the consequences of eustatic processes
on both land and marine environments.
Although the main steps and timing of the MSC are
nowadays broadly clarified (Clauzon et al., 1996; Krijgsman
et al., 1999; Blanc, 2002), many aspects of this major event
remain unresolved or unclear, and are still highly debated.
The first stage of the Crisis (the ‘marginal evaporites’ from
Clauzon et al., 1996) has been widely documented owing to
the easy access to several markers of the Crisis onshore
(Rouchy et al., 1998, and references therein). Conversely,
studies devoted to the deep sea deposits of the second phase
(the ‘basinal evaporites’ from Clauzon et al., 1996) are still
limited (Dos Reis, 2001; Dos Reis et al., 2004a; Lofi et al.,
2005). One of the most enigmatic points is the importance
and the extent of the clastic detrital deposits resulting from
the massive erosion of the continents during the second step
(from w5.60 to 5.32 Ma). Whereas MSC events recorded
in the upper parts of the margins are well described
Marine and Petroleum Geology 22 (2005) 757–773
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F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773758
(e.g. the systematic cross-cutting of deep valleys and
canyons by tributary rivers all around the Mediterranean
shores, and the subsequent infill of drowned valleys by
marine and later fluvial sediments during Pliocene, called
the ‘Zanclean flooding’), the age, nature and/or extent of the
deposits along the lower margin and in the deep basins are
poorly known (Clauzon et al., 1996). For instance, no
drilling has yet recovered the whole deep evaporites
sequence. So far, seismic lines collected at the foot of the
margins are generally unable to resolve the sedimentary
sequences associated to the MSC and their chronostrati-
graphic relationships with the mobile salt and Upper
Evaporites layers (Gorini et al., 1993; Mauffret et al.,
1995, 2001; Seranne et al., 1995; Seranne, 1999). This is
due to several reasons: (1) the sharply-different sedimentary
processes allowing for the deposition of salt (i.e. evapor-
ation process) and clastic sediments (i.e. gravity-driven
process), and the subsequent highly-variable and poorly
controlled deep sea deposition rates (Blanc, 2000); (2) the
salt mobility, which induces thin-skinned tectonics on the
continental slope and at the foot of the margin, from
extensional to contractional deformation in the overlying
brittle sedimentary units (i.e. the Upper Evaporites and the
Plio-Quaternary cover; Gaullier, 1993; Gaullier and
Bellaiche, 1996; Mauffret et al., 2001; Dos Reis et al.,
2004a; Dos Reis et al., this issue); (3) the lack of
stratigraphic control on the seismic facies; and (4) the
lesser interest of the community for the MSC offshore,
relative to other topics such as the deep structure of
extensional margins (Mauffret et al., 1995, 2001) or Plio-
Quaternary sediment dynamics near large canyons (e.g.
Migeon et al., 2001).
In order to tackle these outstanding questions, we
collected in September 2001 a new set of seismic data off
western Sardinia onboard the R/V ‘Tethys II’ (SESAME
cruise, Fig. 1a). Combined with previous seismic lines,
these data document in detail the central part of the
Sardinian passive margin, and especially in and around one
of the most important canyons of the margin, the Oristano
canyon (Thomas and Gennesseaux, 1986).
The western Sardinia margin is perhaps one of the best
area to study the MSC: (1) due to the insulary location of
Sardinia, the Plio-Quaternary sediment supply is weak
compared with other sedimented margins such as the Gulf
of Lion margin; (2) the sedimentary input since Oligocene
has been captured by onshore grabens (Casula et al., 2001;
Thomas et al., 1988); (3) no major tectonic deformation
occurred after the rifting stage of the Mediterranean basin,
allowing for a better preservation of the Messinian erosional
surface and deposits compared to the other margins which
were recently reactivated (Chaumillon et al., 1994); and (4)
the salt-related deformation consists into normal faults with
moderate vertical throws, which do not disrupt the
continuity of the Messinian and post-Messinian units across
the deformation zone, by contrast with most of the
Mediterranean margins.
In this paper, our first objective is to evidence and
characterize the detrital bodies deposited along the deep
margin during the Messinian low-stand sea-level. A second
objective is to better assess the chronological relationships
between the Messinian evaporites sequence and these
detrital bodies. In particular, we show that at the deep
margin, the detritals postdate the Messinian salt deposition.
Our seismic lines, which are limited to the deep margin,
however, do not image the seaward termination of the
detrital fans and their latetal transition to the deep basin
evaporates located seaward from the study area. This lateral
transition is thus discussed on the base of seismic lines
located at the deep basin offshore Saint Tropez (northern
Ligurian margin), in a similar context during Messinian
time.
2. Seismic data set
This paper is based on two surveys offshore western
Sardinia: respectively, the ‘SARD’85’ cruise, conducted in
1985 on the R/V ‘Catherine Laurence’, and the ‘SESAME’
cruise, carried out in September 2001 on the R/V ‘Tethys II’
(INSU-CNRS, France). The ‘SARD’85’ cruise collected
1250 km of single-channel seismic lines (Thomas and
Gennesseaux, 1986; Fig. 1a). The ‘SESAME’ cruise
collected 13 additional seismic lines using a 25 m long
4-channel streamer with a mini-GI air-gun shooting every
12–12.5 m at 140 bar, allowing for a 4-fold coverage.
Seismic processing includes gain recovery, normal move
out correction, stack, post-stack migration in the FK
domain, and band-pass filtering. For the final plot, a
coherency filter was applied to the data, as well as a
dynamic equalization and a filtering in the FK domain.
3. Morphological and tectonic setting
The western Sardinia margin can be divided into three
morphological domains (Casula et al., 2001, and references
therein). First, a 90 km wide amphitheatre facing Oristano is
centered on the margin (Fig. 1b). It provides a smooth
transition between the narrow continental shelf, 25 km
wide, and the deep basin, at a 2850-m water-depth. The
Oristano Amphitheatre is bounded to the north and to the
south by a continental shelf twice wider, which is connected
to the deep basin by two steep slopes, the Nurra and the
Sulcis scarps, respectively, (Thomas et al., 1988; Fig. 1b).
These morphological domains reveal a first order canyon/
interfluve system. Inside the Oristano Amphitheatre, a net of
convergent valleys defines a second order canyon/inter-
fluves system (Fig. 1b).
The morphology of the Oristano Amphitheatre directly
results from the formation of the margin during Oligo-
Miocene times. Back-arc transtensional tectonics, linked
to the northwestern Mediterranean opening, reactivated
Fig. 1. (a) Location on the bathymetric map of the seismic lines recorded during ‘SARD’85’ (thin lines) and ‘SESAME 01’ (thick lines) cruises conducted on
the western Sardinia margin (northwestern Mediterranean Sea). The seismic sections presented in the paper are underlined in grey. The stars indicate the
location of the DSDP Leg XIII sites (from Ryan et al., 1973). (b) Main structural features inherited from the Oligo-Miocene rifting of the western Sardinia
margin, shown on the bathymetry (after Thomas et al., 1988; Lecca et al., 1997). Such structural features control the present day morphology. The Oristano
Amphitheatre is bounded to the north and to the south by the Nurra and the Sulcis scarps, respectively, drawing a first order fluve/interfluves system. Inside the
Oristano Amphitheatre, a second order fluves/interfluves system is drawn by small canyons (thin dotted lines). The N1208 Scarp crosses the whole margin and
corresponds to a main relief in the acoustic basement, bounding the northern flank of the main canyon.
F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773 759
NE–SW and NW–SE late-Hercynian structures (Thomas
and Gennesseaux, 1986; Thomas et al., 1988; Fais et al.,
1996). The formation of the Oristano Amphitheatre is
controlled by NNE–SSW listric normal faults and NW–SE
transfer faults, which delineate horsts and half-grabens in the
metamorphic basement, between the Nurra and the Sulcis
scarps (Thomas et al., 1988; Fais et al., 1996; Lecca et al.,
1997). Moreover, the deepest part of the amphitheatre is
superimposed to a main graben bounded to the north by a
steep scarp trending N1208 (Fig. 1b). Such scarp is
controlled by a major transfer fault that crosses the entire
margin (Thomas et al., 1988). During the rifting and drifting
stages, the grabens were filled by Oligo-Miocene sediment
sequences, sometimes 1500 m thick (Thomas and Gennes-
seaux, 1986; Thomas et al., 1988; Fais et al., 1996; Lecca
et al., 1997; Casula et al., 2001). In the study area, the margin
is stable since the Messinian time, while the tectonics still
affects the central part of the island (Casula et al., 2001).
4. Seismic stratigraphy of the study area
Beneath the continental margin, the deepest acoustic
units observable on the seismic lines correspond to the pre-
Messinian units, i.e. the metamorphic basement (MB)
structured in horsts and grabens, and the Oligo-Miocene
sediments (OM) infilling the grabens (Fig. 2a). In the deep
basin, the basement cannot be properly imaged by the
SESAME and SARD85 seismic lines, since the Messinian
salt layer acts as a screen for the seismic waves.
In the upper part of the continental slope, the pre-
Messinian units are truncated by the Messinian erosional
Fig. 2. Main seismic units described in the paper (a) inside the Oristano Amphitheatre and (b) outside the Oristano Amphitheatre, in the area drilled during
DSDP Leg XIII (see location in Fig. 1). Pre-Messinian units: Metamorphic Basement (MB); Oligo-Miocene sequences (OM). Messinien units: Salt (S); Upper
Evaporites (UE); Detrital units according to the present study (C), (C 0), and (T). Post-Messinian units: Plio-Quaternary sequence (PQ). Two-Way Traveltime
(TWT).
F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773760
F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773 761
surface, which is characterized by a strong reflector
(Fig. 2a). The pre-Messinian units are here directly overlaid
by the Plio-Quaternary unit (PQ), which can be recognized
all around the Mediterranean basin owing to a typical
acoustic facies, transparent at its base and becoming more
reflective upward (Rehault et al., 1984; Fig. 2a,b). In the
lower part of the slope and in the adjacent part of the deep
basin, additional units are observed between the pre-
Messinian and the PQ sequences (Fig. 2a,b). Such units
(called S, UE, C, C 0, and T) are related to the Messinian
event on the western Sardinia margin and constitute the
main purpose of this study. They will be accurately
described in Section 5.
5. The Messinian acoustic units: geometry
and distribution
5.1. The S unit (salt layer)
The Messinian salt layer, identified as the S unit on the
seismic lines, generally corresponds to the deepest clearly
observable seismic unit in the basin (Fig. 2a and b). The top
of the salt is characterized by a strong reflection, due to a
high contrast of impedance between the salt and the
overlying sediments. The S unit is represented by a rather
transparent acoustic facies showing internal reflectors with
low amplitudes, typically observed in the whole Mediterra-
nean deep basins (Rehault et al., 1984; Gaullier et al., 2000;
Dos Reis et al., 2004a). In some places, the salt layer forms
diapirs disrupting the uppermost layers on the seismic
sections (Fig. 2b). Near the margin, the diapirs generally
affect reflectors of the base of the brittle sediment cover
only; seaward, the diapirs grow through the youngest layers
and sometimes reach the sea-bottom. Due to this salt
diapirism, the top of the S unit depicts highly variable
depths (between 4.5 s two way travel time (TWT) and 3.7 s
TWT) in the deep basin, and numerous side-scared echoes
are generated. Such artifacts can hardly be reduced by
numerical processing of the data and prevent us from an
accurate interpretation of the data within the deep basin.
Close to the margin, the salt layer is less deformed, with a
layer top at 4.2 s TWT, gently rising up to 3.9–4.0 s TWT
against the margin’s acoustic basement, where it pinches out
(Figs. 2–6).
5.2. The C unit
The C unit is observed beneath the Oristano Amphithea-
tre and in the deep basin all along the western Sardinia
margin (Fig. 6). It is characterized by a reflective acoustic
facies composed of sub-parallel and irregular reflectors of
high energy (Fig. 3–5), showing dominant frequencies
around 50–70 Hz (Fig. 7).
In the deep basin, close to the continental margin, C unit
corresponds to sub-horizontal reflectors and lies over
the salt layer, with a gradual transition between the two
facies characterized by an increase of the internal layering
in the salt layer, from bottom to top of the layer (Fig. 3c).
At depths shallower than 3.9 s TWT, the salt layer
disappears and C unit lies directly over the acoustic
basement of the margin with a low deposit angle, between
2 and 38 (Figs. 2, 4 and 5). Except for the seismic lines shot
along the Oristano canyon axis (for example SM08, Fig. 5),
C unit lies on a depositional surface, which is apparently
formed by the western flank of the deepest structural block
of the margin, showing an average slope of 8–98. The C
unit reflectors are dipping both toward the deep basin and
toward the canyons axis, with an average angle of 5.3G0.38 (Figs. 3 and 4).
The thickness of C unit appears rather constant within the
few kilometers of the deep basin explored during the
SESAME cruise, around 200 ms TWT, although its base is
not always clearly visible, because of the gradual transition
to the salt layer below. Conversely, its thickness signifi-
cantly varies on the continental slope (Figs. 3 and 6): C unit
is 50–100 ms TWT thick on the second order interfluves of
the Oristano Amphitheatre, whereas it reaches a maximum
thickness of 200 ms TWT at the canyons axis, similar to the
thickness observed in the deep basin.
The C unit tapers off in its upper part on the margin
acoustic basement, at a variable depth, depending on its
position relative to the canyons: the C wedge is observed at
2.4–2.6 s TWT depth in the canyon axis, and its depth
increases gradually to 3.1 s TWT away from the canyons,
since the C unit is more developed in the canyons. Close to
the wedge, the top of C unit is often marked by toplaps,
whereas it is generally concordant with the above T unit
elsewhere (Fig. 4).
Upslope the C wedge, some deposits are still observed in
the canyons axis, but present a chaotic facies and the top of
the layer displays an irregular shape. It is impossible from
our seismic lines to tell the time relationship between such
chaotic facies and C unit described above.
Outside the Oristano Amphitheatre, the C unit observed
in the deep basin tapers off at 3.3–3.4 s TWT depth against
the margin acoustic basement south of the amphitheatre, and
at 3.8 s TWT along the Nurra scarp, north of the study area.
Along the slope out of the Oristano Amphitheatre, the
acoustic facies of C unit laterally evolves into a different
facies, called the C 0 unit (Fig. 2b), composed of irregular
reflectors showing low frequencies, generally less continu-
ous than those described for the C unit, and becoming
locally chaotic (Fig. 2b). The C 0 unit appears to be thin, with
an average thickness of about 70–100 ms TWT, although its
base is not always clearly visible. When an internal layering
is observed, the reflectors are roughly parallel to the
deposition surface, contrastingly with the C unit. The
deposition surface of C 0 is generally much steeper than the
one described in the Oristano amphitheatre, especially along
the Nurra scarp where the basal slope is of about 108.
Fig. 3. Part of the SM11 seismic line recorded during ‘SESAME 01’ cruise (see location in Fig. 1). The line was shot parallel to the margin, across the main
canyon observed in the Oristano Amphitheatre. (a) Processed seismic line; (b) interpreted section; (c) zoom on the Messinian units. From bottom to top, the
Messinian units correspond on this line to: salt (S), C unit, and T unit. The transition between the Salt and the C unit appears to be gradual. The ductile salt,
while migrating toward the basin (perpendicularly to the SM11 line) is responsible for the deformation in the brittle overlying sedimentary layers. Such
deformation is characterized by extensional features at the edge of the canyon (which surimposes the N1208 Scarp), and by contractional structures in the
canyon axis. Both extensional and contractional features are deeply rooted on a low-frequency reflector which deeps towards the canyon axis, and is interpreted
as the salt decollement. See Fig. 2 caption for units labels.
F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773762
5.3. The T unit
The T unit is observed between the C unit and the Plio-
Quaternary sequence, both in the Oristano Amphitheatre
and in the adjacent part of the deep basin (Fig. 6), gradually
disappears southward, i.e. toward the Sulcis scarp, and lacks
along the Nurra scarp, north of the Oristano amphitheatre
(Fig. 6, and location on Fig. 1).
Fig. 4. Part of SM12 seismic line recorded during the ‘SESAME 01’ cruise (see location in Fig. 1). (a) Processed seismic line; (b) interpreted section; (c) zoom
on the Messinian units. See text for the C and T units description. Note that the erosional surface gradually disappears beneath C unit, and that the landward
termination of T unit is characterized by a rather transparent acoustic facies similar to the facies described for the PQ sequence. The C and T Messinian units are
deformed into small tilted blocks due to the salt migration toward the basin. The salt-related normal faults are deeply rooted on a low frequency reflector which
dips toward the deep basin, and which corresponds to the salt decollement. Even if some salt remains along such decollement, it mainly corresponds to a salt
weld. The uppermost normal salt-related fault indicates the landward limit of deposition of the salt, before it moved away toward the deep basin. The salt
thickness may have reached 300–500 m, based on the vertical throw computed from one side to the other of the salt deformation zone. See Fig. 2 caption for
units labels.
F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773 763
The T unit is characterized by a highly reflective facies
(Figs. 3–5), with average frequencies higher than those
described for the C unit, i.e. around 110 Hz on the SESAME
data (Fig. 7). The seismic facies laterally changes along the
margin: inside the Oristano Amphitheatre, the T reflectors
are homogeneous and continuous on the interfluves,
whereas they become more chaotic and discontinuous
towards the canyons axis (Figs. 3–5). In this particular
case, it is sometimes difficult to distinguish the boundary
between the C and T units (Figs. 3 and 5). Locally, on the
interfluves, the T unit facies becomes more transparent, and
is there similar to the base of the Plio-Quaternary sequence
(see for example Fig. 4, at the upslope termination of T
unit).
In the deep basin, since the T unit is concordant with the
C unit, the distinction between the two corresponding
seismic facies can generally be made by a greater regularity
of the T unit reflectors, and by the frequency spectrum of
the numerical data (Figs. 4 and 7). Moreover, beneath the
continental margin, an angular unconformity generally
marks the limit between the C and T units (Figs. 3 and 4),
whereas T reflectors are always concordant with the Plio-
Quaternary sequence (Figs. 3–5).
Above the C unit, the T unit tapers off on the pre-
Messinian acoustic units of the margin (Figs. 4–6), at a depth
of 2.25 s TWT (w1800 m) at the canyon axis. The wedge’s
depth increases up to 2.8 s TWT (w2100 m) when moving
far from the canyons (Fig. 4). In the deep basin, the unit
thickness is rather constant at around 130–150 ms TWT.
5.4. The Messinian erosional surface
From upslope to downslope, the Messinian erosional
surface marks successively the base of the Plio-Quaternary
sequence and the base of the T unit. It gradually
disappears beneath the C unit wedge. Under the C unit,
Fig. 5. Part of the SM08 seismic line recorded during the ‘SESAME’01’ cruise (see location in Fig. 1) shot along the main canyon axis. (a) Processed seismic
line; (b) interpreted section. See text for the C and T units description. The C and T units here reach their maximum thickness, and little change is observed
between the two facies. See Fig. 3 caption for comments about the salt-related deformation of the post-salt acoustic units. On this line, the salt decollement
ondulates onto the underlying morphology. This break of the decollement basal slope is locally responsible for the reverse faults in the post-Messinian acoustic
units. See Fig. 2 caption for units labels.
F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773764
neither the strong reflector representing the erosional
surface, nor the truncation of the Oligo-Miocene reflectors
are observed deeper than 3.25 s TWT (w2900 m). This
apparent lack of the erosional surface may be due to the
lack of the Messinian erosion in this area or to the
scattering of the seismic energy by the C unit, which could
prevent from further acoustic penetration.
The morphology of the erosion surface is mainly
controlled by the nature of the rocks outcropping on the
margin after the rifting: while the Oligo-Miocene sedimen-
tary basins are preferentially submitted to erosion, the
metamorphic horsts of the margin should have persisted
longer. Indeed, the rifted structure of the margin is still
clearly visible on the normal-striking lines (see for example
Figs. 5 and 6). However, we note that the erosional surface
is generally marked by a slope break located at the pinch-out
of the C unit, suggesting here an imprint of the Messinian
times. Assuming velocities of 1900 and 2500 m/s for PQ
and T units, respectively, the average angle for the erosional
surface upslope C unit C is 2.3–2.58, whereas it reaches 8–98
for the deposition surface beneath the C unit.
6. Salt-related deformation in the study area
The transition zone from the continental margin to the
deep basin is characterized, all along the margin, by 5–
30 km wide deformation zone affecting the C, T and PQ
units (Figs. 4 and 5). These units are cut by listric normal
faults that define small tilted blocs that are 1–3 km wide
(Fig. 4). The vertical throw along the faults is maximum
(reaching values of about 150 m) for the sediments of the C
and T units, and gradually decreases upward into the PQ
sequence, which displays wedge-shaped reflectors. Most of
Fig. 6. Distribution of the T, C and C0 units in the study area. The seaward extension of the C and T units is not known from our data. The C unit and its time-equivalent
C0 unit are observed all along the margin, while the T unit extent is restricted to the vicinity of the main canyons, inside and facing the Oristano Amphitheatre. The red
line indicates the seaward extension of the observed Messinian erosional surface. The yellow line corresponds to the landward extension of the salt layer, before it
migrated toward the deep basin. This boundary is deduced from the location of the root of the uppermost salt-related normal fault, upslope the salt weld (see for
example on Fig. 4). These deltas seem to be fed by a terrestrial fluvial system which underlines the present-day one, as shown by an almost exact superimposition of
the paleo-valleys drawn by the erosional surface and the cones apex on one hand, and by the present-day valley system on the other hand. The detrital units leveled the
pre-existing topography, and are thus responsible for a present-day smooth topography at this place. Upslope from the detrital units, the topography is conversely
contrasted on to the canyons.(For interpretation of the reference to colour in this legend, the reader is referred to the web version of this article.)
F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773 765
the faults reach the sea-bottom, which is slightly vertically
shifted. Perpendicularly to the margin, the cumulated
vertical throw across the deformation zone reaches 0.4–
0.6 ms TWT (i.e. 300–500 m).
Similar salt-related faults are identified on the seismic
lines parallel to the margin, across the major canyon of the
Oristano Amphitheater (Fig. 3). The uppermost normal salt-
related fault fits there with the structural boundary provided
POWER
FREQUENCY (HZ)0 50 100 150 200 250 300 350 400 450 500
1500
010
000
5000
0
POWER
0 50 100 150 200 250 300 350 400 450 500
1500
010
000
5000
0(a)
(b)
Fig. 7. Frequency spectrum computed for (a) the C unit and (b) the T unit,
showing dominant frequencies, respectively, of the order of 50–70 and
110 Hz.
F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773766
by the N1208 Scarp. The vertical shift of the C and T units
between the canyon edge and the canyon axis reaches 0.4 s
TWT in that case (Fig. 3).
The offset of the reflectors generally indicates a normal
displacement component along the faults. However, reverse
faulting is locally observed downslope from normal faults,
where the acoustic basement forms some relief (Fig. 5).
The normal and reverse faults are deeply-rooted on a
low-frequency reflector (Figs. 3–5). This reflector, located
immediately below the C unit, dips towards both the deep
basin, and the Oristano canyon’s axis (Figs. 3–5). It forms
the upslope continuation of the salt wedge on the seismic
lines.
Since sediments overlying a weak basal salt layer can
easily glide and spread under the effects of their own weight
where values of the basal and surface slopes are 38 or less
(Vendeville, 1987; Dos Reis, 2001), the belt of listric
normal growth-faults cutting the C, T units and the PQ
sequence into small tilted blocs in the study area can be
clearly attributed to salt-related deformation, i.e. to gravity
gliding and/or spreading of the brittle sediment cover above
the Messinian decollement. Loading of the Messinian salt
triggered a thin-skinned tectonics, which typically causes
proximal extension (normal growth faults), mid-slope
translation, and distal contraction (folds and diapirs)
(Gaullier and Bellaiche, 1996; Dos Reis et al., 2004a,b).
Gravity gliding occurs if the base of the salt decollement is
dipping downslope (Vendeville, 1987) and, as shown by
recent analogue modeling (Gaullier and Vendeville, in
press), or field observations (Dos Reis et al., 2004a,b, this
issue) gravity spreading can be caused by a differential
sedimentary overload, and especially by detrital wedges.
Upslope, there is nearly no more salt, because it has been
expelled toward the deep basin and replaced by a salt weld
and locally by residual salt rollers (Figs. 4 and 5).
Downslope, the compressive structures locally described
on the seismic lines (Figs. 3 and 5) are also due to gravity
gliding/spreading, and correspond to areas where local
reliefs act as passive buttresses, thus preventing gravity
gliding and spreading. The similar thickness of C and T
units on both sides to deformation belt, together with the
wedge-shape of the PQ reflectors, indicate that the salt
migration towards the basin mainly occurred after the T unit
deposition, during the Plio-Quaternary times. Fault scarps
on the sea-floor show that salt tectonics was still recently
active and indicate that small quantities of salt are probably
still resting at the foot of the faults, which cannot be easily
detected on the seismic data (Figs. 4 and 5).
Along the salt weld lying upslope the salt wedge, the root
of the uppermost normal salt-related fault thus indicates the
landward limit of initial deposition of the massive salt,
before the salt moved toward the deep basin (yellow line on
Fig. 6). The salt landward paleo-boundary classically lies
down roughly parallel to the margin, but shows two
reentrants toward the continent, which coincide with the
two main canyons of the western Sardinia margin (Fig. 6).
The first one, centered on the Oristano Amphitheatre, is 12–
30 km wide, and enters the margin 20 km towards the
continent; the second one, much smaller in size, faces a
smaller canyon, north of the Oristano Amphitheatre.
Bathymetric lows thus already existed at those places,
when the salt emplaced, and were much narrower than the
present-day depressions. The other canyons, of smaller size
and crossing the margin, do not look like salt reentrants
toward the continent.
7. Discussion
7.1. The C unit: a detrital coalescent delta, coeval with
the Messinian low-stand sea-level
7.1.1. Nature of the C unit
We have shown that the C unit is observed on the
continental margin, only within the Oristano Amphitheatre,
and in the part of the deep basin explored during the
SESAME cruise, where it is continuous all along the western
F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773 767
Sardinian margin (Fig. 6). Beneath the continental slope, its
upslope extension is clearly correlated to the canyons
(Fig. 6). Moreover, the geometry of the C unit depicts a
general dipping and thickening towards both regional low
(the deep basin) and local lows (the canyons axes), where its
thickness reaches its maximum value (w300 m.). Accord-
ing to these geometrical criteria, we interpret the C unit as a
detrital deposit accumulated downslope and shaped as
coalescent deltas (Dunne, 1998; Nemec, 1990b; Postma,
1990; Richards et al., 1998). The top of the C unit forms a
rather smooth topography, suggesting that the C sediments
levelled the pre-existing relief of the deposition surface.
The detrital origin of the C unit is also supported by the
existence of an acoustic unit of similar facies described at
the foot of the northern Ligurian margin, below the Plio-
Quaternary sediments (Savoye and Piper, 1991; Bigot-
Cormier et al., 2004). At this location, submersible dives
have shown that this seismic unit corresponds to coarse-
grained sediments, poorly sorted into well-bedded massive
conglomerates (Savoye and Piper, 1991; Sosson and
Guennoc, 1998) emplaced close to the shoreline (Rohais,
2002). Off Sardinia, the same kind of information is
provided by the cores drilled in Hole 133 during DSDP
Leg XIII (location on Figs. 1 and 6), where the C unit is
replaced by its time-equivalent C 0 unit. In the drilling area,
the C 0 unit has been identified as the ‘M-reflectors’ by the
scientific team of Leg XIII (Ryan et al., 1973). These M
reflectors were drilled over 140 m, between the Plio-
Quaternary sediments and the metamorphic basement. The
collected cores correspond to variegated brown silts and
shales including a large number of well-rounded pebbles
and cobbles of terrestrial origin (Ryan et al., 1973). The
detrital elements, which are totally barren of microfossils,
are believed to correspond to debris of the margin basement
itself, eroded and abraded during the Messinian low-stand
sea-level, when the slope was near the wave base or
subaerial exposed (Ryan et al., 1973; Hsu and Ryan, 1973).
They show a lithology similar to that of variegated clastics
often deposited onshore in an arid environment (Ryan et al.,
1973; Hsu and Ryan, 1973).
We thus infer that the C and C 0 units both correspond to
detrital deposits from continental origin. We propose that
different processes of emplacement explain the evolution of
the acoustic facies between both units, probably related to
the different dips of the deposition surface and/or different
types of drainage. While the C 0 unit could characterize
detrital deposits on steep slopes, the C unit seems to
correspond to thicker and more organized detrital bodies in
alluvial deltas, and has developed downslope a wide
drainage basin fed through the Oristano canyons, probably
close to the paleo-shoreline (Massari and Parea, 1990). It is
however, impossible to tell from the available seismic lines
whether the C unit corresponds to shallow-water or to
subaerial coalescent alluvial deltas, since similar geometries
are described in both cases (see for example (Einsele, 1996;
Nemec, 1990a,b; Postma, 1990; Richards et al., 1998) for
sub-marine deltas and (Guzzetti et al., 1997; Garces et al.,
1998; Harvey, 2002) for sub-aerial alluvial fans).
The basal onlaps on the margin basement suggest the
aggradation of the detrital delta, and thus a gradual rise of
the sea-level during the C deposition.
7.1.2. Geometrical and time relationships between the
Messinian deltas and the deep salt layer
During the Messinian low-stand sea-level, the upper
margin is eroded whereas the evaporites precipitate in the
deep basin. Detrital deposits are thus expected to be a time-
equivalent of the evaporites (massive salt and/or Upper
Evaporites), emplaced between the evaporites of the deep
basin on one hand and the erosional surface at the upper
continental slope on the other hand. This general scheme
applies to the western Sardinia margin, but with some
differences.
The salt tectonics observed both in the C and T units
implies that the salt deposits along the margin occurred prior
to the detrital deposits. The anteriority of the salt deposition
in the study area is also supported by the stratigraphic
relationship between the salt and the C unit. The C unit is
observed directly above the landward termination of the
massive salt layer (Figs. 3–5). Moreover, the upward
increase of the internal layering in the massive salt layer
(Fig. 3c) suggests that the detrital deposits coming from the
continent have gradually emplaced at the base of the slope
and have been mixed with the salt layer, to become finally
dominant at the top of the C unit.
Detrital deposits coeval with the proximal salt, if they
exist, should be easily seen on the seismic lines, upslope
from the salt layer in lateral transition with it, that is now
upslope from the salt weld since the proximal salt moved
away toward the deep basin. Indeed, the vertical throw from
one side to the other of the salt-related deformation zone
described for both C and T units, indicates that the salt
thickness reached 300–500 m at the present salt weld,
before the salt moved away. Assuming similar sedimen-
tation rates for salt and coarse grained detrital deposits in the
order of a few centimetres per years, (Blanc, 2000; Rouchy,
2001a,b; Nemec, 1990a; Richards et al., 1998), we can
hardly explain the total lack of detrital body upslope from
the initially-thick salt layer. Another reason for low detrital
supply during salt deposition may be climatic, with regional
aridity marking this period (Bertolani-Marchetti, 1985). The
seismic lines, however, clearly indicate that the detrital
sedimentation on the margin postdates the salt deposition
against the continental slope. The anteriority of the salt
deposition with respect to the Messinian delta emplacement
and the lack of sub-aerial erosion/deposition processes
imply that the reentrants formed by the basal surface of the
salt at the deep slope (Fig. 6) do not correspond to canyons
produced by sub-aerial erosion before the salt precipitated.
We propose that such reentrants are primarily explained by
the initial structure of the margin in horsts and grabens.
Indeed, existence of inherited scarps on both sides of
F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773768
the Oristano Amphitheatre is in particular inferred from
structural data, as detailed above (Thomas and Gennes-
seaux, 1986; Thomas et al., 1988; Fais et al., 1996).
Sub-marine erosion of the slope prior to the major sea-
level drop also probably contributed in emphasizing the
inherited morphology. It has been shown elsewhere that
huge detrital units may emplace as a consequence of sea-
level drops (Nemec, 1990a), as argued during the last major
glaciation in the western area of the Mediterranean Sea. This
implies a transfer of large amounts of sediments across the
margin, which are supposed to erode marine valleys on the
continental slope (Einsele, 1996; Nemec, 1990a). In the Gulf
of Lion, a chaotic acoustic unit interpreted as a detrital unit
is described within the deep basin, close to the continental
slope, and extends 15 km into the deep basin below the salt
layer (Dos Reis et al., 2004b; Lofi et al., 2005). This detrital
unit is explained by a massive emplacement of debris flows
during an early stage of the Messinian sea-drop (Lofi et al.,
2005). In the case of the western Sardinia margin, an
equivalent sedimentary unit could lie below the salt layer,
but cannot be imaged with the available seismic lines.
In summary, we infer from the available seismic lines
that the salt deposits started before the emplacement of the
detrital delta fed by the sub-aerial erosion of the margin. The
salt emplaced in the basin and at the base of the continental
slope, which was initially shaped by the margin deep
structure and later probably emphasized by a stage of sub-
marine erosion, maybe related to the earliest stage of the
Messinian sea-level drop. The salt deposited first with little
or nearly no detrital element as shown by its ductile
behaviour allowing for its later migration toward the basin.
Then, the detrital elements gradually came from the upper
margin, being progressively incorporated into the salt layer,
and thus changing its mechanical behaviour. Finally,
sedimentation at the deep margin became mostly composed
of detrital elements to form the C unit. At that time, the
evaporites (salt and/or Upper Evaporites) were probably
still accumulating further away into the basin, their
landward limit being shifted offshore whilst the delta was
building up.
The stratigraphic relationship between the salt and the
detrital delta corresponding to the C unit may be related to a
delay in the record of the drastic Messinian environmental
change. On one hand, the initiation of the erosion-transfer-
deposition processes requires a sea-level drop at the margin.
It probably takes time to settle and produce detrital units
sufficiently important to be detected on the seismic lines. On
the other hand, the salt is expected to precipitate at a
relatively high sea-level, immediately once the requested
saturation is reached (Blanc, 2000; Martin et al., 2001), and
then before and/or during the sea-level drop. Such quick
response of the salt precipitation to the closing of the open
connections with the surrounding oceanic masses, together
with the high sedimentation rates expected for the salt, on
the order of a few centimeters by year, may explain why the
C unit appears later, over the salt.
7.2. The T unit: a sub-marine detrital fan
7.2.1. Nature of the T unit
The stratigraphic location of the T unit, between the C
unit and the PQ sequence, is similar to that of the Upper
Evaporites described at Hole 134 during DSDP Leg XIII
(Ryan et al., 1973). The acoustic facies (i.e. highly
reflective) and the thickness of the both units are also
similar, suggesting that the T unit may correspond to the
drilled Upper Evaporites. Such interpretation would agree
with previous studies, which described transgressive Upper
Evaporites over the continental margins in western
Mediterranean (Rehault, 1981; Rehault et al., 1984; Thomas
et al., 1988).
Within the deep basin, the drilled Upper Evaporites are
described as an interbedding of playa salts made of banded
halite alternating with laminae of anhydrite on one hand,
and marls on the other hand (Ryan et al., 1973). Towards the
slope, the levels include nodular gypsum and dolomitic silts,
which characterize a supratidal sabkha observed at a present
3100 m depth (Ryan et al., 1973). While the evaporitic
levels constitute the landward end of the Messinian brine
pool, the faunas included in the alternating marls indicate a
marine open environment (Ryan et al., 1973). The Upper
Evaporites are thus interpreted as the last markers of the
Messinian low-stand sea-level, with regularly spaced
incursions of the sea.
The Upper Evaporites nature, with alternating playa and
open marine environments, is thus strongly controlled by
the water depth at a given time, implying a rather constant
depth of deposition along the whole margin. In the Oristano
Amphitheatre area, the T unit is described from 3000 m
depth in the deepest part of the slope, up to 1880 m in the
canyon axis. The T unit is, however, lacking out of the
Oristano Amphitheatre at similar depths, and there are no
evaporite deposits in Hole 133, where the Messinian level is
lying at 2623 m depth on the margin basement, suggesting
different types of sedimentation in the Oristano region and
away from it, at similar depths. Moreover, the positive
correlation between the occurrence of the T unit and the
canyons location strongly suggests that the presence of the T
unit is controlled by the terrigeneous feeding from the
uppermost margin rather than by sea-level changes. The
variability of the acoustic facies of the T unit in and outside
of the canyons, together with the layer thickening and
dipping toward the canyons also suggest a detrital nature of
the unit. This interpretation is supported by the similarity of
the acoustic facies of T and C units in the canyons axis,
where it difficult to decipher the two units from one another.
Therefore, we infer from our data that the T unit mainly
corresponds to a detrital fan.
Despite the acoustic facies described for the T unit
presents numerous similarities with that of the C unit,
especially in the canyons axis, the clear angular unconfor-
mity observed almost everywhere between the C and T
units, together with the increased continuity of the reflectors
F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773 769
depicted for the T unit, suggest a significant depositional
change between these two units.
Some arguments favour a submarine origin for the T unit,
and suggest that the sea-level has significantly risen after the
C deposition. These arguments are: (1) the generalized
concordance between the T unit and the Plio-Quaternary
sequence, (2) the local changes in facies observed within the
T unit, which becomes sometimes transparent, similarly to
the base of the Plio-Quaternary sequence; and finally (3) the
location of the wedge of the T unit on the continental slope,
which always lies upslope from the C unit wedge, on the
erosional surface (Fig. 6).
On the northern Ligurian margin, submersible dives
observed a detrital unit over the massive conglomerates
attributed to the C unit, made of coarse grained conglom-
erates alternating with levels of silts and sandstones (Savoye
and Piper, 1991; Bigot-Courmier et al., 2004; Sosson and
Guennoc, 1998) in increasing proportion when moving
upward in the unit (Savoye and Piper, 1991; Rohais, 2002).
The element organization indicates a sub-marine reworking
of pre-existing deltas (Savoye and Piper, 1991) and a
deepening of the detrital emplacement relative to the
underlying massive conglomerates (Rohais, 2002). Seismic
lines shot in the diving areas show that this detrital unit
corresponds to a highly reflective acoustic unit, which is
locally discordant on the underlying acoustic unit inter-
preted as the low-stand Messinian delta (Bigot-Cormier
et al., 2004; Rohais, 2002; Camera, 2002; Sage et al., 2002).
The reflective seismic unit is observed in the vicinity of the
canyons, but gradually disappears on the interfluves (Sage
et al., 2002; Camera, 2002). Following the Sardinia
example, the existence of this detrital unit thus seems to
be conditioned by the alluvial supply.
Therefore, we propose that the T unit corresponds to a
detrital unit fed by the margin through the valleys of the
Oristano Amphitheatre. We explain the angular unconfor-
mity between the both C and T detrital units by a sea-level
rise, and an open-marine environment for the T unit, that
predates deposition of the classical PQ sequence. We
explain the facies variation along the margin by a submarine
redistribution of the alluvial sediments supply. Indeed,
while the coarse grained elements were deposited in the
canyon axis, the thinner elements were transported laterally
rather far away from the canyon axis, (Bardaji et al., 1990).
The restricted extension of the T unit in comparison with
that of the underlying C unit could mark a decreasing
surface of the margin area submitted to erosion owing to the
sea rising. The restriction in the detrital supply related to
sea-level rise may be particularly clear on the Sardinia
margin because most of the feeding is provided here by the
erosion of the margin itself, since the alluvial sediments
provided by the hinterland were captured by the grabens
onshore (Thomas and Gennesseaux, 1986; Casula et al.,
2001; Lecca et al., 1997). The restricted extension of the T
unit may also be due to a channeling of the transported
elements, subsequent to the sea-level rise. We therefore
propose that the lack of T unit inferred from the seismic
lines out of the Oristano Amphitheatre is mainly related to
the absence of large feeding channels able to provide and
transfer detrital sediments across the margin.
7.2.2. Geometrical and time relationships between T unit
and the deep basin evaporites
Because of the lack of seismic line joining the Oristano
Amphitheatre to the area drilled during DSDP Leg XIII, it is
impossible to directly observe the stratigraphic relationship
between the T unit on one hand, which is observed beneath
the continental slope inside the Oristano Amphitheatre and
the adjacent part of the deep basin, and the Upper Evaporites
on the other hand, which were drilled in the deep basin. Two
distinct interpretations are proposed below, which are both
compatible with the observation above.
7.2.2.1. Is the T unit coeval with the Upper Evaporites?. The
stratigraphic location of the T unit, between the PQ
sequence and the C unit, together with the unit thickness
and depth, suggest that T could be considered as a time-
equivalent of the Upper Evaporites, although we assumed
they are different in nature. In the Upper Evaporites, the
marls levels include assemblages of siliceous microfossils,
which suggest that an open marine environment existed
intermittently for short periods of time at the end of the
MSC. The Upper Evaporites are thus believed to mark the
initiation of the transgression at the end of the Messinian
crisis (Ryan et al., 1973), implying a globally higher sea-
level than at earlier times. The T unit assumed to be
deposited in a marine environment (i.e. deeper than for C
unit) would thus correspond to the time-equivalent of the
Upper Evaporites. Nevertheless, the T unit is observed up to
1800 m depth in its uppermost part, whereas the Upper
Evaporites are described at about 3100 m depth. This depth
difference in the order of 1000 m can be explained by two
factors: (1) a differential subsidence between the margin and
the deep basin since the Messinian times, and (2) the
collapse of the post-salt units at the deep basin edge because
of the migration of the underlying salt toward the basin.
According to previous studies conducted on the northern
Liguro-Provencal basin, the subsidence at the foot of the
margin since the Messinian age could reach 800–1000 m in
amplitude (Savoye and Piper, 1991), and could explain a
differential depth in the order of 400–500 m between the
upslope part of the T unit and the deep basin edge. The
vertical shift across the salt-related deformed area at
the margin-basin connection also gives a 400–550 m
collapse of post-salt units lying downslope from the
deformed area linked to the salt tectonics. The present-day
depths of both T and UE units are thus compatible with a
similar depth of deposit at Messinian times.
In that case, the detrital T unit would be the upward
continuation of the C unit, deposited whilst the uppermost
margin was still submitted to erosion (Fig. 8a). It is then
coeval with the initiation of the transgression, i.e. with
Fig. 8. Restoration of the acoustic units described in the paper, before the
salt tectonics. According to the seismic observations, we propose two
possible interpretations for the stratigraphic relationships between the
detrital delta emplaced off the Oristano Amphitheatre and the Evaporites
drilled south of the Amphitheatre, in the deep basin. In both interpretations,
the massive salt layer emplaced at the deep margin before the detrital deltas
feeded by the subaerial erosion of the margin. (a) The T unit is coeval with
the Upper Evaporites, both characterizing the end of the low-stand
Messinian sea-level, with alternation of open marine and playa
environments. (b) The uppermost part of the C unit is coeval with the
Upper Evaporites, the T unit corresponding to the first Pliocene high-stand
sea-level deposit at the canyon axis.
F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773770
the deep Upper Evaporites, but is still at a low-stand sea-
level. In this interpretation, we cannot exclude that the
detrital T unit locally contains some evaporites, although
evaporites were never sampled on the basin margins
(Fig. 8a).
7.2.2.2. Is the T unit Pliocene?. Although the stratigraphic
relationship inferred above between T unit and the Upper
Evaporites appears satisfactory when comparing the seismic
lines in the Oristano Amphitheatre and in the drilling area, it
assumes that the type of sedimentation is identical whether it
takes place off a wide feeding system (Oristano Amphithea-
tre) or away from it (drilling area). Because there are no
seismic lines connecting the two areas, we can compare our
observation with available seismic lines shot parallel to the
northern Ligurian Margin, seaward from the Saint-Tropez
canyon, in order to appreciate how the acoustic units vary in
the deep basin when crossing a major canyon (Fig. 9).
Although these two areas are far away from one another,
similar acoustic sequences are observed at the basin edge,
both in the canyon axis and away from it.
In the deep basin, while approaching the canyon axis, a
more or less chaotic and highly reflective unit gradually
emplaces and thickens between the salt and the Upper
Evaporites (C1 and C2 on Fig. 9). Based on its facies
and stratigraphic location, such unit is interpreted as
the SARD’85 and SESAME C unit equivalent, emplaced
above the salt layer. Toward the basin, C2 unit laterally
passes to the Upper Evaporites (Fig. 9). The coeval Upper
Evaporites and C2 unit are overlaid by the PQ sequence,
which is transparent at its base away from the canyon (base
of the PQ sequence on Fig. 9), and becomes highly reflective
at the canyon axis (P on Fig. 9), with a seismic facies similar
to the T unit observed along the western Sardinian margin.
Off Saint-Tropez canyon, the lateral variations of the
different units since the Messinian times can be explained
by the sedimentation of terrigeneous influx in the canyon
axis, which laterally passes to the basinal sedimentation.
Over the salt layer, the C1 unit extends far from the canyon
axis during the main Messinian erosion stage. While the
Upper Evaporites deposit in the deep basin, the erosion and
its related deposition continue (C2 unit), but are restricted to
the vicinity of the canyon axis. Finally, a channel-levee
system emplaces during the Pliocene at a deep sea-level.
The observed variation of the Pliocene acoustic facies
perpendicularly to the canyon is explained by the transition
between coarse grained sediments lying at the canyon axis
and its lateral levees composed of thinner sediments, as
previously observed in the Var deep sea fan system for
instance (Savoye et al., 1993; Migeon et al., 2001).
It is clear from this example that similar acoustic facies
successions can be explained differently depending on their
position relative to the canyon. In particular, the Upper
Evaporites facies commonly described in the deep
Mediterranean basin can be similar to that of the Pliocene
sequence observed at the canyons axis, corresponding to
coarse-grained sediments. Such control of the deep basin
sedimentation by the canyons feeding may also be expected
off western Sardinia, since the Oristano area concentrates
most of the channels of the margin. Therefore, we cannot
exclude that the T unit may correspond to a Pliocene
sedimentation, passing laterally to the transparent lower
Pliocene, explaining the observed changes in the T acoustic
facies. In that case, it corresponds to deep water sediments,
emplaced at a canyon axis (Fig. 8b), and the Upper
Evaporites are coeval with the uppermost part of the C
unit (Fig. 8b).
At the drilling sites of Leg XIII, the lowermost Pliocene
is lacking, indicating a gap of 80,000 years at the base of the
PQ sequence (Ryan et al., 1973). The above sediments
correspond to Pliocene nannofossil calcareous marl oozes
including large rounded cobbles of metagreywacke and
numerous fragments of feldspathic schist (Ryan, 1973).
These marls are interpreted as pelagic marine sediments,
except for some allochtonous fragments of rocks (Ryan,
1973; Hsu et al., 1973). Some of the lower Pliocene
collected cores display moreover contemporaneous defor-
mational structures including folding and overturned strata
that are intercalated into layers typical of current-controlled
deposition. The deformation of the lower Pliocene sedi-
ments, together with the occurrence of fragments of rocks in
the layer tend to indicate that part of the lower Pliocene was
Fig. 9. Seismic line recorded during the ‘MESEA 90’ cruise (N/O Jean Charcot). The line was shot in the deep basin, parallel to the margin and perpendicularly
to the major canyon of Saint-Tropez. The acoustic units are controlled by their location relative to the canyon axis. At Messinian times, during the low-stand
sea-level, sedimentation is dominated by deposits of evaporites, as in most deep Mediterranean basins, with some detrital supply at the canyon axis. At Pliocene
time, the sedimentation is controlled by the canyon supply and is typical of channel-levee systems described around the Mediterranean basins.
F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773 771
emplaced as large slumps from the continental slope (Ryan
et al., 1973; Hsu and Ryan, 1973). Consequently, if it is
Pliocene in age, the detrital T unit may be a time-equivalent
either to the stratigraphic gap or to the following slumping
stage both observed in the drilling area, but located in the
Oristano Amphitheatre region which was widely fed by the
margin alluvial system. It could be a deep fan deposit of very
limited size associated with turbity currents and debris flows
triggered by the rapid Pliocene flooding.(Fig. 8b)
8. Conclusions
A synthesis of new and previous seismic data collected
off the western Sardinian margin accurately image the
effects of the Messinian Salinity Crisis from the salt deposit
up to the Pliocene flooding.
In this study, we characterize for the first time the
Messinian detrital bodies emplaced in the lower part of the
western Sardinian margin, in response to the sub-aerial
erosion of the continental slope. The detrital deposits are
organized into two successive alluvial fans (C and T units),
reaching together 500–600 m of maximum thickness. We
infer that both are fed by the subaerial erosion of the margin
itself owing to the upstream capture of most of the
hinterland sediments. The earliest sedimentary body (C
unit) is observed continuously all along the margin and
the adjacent part of the deep basin, whereas the youngest
one (T unit) appears to be restricted to the vicinity of the
detrital supplying systems. This upward evolution is
attributed to a sea-level rise that has two main effects: (1)
to limit the surface of the margin submitted to erosion, and
(2) to allow for a channeling of the sediments in an open
marine environment. This interpretation is in good
agreement with results from dives conducted along the
northern Ligurian margin where similar seismic units are
made of detrital fan conglomerates deposited close to the
shoreline, showing an upward deepening of the deposit
environment. The earliest detrital delta (C unit) is thus
inferred to develop as long as the sea-level remained at its
lowest stand (i.e. coeval with the salt layer, and/or with the
Upper Evaporites), since the second one (T unit) can be
interpreted in two different ways: the T unit could be the
upward continuity of C unit, deposited under slightly higher
sea-level at the end of the Messinian low-stand sea-level,
and corresponds in that case to a time equivalent of the
Upper Evaporites (Fig. 8a); the T unit could also be
interpreted as the first coarse grained sediments emplaced
off the major canyon at Pliocene time, once the sea-level
rose up. In that case, the uppermost part of C unit would be
the time-equivalent of the Upper Evaporites (Fig. 8b).
We also show evidence for (1) a vertical transition
between the salt layer and the detrital delta, and (2) salt
tectonics, which affects both C and T detrital units. This
F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773772
implies that the salt began to emplace at the base of the
continental slope before the alluvial fan. The collapse of the
sedimentary layers deposited over the salt is related to salt
flows toward the basin and indicates that 300–500 m of
ductile salt (i.e. salt containing little detritic fraction) were
deposited before the low-stand Messinian delta emplaced.
This result supports the idea suggested by recent modeling
that the salt began to precipitate at a rather high stand sea-
level, as soon as the saturation required for chlorite
precipitation was reached. The seismic lines, which are
restricted to the vicinity of the continental margin, do not
provide information on the stratigraphic relationship
between the seaward termination of the detrital bodies and
the evaporitic sequences, which lies out of the study area.
We can however, suppose that the evaporites (massive salt
and/or Upper Evaporites) still precipitate seaward of the
detrital fan, as long as the sea-level is low. Detrital
emplacement at the deep margin thus probably moved the
landward extension of the deep evaporites toward the deep
basin.
The sub-aerial Messinian erosion is marked by a sharp
reflector which is evidenced on our seismic lines down to
the apex of the Messinian alluvial fan, at around 2000 m
depth. At this depth, a general slope break together with a
change in the slope of the canyons flanks suggest that the
erosion surface progressively becomes downslope a
depositional surface, which could be either sub-aerial or
sub-marine. This could indicate that the shoreline was stable
around this depth for most of the duration of the crisis,
during the unit C edification. There is no evidence from our
data, that the sea-level was lying much deeper under C unit
for a long time, even before the salt deposition. The lack of
detrital bodies on the landward termination of the salt layer
leads us to propose that the pre-salt morphology is due to
sub-marine erosion emphasizing an irregular morphology
inherited from the Oligo-Miocene geological history of the
margin.
Acknowledgements
The SESAME cruise was supported by ‘ECLIPSE’ and
‘OCEANS’ and ‘GDR MARGES’ INSU/CNRS programs.
We sincerely thank W.B.F. Ryan and anonymous reviewer
who both helped us to improve the text and the figures of our
manuscript. We also wish to thank Alain Moreau for his
technical support despite a painful back, as well as Lucciano
Lecca and Andrea Billi, for their participation to the data
acquisition. We thank the captain of the R/V ‘Tethys II’
(INSU/CNRS) Alain Stephan and his crew for the data
acquisition and the nice atmosphere on board in spite of bad
meteorological conditions. We also warmly acknowledge
Georges Clauzon, Johanna Lofi and Jean-Pierre Rehault for
fruitful discussions on the seismic lines. Finally, we thank
G. Bellaiche for providing seismic lines from MESEA
cruise’90. This is UMR 6538 contribution N8 757.
References
Bardaji, T., Dabrio, C.J., Goy, J.L., Somoza, L., Zazo, C., 1990. Pleistocene
fan deltas in southeastern Iberian peninsula: sedimentary controls and
sea-level changes. In: Colella, A., Prior, D.B. (Eds.), Coarse-grained
Deltas Spec. Publs Int. Ass. Sediment., vol. 10. Blackwell Scientific
Publications, Oxford, pp. 129–154.
Bertolani-Marchetti, D., 1985. Pollen paleoclimatology in the Mediterra-
nean since Messinian time. In: Stanley, D.J., Wesel, F.C. (Eds.),
Geological Evolution of the Mediterranean Basin. Springer, New York,
pp. 525–543.
Bigot-Cormier, F., Sage, F., Sosson, M., Deverchere, J., Ferrandini, M.,
Guennoc, P., Popoff, M., Stephan, J.-F., 2004. Deformations pliocenes
de la marge nord-Ligure (France): consequences d’un chevauchement
crustal sud-alpin. Bull. Soc. Geol. De France 175 (2), 197–211.
Blanc, P.-L., 2000. Of sills and straits: a quantitative assessment of the
Messinian Salinity Crisis. Dea-Sea Research I 47, 1429–1460.
Blanc, P.-L., 2002. The opening of the Plio-Quaternary Gibraltar Strait:
assessing the size of a cataclysm. Geodin. Acta 15, 303–317.
Camera, L., 2002. Caracterisation de l’erosion et des produits de l’erosion
lies a la Crise de Salinite Messinienne dans le domaine marin Nord-
Ligure—relations entre marge et bassin. Rapport de D.E.A., Universite
Pierre et Marie Curie, Paris. 39p.
Casula, G., Cherchi, A., Montadert, L., Murru, M., Sarria, E., 2001. The
Cenozoic graben system of Sardinia (Italy): geodynamic evolution from
new seismic and field data. Marine and Petroleum Geology. 18, 863–
888.
Chaumillon, E., Deverchere, J., Rehault, J.-P., Gueguen, E. 1994.
Reactivation tectonique et flexure de la marge continentale Ligure
(Mediterranee Occidentale). C.R. Acad. Sci., Paris, 319(serie II), 675–
682.
Clauzon, G., 1982. Le canyon messinien du Rhone: une preuve decisive du
’dessicated deep-basin model’ (Hsu, Cita et Ryan, 1973). Bull. Soc.
Geol. France 24, 231–246.
Clauzon, G., Suc, J.-P., Gautier, F., Berger, A., Loutre, M.-F., 1996.
Alternate interpretation of the Messinian salinity crisis: controversy
resolved? Geology 24 (4), 363–366.
Dos Reis, A.T., 2001. La tectonique salifere et son influence sur
l’architecture sedimentaire quaternaire de la marge du Golfe du Lion
en Mediterranee Occidentale. These de l’Universite Paris VI, 2
volumes, 373 p.
Dos Reis, A.T., Gorini, C., Mauffret, A., Weibull, W., 2004a. Salt tectonics,
a controlling factor on the development of the Marseilles and Grand-
Rhone sedimentary ridges, Gulf of Lions, western Mediterranean Sea.
C. R. Geosci. 336 (2), 143–150.
Dos Reis, A.T., Gorini, C., Mauffret, A., Weibull, W., 2004b.
Stratigraphic architecture of the Pyreneo-Languedocian submarine
fan, Gulf of Lions, western Mediterranean Sea. C. R. Geosci. 336
(2), 125–133.
Dos Reis, A.T., Gorini, C., Mauffret, A. Implications of salt-sediment
interactions on the architecture of the Gulf of Lions deep-water
sedimentary systems-Western Mediterranean Sea. Marine and Pet-
roleum Geology, this issue, doi:10.1016/j.marpetgeo.2005.03.006.
Dunne, L.A., 1988. Fan-Deltas and braid deltas: varieties of coarse-grained
deltas—Discussion. Geol. Soc. Am. Bull. 100, 1308–1309.
Einsele, G., 1996. Event deposits: The role of sediment supply and relative
sea-level changes-overview. Sed. Geology 104, 11–37.
Fais, S., Klingele, E.E., Lecca, L., 1996. Oligo-Miocene half graben
structure in Western Sardinian Shelf (western Mediterranean):
Reflexion seismic and aeromagnetic data comparison. Marine Geology
133, 203–222.
Garces, M., Krijgsman, W., Agusti, J., 1998. Chronology of the late
Turolian deposits of the Fortuna basin (SE Spain): implications for
the Messinian evolution of the eastern Betics. Earth Plan. Sci. Let.
163, 69–81.
F. Sage et al. / Marine and Petroleum Geology 22 (2005) 757–773 773
Gaullier, V., 1993. Diapirisme salifere et dynamique sedimentaire dans le
bassin Liguro-Provencal: donnees sismiques et modeles analogiques.
These de doctorat, Universite Pierre et Marie Curie, Paris. p. 327.
Gaullier, V., Bellaiche, G., 1996. Diapirisme liguro-provencal: les effets
d’une topographie residuelle sous le sel messinien. Apports de la
modelisation analogique. C.R. Acad. Sc., Paris 332 (serie IIa), 213–220.
Gaullier, V., Vendeville, B., in press. Salt tectonics drwein by Sediment
progradation. Part II: Radial spreading of sedimentary lobes prograding
above salt. AAPG Bulletin, in press.
Gaullier, V., Mart, Y., Bellaiche, G., Mascle, J., Vendeville, B., Zitter, T.,
the second leg ‘PRISMED II’ scientific party, 2000. Salt tectonics in
and around the Nile Deep-Sea Fan: insights from ‘PRISMED II’ cruise.
In: Vendeville, B., Mart, Y., Vigneresse, J.-L. (Eds.), Salt, Shale and
Igneous Diapirs in and around Europe, vol. 174. Geological Society,
London, pp. 111–129 (special publications).
Gorini, C., Le Marrec, A., Mauffret, A., 1993. Contribution to the structural
and sedimentary history of the gulf of Lions (western Mediterranean),
from the ECORS profiles, industrial seismic profiles and well data. Bull.
Soc. Geol. France 164 (3), 353–363.
Guzzetti, F., Marchetti, M., Reichenbach, P., 1997. Large alluvial fans in
the north-central Po Plain (Northern Italy). Geomorph 18, 119–136.
Harvey, A.M., 2002. The role of base-level change in the dissection of
alluvial fans: case studies from southeast Spain and Nevada. Geomorph
45, 67–87.
Hsu, K.J., Ryan, W.B.F., 1973. Summary of the evidence for extensional
and compressional tectonics in the Mediterranean. In: Kaneps, A.G.
(Ed.), Deep Sea Drilling Project, Leg 13. National Science Foundation,
Arlington, VA, pp. 1011–1020.
Hsu, K.J., Ryan, W.B.F., Cita, M.B., 1973. Late Miocene dessication of the
Mediterranean. Nature 242, 240–244.
Krijgsman, W., Hilgen, F.J., Raffi, I., Sierro, F.J., Wilson, D.S., 1999.
Chronology, causes, and progression of the Messinian salinity crisis.
Nature 400, 652–655.
Lecca, L., Lonis, R., Luxoro, S., Melis, E., Secchi, F., Brotzu, P., 1997.
Oligo-Miocene volcanic sequences and rifting stages in Sardinia: a
review. Per. Miner. 66, 7–61.
Lofi, J., Gorini, C., Berne, S., Clauzon, G., Dos Reis, A.T., Ryan, W.B.F.,
Steckler M.S., in press. Erosional processes and paleo-environmental
changes in the Western Gulf of Lion (SW France) during the Messinian
Salinity Crisis. Marine Geology, in press.
Martin, J.M., Braga, J.C., Beltzer, C., 2001. The Messinian Guadalhorce
corridor: the last normative, atlantic gateway. Terra Nova 13 (6), 418–
424.
Massari, F., Parea, G.C., 1990. Wave-dominated Gilbert-type deltas in the
hinterland of the Gulf of Taranto (Quaternary, southern Italy). In:
Colella, A., Prior, D.B. (Eds.), Coarse-grained Deltas Spec. Publs Int.
Ass. Sediment., vol. 10. Blackwell Scientific Publications, Oxford,
pp. 311–331.
Mauffret, A., Pascal, G., Maillard, A., Gorini, C., 1995. Tectonics and deep
structure of the north-western Mediterranean Basin. Mar. Pet. Geol. 12
(6), 645–666.
Mauffret, A., Durand de Grossouvre, B., Dos Reis, A.T., Gorini, C.,
Nercessian, A., 2001. Structural geometry in the eastern Pyrenees and
western Gulf of Lion (Western Mediterranean). J. Struct. Geol. 23 (11),
1701–1726.
Migeon, S., Savoye, B., Zanella, E., Mulder, T., Faiugeres, J.-C.,
Weber, O., 2001. Detailed seismic reflexion and sedimentary study of
turbidite sediment waves on the Var Sedimentary Ridge (SE France):
significance for sediment transport and deposition and for the
mechanisms for sediment-wave construction. Mar. Pet. Geol. 18,
179–208.
Nemec, W., 1990a. Aspects of sediment movement on steep delta slopes.
In: Colella, A., Prior, D.B. (Eds.), Coarse-grained Deltas Spec. Publs
Int. Ass. sediment., vol. 10. Blackwell Scientific Publications, Oxford,
pp. 29–73.
Nemec, W., 1990b. Deltas—remarks on terminology and classification. In:
Colella, A., Prior, D.B. (Eds.), Coarse-grained Deltas Spec. Publs Int.
Ass. sediment., vol. 10. Blackwell Scientific Publications, Oxford,
pp. 3–12.
Postma, G., 1990. Depositional architecture and facies of river and fan
deltas: a synthesis. In: Colella, A., Prior, D.B. (Eds.), Coarse-grained
Deltas spec. Publs Int. Ass. Sediment., vol. 10. Blackwell Scientific
Publications, Oxford, pp. 13–27.
Rehault, J.-P., 1981. Evolution tectonique et sedimentaire du bassin Ligure
(Mediterranee Occidentale). These d’etat, Universite Pierre et Marie
Curie, Paris. p. 128.
Rehault, J.-P., Boillot, G., Mauffret, A., 1984. The Western Mediterranean
basin geological evolution. Mar. Geol. 55, 447–477.
Richards, M., Bowman, M., Reading, H., 1998. Submarine-fan systems I:
characterization and stratigraphic prediction. Mar. Pet. Geol. 15, 689–
717.
Rohais, S. (2002). Synthese geologique de la Baie des Anges, Mer Ligure
(Mer Mediterranee): conception et exploitation d’un systeme d’infor-
mation geographique en geologie marine. Memoire d’ingenieur de
l’IGAL no.181, IFREMER, Cergy-Pontoise. p. 152.
Rouchy, J.-M., 2001a. Environnements evaporitiques actuels. Geochroni-
que 80, 15–18.
Rouchy, J.-M., 2001b. Le fonctionnement des bassins evaporitiques.
Geochronique 80, 14.
Rouchy, J.-M., Taberner, C., Blanc-Valleron, M.-M., Sprovieri, R.,
Russell, M., Pierre, C., Di Stefano, E., Pueyo, J.J., Caruso, A.,
Dinares, J., Gomis-Coll, E., Cespuglio, G., Wolff, G., Ditchfield, P.,
Santisteban, C., Pestrea, S., Combourieu-Nebout, N., Santisteban, S.,
Grimalt, J.O., 1998. Sedimentary and diagenetic markers of the
restriction in a marine basin: the Lorca basin (SE Spain) during the
Messinian. Sediment. Geol. 121, 23–55.
Ryan, W.B.F., 1973. Geodynamic implications of the Messinian crisis of
salinity. In: Drogger, C.W. (Ed.), Messinian Events in the Mediterra-
nean. North Holland, Amsterdam, pp. 26–38.
Ryan, W.B.F., Hsu, K.J., Cita, M.B., Dumitrica, P., Lort, J., Maync, W.,
Nesteroff, W.D., Pautot, G., Stardner, H., Wesel, F.C., 1973. Boundary
of Sardinia slope with Balearic Abyssal Plain—sites 133 and 134. In:
Ryan, W.B.F., Hsu & al, K.J. (Eds.), Init. Rep. DSDP 13, pp. 465–514.
Sage, F., Deverchere, J., Bigot-Cormier, F., Gaullier, V., Gorini, C.,
Maillard, A., Von Gronefeld, G., 2002. The sedimentary markers of the
messinian crisis around the Liguro-Provencal Basin. Western Medi-
terranean, EGS, 27th General Assembly, vol. 4.
Savoye, B., Piper, D.J.W., 1991. The Messinian event on the margin of the
Mediterranean Sea in Nice area, southern France. Mar. Geol. 97, 279–
304.
Savoye, B., Piper, D.J.W., Droz, L., 1993. Plio-Pleistocene evolution of the
Var deep-sea fan off French Riviera. Mar. Pet. Geol. 10, 550–571.
Seranne, M., 1999. The Gulf of Lion continental margin (NW
Mediterranean) revisited by IBS: an overview. In: Durand, B.,
Jolivet, L., Horvath, G.F., Seranne, M. (Eds.), The Mediterranean
Basins: Tertiary Extension within the Alpine Orogen, vol. 156.
Geological Society Special Publication, London, pp. 15–37.
Seranne, M., Benedicto, A., Labaume, P., Truffert, C., Pascal, G., 1995.
Structural style and evolution of the Gulf of Lion Oligo-Miocene
rifting: role of the Pyrenean orogeny. Mar. Pet. Geol. 12 (8), 809–820.
Sosson, M., Guennoc, P., et l’equipa CYCLICE, 1997. Nouvelles
contraintes geologiques sur l’evolution du bassin ligure: resultats de
la campagne de plongee CYCLICE (1997) RST1998, pp 196–197.
Thomas, B., Gennesseaux, M., 1986. A two-stage rifting in the basins of the
Corsica-Sardinia straits. Mar. Geol. 72, 225–239.
Thomas, B., Lecca, L., Gennesseaux, M., 1988. Structuration et
morphogenese de la marge occidentale de la Sardaigne au Cenozoıque.
C.R. Acad. Sci. Paris 306 (Serie. II), 903–910.
Vendeville, B., 1987. Champs de failles et tectonique en extension:
modelisation experimentale. These de doctorat, Memoires et Documents
du Centre Armoricain d’Etudes Structurales des Socles (CAESS),
Rennes, vol. 15.
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