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.

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