Acknowledgements The authors would like to acknowledge warmly everyone who brought support and assistance during
TOTAL, BRGM and Action Marges (Action coordonnée CNRS-INSU; TOTAL; IFREMER; BRGM; IFP)
the preparation of this atlas. Thanks are due to the INSU (Institut National des Sciences de l’Univers)-
are acknowledged for their support to the printing of the Atlas.
CNRS (Centre National de la Recherche Scientifique) ECLIPSE French research program (20012004 and 2004-2007), which is at the origin of this project. Part of this work was funded by the following programs or partners: GDR (Groupe De Recherche) “Marges”, INSU-CNRS, ACI (Action Concertée Incitative) Algeria, Ministry of Research and High Education France, GEMME and SAGA. The BRGM's research works have been funded in the framework of the Geological Mapping Program of France - "Land-Sea transition" project. The authors wish to thank IFREMER for giving us access to seismic data from the BlaSON and Calmar surveys. We are grateful to Yves Lagabrielle, for his receptiveness to our project in its initial form, his precious advices and his encouraging words throughout the development of the atlas. Françoise Rangin is acknowledged for her help, careful reading and constant support during the editing process. Philippe Rossi and Olivier Lacombe are also acknowledged. François Guillocheau and an anonymous reviewer are thanked for their constructive comments on the first version of this atlas. Thanks are also due to Joelle Gastambide for the design of the atlas cover and to Simon Barry for the English corrections. The Messinian community is wide, bringing together several disciplines and several approaches in order to better understand the Messinian salinity crisis event. We are thankful to all our “Messinian” colleagues and friends for discussions during congresses and meetings. Special thanks are due to George Clauzon, William B.F. Ryan and Jean-Pierre Suc who kindly shared with us their knowledge and their passion for this fascinating event. All those mentioned above considerably helped us to achieve this global work, that we hope to be as consistent as possible.
SEISMIC ATLAS OF THE "MESSINIAN SALINITY CRISIS" MARKERS IN THE MEDITERRANEAN AND BLACK SEAS J. Lofi1, J. Déverchère2, V. Gaullier3, H. Gillet4, C. Gorini5, P. Guennoc6, L. Loncke3, A. Maillard7, F. Sage8, I. Thinon6
With contributions from J. Benkhelil3, S. Berné3,9, C. Bertoni10,11, L. Camera8, J.A. Cartwright11, A. Capron2, J.-J. Cornée1, S. Dümmong12, C. Hübscher12, G. Lericolais9, P. Le Roy2, S. Leroy13, J. Mascle8, A. Mauffret5, B. Mercier de Lépinay8, E. M. Obone-ZuéObame3, J.-P. Rehault2, B. Savoye9, N. Sellier13, E. Tahchi3, G. Von Gronefeld8, A.K. Yelles14.
1 Géosciences
Montpellier, cc60, Bât. 22, Université de Montpellier 2, Place Eugène Bataillon, 34095 Montpellier cedex 05, France Européenne de Bretagne, Domaines Océaniques, UMR 6538 CNRS, Université de Brest (UBO), IUEM, Place N. Copernic, 29280 Plouzané, France 3 Laboratoire IMAGES, E.A. 4218, Université de Perpignan, Via Domitia, 52 avenue Paul Alduy, 66860 Perpignan cedex, France 4 UMR 5805-EPOC, Université Bordeaux 1, 33405 Talence, France 5 ISTeP, Université Pierre et Marie Curie-Paris 6, 4 place Jussieu, 75252 Paris cedex 05, France 6 BRGM, GEO-GBS, 3 avenue Claude Guillemin, BP 6009, 45060 Orléans cedex 02, France 7 LMTG, Université de Toulouse-CNRS-IRD-OMP, 14 avenue Ed. Belin, 31400 Toulouse, France 8 Géoazur, Université Pierre et Marie Curie-Paris 6, UNS-CNRS-IRD-OCA, B.P. 48, 06235 Villefranche-sur-Mer cedex, France 9 IFREMER, Géosciences Marines, Technopôle Brest-Iroise, BP 70, 29280 Plouzané, France 10 Repsol-YPF, Paseo de la Castellana 278-280, 28046 Madrid, Spain 11 3DLab, School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff, UK 12 University of Hamburg, Bundesstrasse 55, 20146 Hamburg, Germany 13 Laboratoire PBDS, Université de Lille 1, UMR 8110, SN5, 59655 Villeneuve d'Ascq cedex, France 14 C.R.A.A.G. (Centre de Recherche en Astronomie, Astrophysique, et Géophysique), Route de l’Observatoire, BP63, Bouzaréah, Algiers, Algeria 2 Université
1
CONTENTS
3.A- Regional setting 3.B- MSC surfaces 3.C- MSC basinal units 3.D- MSC clastics 4. Provençal Basin 4.A- Regional setting 4.B- MSC surfaces and basinal units 4.C- Salt tectonics 5. Northern Ligurian Margin 5.A- Regional setting 5.B- Slope MSC surfaces and units 5.C- MSC basinal units 6. East-Corsica Basin 6.A- Regional setting 6.B- MSC surfaces 6.C- MSC basinal units 7. West-Corsica Basin 7.A- Regional setting 7.B- MSC surfaces 7.C- MSC basinal units 8. Western Sardinia 8.A- Regional setting 8.B- MSC surfaces and basinal units 9. Nile Deep Sea Fan 9.A- Regional setting 9.B- MSC surfaces and clastics 9.C- MSC basinal units 10. Levant Basin 10.A- Regional setting (1) 10.B- Regional setting (2) 10.C- MSC surfaces and clastics 10.D- MSC basinal units 10.E- Salt tectonics and fluid dynamics 11. Cyprus Arc 11.A- Regional setting 11.B- MSC surfaces 11.C- MSC basinal units 12. Florence Ridge and South Antalya Basin 12.A- Regional setting 12.B- MSC surfaces and clastics 12.C- MSC basinal units 13. Western Black Sea 13.A- Regional setting 13.B- MSC surfaces
Acknowledgements Forewords ............................................................................................................................................. 3 Use of the seismic atlas of the Messinian markers in the Mediterranean and Black seas.................... 4 1.
Introduction .................................................................................................................................. 5
2.
The Messinian salinity crisis event: main facts and uncertainties ............................................... 5
3.
Why studying the MSC in the offshore area? .............................................................................. 6
4.
Atlas design ................................................................................................................................. 7 4.1. Messinian marker nomenclature and colour code ............................................................. 7 4.2. Organisation ...................................................................................................................... 9 4.3. References to figures and numerotation.......................................................................... 10
5.
Description of the Messinian markers in the offshore domain ................................................... 10 5.1. Messinian salinity crisis surfaces ..................................................................................... 10 5.1.1. Margin Erosion Surface (MES) .............................................................................. 10 5.1.2. Bottom Surface (BS) and Bottom Erosion Surface (BES)...................................... 12 5.1.3. Intermediate Erosion Surfaces (IES)...................................................................... 13 5.1.4. Top Surface (TS) and Top Erosion Surface (TES) ................................................ 14 5.2. Messinian salinity crisis units ........................................................................................... 15 5.2.1. Lower Unit (LU) ...................................................................................................... 15 5.2.2. Mobile Unit (MU) .................................................................................................... 16 5.2.3. Upper Unit (UU) ..................................................................................................... 18 5.2.4. Bedded Units (BU) ................................................................................................. 19 5.2.5. Complex Units (CU) ............................................................................................... 19 5.3. Limits of the offshore approach ....................................................................................... 20
6.
Comparing the MSC markers at the scale of the Mediterranean area ...................................... 21
7.
Conclusions and perspectives ................................................................................................... 23
ILLUSTRATIONS ................................................................................................................................ 25 Synthesis Map 1. Algerian Margin 1.A- Regional setting 1.B- MSC surfaces and clastics 1.C- MSC basinal units 2. Valencia Basin 2.A- Regional setting 2.B- MSC surfaces 2.C- MSC basinal units 2.D- MSC clastics 3. Gulf of Lions
REFERENCES ....................................................................................................................................67 2
FOREWORDS Although the Messinian salinity crisis is commonly treated as a pan-Mediterranean event, its impact was far reaching. More than five percent of the salt of the global ocean was extracted by evaporation in a fraction of a million years. This salt is now archived in the enormous deposit so clearly portrayed in the seismic reflection profiles of this comprehensive atlas. Marine aquatic life within the Mediterranean, Black and Red Seas was extinguished. Brine stratification induced anoxia, resulting in vast storage of carbon. The drop in atmospheric CO2 so thoroughly chilled Earth that glaciers started to grow on Greenland and shed icebergs into the North Atlantic. Extreme temperatures seared the surfaces of saltpans. Alluvial plains appeared at elevations thousands of meters below former shorelines. The loss of the weight of water removed by evaporation and the addition of load from concentrating brines deformed the edges of surrounding continents. But of all the impacts of a sea drying up, the change in the shape of the landscape has left the most dramatic scenery. Yet, until the publication of this atlas, this physical world has been hidden by water and five million years of subsequent sediment cover. Now with the atlas in your lap, you can travel back through time to see the ravages of erosion and the consequences of brine precipitation profile-by-profile and map-by-map stretching from Iberia to the Levant. I hope you enjoy your explorations. There is treasure to be found in this imagery. Guided by the skillful interpretation of the authors, I am confident that all of us will gain new knowledge of this brief, but extraordinary event, that we call the Messinian salinity crisis.
William B. F. Ryan
3
Use of the seismic atlas of the Messinian markers in the Mediterranean and Black seas
It is recommended that reference to the whole atlas should be made as follows: Lofi J., Déverchère J., Gaullier V., Gillet H., Gorini C., Guennoc P., Loncke L., Maillard A., Sage F. & Thinon I. (2011). –
This atlas provides an overview of the most important characteristics of the offshore Messinian
Atlas of the Messinian seismic markers in the Mediterranean and Black seas. – Mém. Soc. géol.. fr.,
salinity crisis (MSC) seismic markers. Throughout several study areas located in the Mediterranean
n.s., 179, and World Geological Map Commission, 72p.
and Black seas, this collective work aims at evidencing the most significant features (seismic facies,
Reference to part of this atlas should be made as follows (see also Table A, below): e.g. Loncke L.,
geometry and extent) of Messinian surfaces and depositional units in the offshore domain. However,
Gaullier V., Camera L. & Mascle J. (2011). – Nile deep sea fan, detritism and erosion. In: J. Lofi et
this atlas does not intend to provide a complete description of all what is known on the MSC and on
al., Eds, Atlas of the Messinian seismic markers in the Mediterranean and Black seas. – Mém. Soc.
the geology of each study area. Accordingly, illustrations in the atlas should be used for a global
géol.. fr., n.s., 179, and World Geological Map Commission, 72p.
description of the offshore imprints of the MSC at the scale of the Mediterranean and Black seas, or for local information or site-specific general interpretations. The listed references should be used for more detailed or additional information. The atlas summarizes geophysical investigations that have been made over many years by a large number of marine geophysicists. It aims to share the geological interpretation of seismic reflection data and to be a reference work that can be used by teachers and researchers working on the Messinian event.
STUDY AREAS 1. Algerian margin 2. Valencia through 3. gulf of Lions 4. Ligurian margin 5. Provençal margin 6. western Corsica 7. eastern Corsica 8. western Sardinia 9. Nile deep sea fan 10. Levant basin eastern Mediterranean 11. Cyprus arc 12. Florence ridge and Antalya basin Black sea 13. Romanian shelf and Black sea Synthesis of the Messinian markers in the Mediterranean and Black seas western Mediterranean
AUTHORS Capron A., Déverchère J., Gaullier V., Le Roy S., Mercier de Lépinay B., Yelles A.K. /Obone-Zué-Obame E.M., Gaullier V., Déverchère J., Capron A., Mercier de Lépinay B., Le Roy S., Yelles A.K. /Capron A., Déverchère J., Obone-Zué-Obame E.M., Gaullier V., Mercier de Lépinay B., Le Roy S., Yelles A.K. Maillard A. and Mauffret A. Lofi J., Berné S. and Gorini C. Sage F. and Déverchère J. Obone-Zué-Obame E.M., Gaullier V., Sage F., Maillard A. and Déverchère J. Guennoc P., Réhault J.-P. and Thinon I. Thinon I., Réhault J.-P. and Guennoc P. Sage F., Déverchère J. and Von Gronefeld G. Loncke L., Gaullier V., Camera L. and Mascle J. Bertoni C. and Cartwright J.A./ Hübscher C. and Dümmong S./ Dümmong S. and Hübscher C. Maillard A., Tahchi E. and Benkhelil J. Loncke L., Sellier N. and Mascle J. Gillet H., Lericolais G. and Réhault J.-P. Maillard A., Lofi J., Déverchère J., Gaullier V., Loncke L., Sage F., Thinon I., Guennoc P., Gillet H. and Gorini C.
Table A: List of the illustrations presented in the atlas of the Messinian salinity crisis seismic markers in the Mediterranean and Black seas with reference to their authors. 4
1. Introduction
a new global and consistent terminology for MSC markers in the entire offshore Mediterranean area in order to avoid nomenclatural problems and (3) to make this information accessible to the non-
The Messinian salinity crisis (hereafter referred as MSC) is a huge, outstanding succession of events
geophysicist community. Interpreted seismic data were therefore carefully selected according to their
that has deeply modified the Mediterranean region environment within a relatively limited time span.
quality, position and significance in order to reach these objectives, and are presented here. Raw and
Since the discovery of the thick evaporitic Messinian layer of the deep basin in the seventies [Hsü et
interpreted data seismic profiles are available in the CR-Rom.
al., 1973], the MSC has been the object of a large number of studies, especially on land, as it offers a unique opportunity to analyze and understand the controlling factors and the paleogeographic,
2. The Messinian salinity crisis event: main facts and uncertainties
paleoenvironmental, sedimentary and geodynamic consequences of a large-scale sea-level drop
Less than 6 Ma ago, the Pan-Mediterranean realm underwent rapid and dramatic paleo-
(> 1 km) of uncommon amplitude on Earth [e.g. McKenzie, 1999, and references therein].
environmental changes known as the Messinian salinity crisis [Hsü et al., 1973]. This short-term
The atlas of the Messinian seismic markers in the Mediterranean and Black seas provides a
event at the geological scale [~5.96-5.32 Ma, Gautier et al., 1994; Krijgsman et al., 1999a] results
summary of the most important observations available from seismic profiles on the MSC markers in
from the progressive closure of the two-way connection between the Atlantic ocean and the
the offshore domains of the Mediterranean and Black seas. This work has been initiated in the
Mediterranean sea [e.g. Benson et al., 1991; Krijgsman et al., 1999b; Govers, 2009; Ryan, 2011].
framework of the French research programs “ECLIPSE” 1 and 2 (Institut National des Sciences de
Among the most important characteristics of this event, we note: (1) a reduction of the Atlantic water
l’Univers, Centre National de la Recherche Scientifique). These two successive projects aimed to
supply resulting in an increased salinity and in the deposition of evaporites within shallow water
define and quantify the responses of the Mediterranean environment to the MSC event, from an
marginal/peripheral basins (today located onshore and isolated from the deep basins); (2) a
ecological point of view as well as from a sedimentary point of view. A multidisciplinary approach has
subsequent major sea-level fall exceeding 1500 m [Ryan and Cita, 1978] and resulting in the
thus been deliberately put forward. In the offshore area, the main objective was to define the
massive erosion of the margins and the development of deep subaerial canyons [Chumakov, 1973;
consequences of the MSC, particularly concerning the relationship between erosion surfaces and
Clauzon, 1973; Guennoc et al., 2000]; (3) the accumulation of the product of the erosion in the
depositional units. It rapidly came out that a better understanding of the crisis required an accurate
downslope domain of the margins [Rizzini et al., 1978; Barber, 1981; Savoye and Piper, 1991; Lofi et
comparisons from one study area to another and the identification and quantification of the main
al., 2005; Sage et al., 2005; Maillard et al., 2006]; (4) the deposition of thick (> 1 km) evaporitic
tectonic and sedimentary changes encountered on the various Mediterranean and Black sea basins
sequences above the deep Mediterranean abyssal plains [Montadert et al., 1970; Hsü et al., 1973];
and margins since 6 Ma. For this purpose, an integrated study at the scale of the Mediterranean area
and (5) a very rapid refilling of the Mediterranean basin during the Latest Miocene/Lower Pliocene,
has been initiated in 2001. This new approach, based on multi-site studies, lies on a comparative
following the re-opening of the communication with the Atlantic ocean at the Gibraltar straight
seismic analysis of the MSC surfaces and depositional units recovered from several offshore areas
[McKenzie, 1999; Blanc, 2002; Loget and Van Den Driessche, 2006]. For more details concerning
[Lofi et al., 2008b]. This allows us to discuss the impact of the MSC on margin segments and basins
the modalities and scenario of the MSC, the history of its discovery and the evolution of the main
that have various geodynamical, structural and geological backgrounds [Lofi et al., 2011]. In this
ideas since the 70’s, one can refer to recent publications, which fully summarize what the MSC is [eg.
framework, the purpose of the atlas is to summarize, in one publication with a common format, the
Rouchy and Caruso, 2006; CIESM, 2008; Ryan, 2009].
most relevant information that has been collected over many years from the offshore domain, and not to discuss either the absolute time succession of the MSC events, or the processes responsible for
As summarized by Rouchy and Caruso [2006], numerous recent MSC scenarii enhance differences
their occurrence in space and time. More precisely, our objectives are: (1) to image the Messinian
in our understanding of the crisis, essentially regarding the timing and numbers of the drawdown
seismic markers on the main margins and in the Mediterranean and Black sea basins; (2) to propose
phases (and related erosions) and the chronology of the deposition of the evaporitic units in the 5
different Mediterranean settings (fig. A). A near-consensus (that still need to be tested) has recently
recognition and understanding of these Messinian markers [eg. Lofi et al., 2005; Sage et al., 2005;
been proposed [CIESM, 2008] around an adaptation of the deep-desiccated basin model [Hsü et al.,
Maillard et al., 2006; Bertoni and Cartwright, 2006, 2007a; Gillet et al., 2007; Lofi et al., 2008a; Lofi et
1973] and of the two-step model of Clauzon et al. [1996]. Several points are however still under
al., 2011]. Indeed, until the nineties, seismic data used to study the deep Mediterranean basins were
debate and the detailed modalities of the crisis are not fully established. Disagreements are mainly
mostly acquired with seismic sources of moderate strength and with 6 channel streamers. The atlas
linked to the fact that most of the works dealing with the Messinian salinity crisis are based on
of the Messinian seismic markers in the Mediterranean and Black seas provides here a summary of
outcrops actually located onshore and isolated from the deep basins (e.g. Morocco, Cyprus, Spain,
the most important observations and recent advances available from seismic profiles for the
Italy…). Some of those outcrops contain MSC evaporites that accumulated in shallow water
Messinian salinity crisis markers.
marginal/peripheral basins (e.g. Spanish basins). These deposits predate the drawdown phase and are not coeval with the Messinian successions of the deep basin [Clauzon et al., 1996; CIESM, 2008]. In addition, these deposits contain incomplete Messinian successions because the marginal basins (and the continental margins) are above the main depressions and have undergone subaerial erosion during the drawdown phase. For these reasons, these outcrops cannot help constraining the Messinian event during the drawdown. Some other MSC deposits, outcropping in the Central Sicilian basin (containing halite), are considered by some authors as part of the deep basin MSC succession [Decima and Wezel, 1973; Ryan and Cita, 1978; Roveri et al., 2008a,b; Ryan, 2009], whereas some other authors propose that they belong to a deeper, but still marginal, setting [Clauzon et al., 1996]. Because of tectonics, those outcrops are now totally disconnected from the deep basin Messinian sequence, and no stratigraphical, geographical, geometrical or sedimentological correspondences can be directly established with offshore MSC deposits. Correlations with the deep basin are thus complex, preventing from constructing an integrated scenario of the MSC that would provide a time
Figure A. – Some examples of different scenarios of the MSC [Roveri et al., 2008b; modified from Rouchy and Caruso, 2006].
link between the evaporites of the marginal basins, the erosion of the margins, and the thick depositional units of the abyssal plains.
3. Why studying the MSC in the offshore area?
During the crisis, the drawdown gives a new configuration to the Mediterranean basin and creates a succession of morphological and sedimentological changes. A major contrast exists between the
At the present time, for obvious reasons of data accessibility, most of the works dealing with the MSC
surficial and isostasic responses of the margins to the crisis and that of the deep basins: the former,
event are based on the study of outcrops located onshore (Sicily, Morocco, Cyprus, Spain, Italy…).
mostly emerged, have been largely eroded whereas the latter accumulated sediments mostly made
However, as discussed above, the whole MSC event cannot be studied directly from these series,
of thick evaporites (up to 1.5 sec twtt (two-way travel time)) and clastics arising from the erosion of
either because most of them are not contemporary with the MSC deposits accumulated offshore, or
the margins. Consequently, the seismic markers of the Messinian salinity crisis in the offshore
because they are tectonically/geometrically disconnected from them. At the present time however,
domain correspond to erosion surfaces, depositional units and associated bounding surfaces. Recent
most of the MSC scenarii are based on field works that rarely rest on observations from the offshore
works based on better quality seismic section analysis have allowed some important advances in the
domain, because of (1) scale integration problems (limitation in resolution and lateral correlations), 6
(2) the scarcity and heterogeneity of marine seismic data available today, and (3) the ambiguous
the Mobile Unit (MU, mainly halite), which is generally the easiest unit to evidence on the seismic
labelling of the MSC units (regarding e.g. the so-called lower evaporites), leading to some confusions
profiles thanks to its transparent facies and associated plastic deformation, producing diapirs and
and ambiguities. Studying the MSC in the offshore area however offers one major advantage. If the
listric faults in the overlying sediments [eg. Dos Reis et al., 2005; Gaullier et al., 2006; Loncke et al.,
margins have been mainly eroded as a result of the sea-level fall, the deepest part of the
2006]. We then define a Lower Unit LU below MU, and an Upper Unit UU above MU (fig. B and tab.
Mediterranean basin accumulated MSC units that are several kilometers thick (see section 5.2).
B). On the margins, two other units are defined according to their acoustic facies, geometry, an
These units offer the best (and the only) manner allowing to document what happened during the
spatial extent: a Complex Unit (CU) which appears closely related to the sediment supply sources
drawdown phase, when the margins were only recording erosion. In addition, because the deepest
during the MSC; a Bedded Unit (BU), observed only locally, and geometrically disconnected from the
areas have probably never been fully desiccated, the offshore MSC units probably locally constitute a
other Messinian units. The MSC surfaces have been defined based on their relationship with the Pre-
continuous record of the entire MSC event (i.e. before, during and after the drawdown phase).
MSC units, the offshore MSC units, and the Plio-Quaternary sedimentary cover. The Messinian units are bounded at top and bottom by a Bottom Surface (BS) and a Top Surface (TS), named BES and
4. Atlas design
TES respectively where they present erosion indications (fig. B and tab. B). IES are an intermediate surfaces observed within the MSC deposition units.
The purpose of this chapter is to describe the content of the atlas of the Messinian seismic markers
A variety of illustrations is used to describe the Messinian markers in each regional area. Seismic
in the Mediterranean and Black seas and to present the labeling of the MSC seismic markers.
4.1.
profiles with enlargements are used to illustrate the seismic facies of the markers and their
Messinian marker nomenclature and colour code
geometrical relationships. Maps are used to show the location and extent of the markers and the
In this atlas, we propose a new global and consistent terminology for Messinian markers (MSC
thicknesses of the depositional units. Most of the illustrations in the atlas use colours to emphasize
surfaces and depositional units) identified from seismic reflection profiles in the entire offshore
the information and to increase the clarity and understanding of the data or interpretations that are
Mediterranean region (see fig. B and tab. B). In the deep basin, the terminology of the depositional
presented. To help maintain consistency, each Messinian seismic marker is assigned a unique colour
units is based on their seismic facies and/or the geometrical relationship of the units with respect to
that is used throughout the atlas to map the marker wherever it appears in the different areas (see tab. B). Figure B. – Dip schematic section of the gulf of Lion margin, from the continental shelf to the deep basin. During the MSC, the margin has been deeply eroded (creation of the MES) whereas the deep basin accumulated sediments (LU, MU, UU and CU) bounded below and above by the BS/BES and TS/TES respectively. The present day geometry of the MSC markers reflect the post-MSC evolution of the margin (subsidence, compaction, salt tectonics). 7
With evidence for erosion
DEPOSITIONAL UNITS
BOUNDING SURFACES
Without evidence for erosion
Upper/ Lower Intermediate Deep Land Middle slope depth basin basin shelf slope
Mediterranean basins Black sea West East
Labeling Name
Colour code
TS
Green line (dotted)
No
BS
Orange Bottom Surface line (dotted)
No
Yes
Yes
Yes
Yes
Yes
Yes
MES
Margin Erosion Red line Surface
Yes
Yes
No
No
No
Yes
Yes
Green line
No
Yes
Yes
Yes
Yes
Yes
Yes
IES
Intermediate Yellow Erosion Surface line
No
Yes
Yes
Yes
No?
Yes
No
BES
Bottom Erosion Orange Surface line
No
Yes
Yes
Yes
Yes
Yes
Yes
Top Surface
Top Erosion Surfaces
TES
Yes
Yes
Yes
Yes
Yes
Yes
Seismic expression
M horizon or M surface [Ryan, 1973; 1978]
No
- at the bottom of MSC deposits. - above Pre-MSC deposits.
N reflector [Mart and Ben Gai, 1982]
Widespread erosional or angular Yes discordance with subaerial drainage pattern topography.
- beneath PQ - top of the pre-MSC deposits.
M horizon, M surface [Ryan, 1973]; H horizon [Mauffret et al., 1973; Montadert et al., 1970]; Messinian erosional surface [Barber, 1981]
Conformable surfaces or angular discordances.
- beneath PQ - top of MSC deposits. Erosional discordances very well expressed in intermediate-depth basins and in the eastern Mediterranean basin No (excepted IES). - within UU or CU - flat with drainage pattern and valleys (TES). - gullied morphologies and drainage - bottom of MSC deposits. No pattern (BES). - above pre-MSC deposits.
Grey
No
Yes
Yes
Yes
No
Yes
Yes
No
Either chaotic, more or less bedded, transparent or disorganized.
BU
Bedded Unit
Blue
No
Yes
Yes
Yes
No
Yes
?
?
More or less bedded unit. Locally chaotic.
Upper Unit
Green
No
No
Yes
Yes
Yes
Yes
No?
No
High frequency, high amplitude continuous reflections.
UU1
M horizon [Ryan, 1973]; Younger M surface [Ryan, 1973];
No
Complex Unit
UU2
Previous labeling
- beneath PQ - at the top of MSC deposits.
No
CU
UU
Geometrical characteristics
(sec twtt)
Type of marker
LOCATION
Mean thickness
MSC SEISMIC MARKERS
MU
Mobile Unit
Yellow
No
No
Yes/No
No
Yes
Yes
Yes
- transparent facies (with internal reflections in the Eastern basin) No - plastic deformation (listric faults, anticlines and diapirs)
LU
Lower Unit
Purple
No
No
Yes/No
No
Yes
Yes
No?
No
Low frequency, high continuity reflections.
variable spatio-temporal relationship with other MSC depositional units. Often fan shaped at the outlet of the Messinian thalwegs. - geometrically disconnected from the other MSC units - in onlap on the margin feet - beneath PQ - above MU - in onlap on the margin feet - in onlap on the margin slopes - above LU and below UU (western basin) - above pre-MSC deposits. - below MU. - in onlap on the margin feet
Table B. – Labeling and colour code used in the atlas of the Messinian salinity crisis seismic markers in the Mediterranean and Black seas. 8
none Basal discordance [Ryan, 1978]; N surface [Ryan, 1978]; N reflector [Mart and Ben Gai, 1982]; BES [Maillard et al., 2006] 0.15 0.7
Messinian detritals [Lofi et al., 2005]
0.15
Messinian formation [Aleria group, 1979]
0.3 0.5
M reflectors [Ryan and Cita, 1978]; Upper Evaporites [Mauffret et al., 1973]
0.5 (west) couche fluante [Montadert et al., and 1.0 1970]; salt [Auzende et al., 1971] (east) 0.25 0.35
N reflectors [Ryan, 1973]
4.2.
In this atlas, the seismic profiles are used as a tool, and the geometry of reflections and the seismic
Organisation
facies have been voluntarily put forward. Thus, seismic profiles are the primary types of illustrations
The atlas consists of one explanatory note and 41 illustrative sheets organized under 13 study
in each atlas sheet. Interpreted and non-interpreted seismic profiles in the atlas are available on CD.
areas (fig. C). Each of the 13 areas begins with an overview of the geological, sedimentary and
They are supplemented by line drawings, maps or graphs as needed to describe the depositional
architectural context through a regional setting sheet. This overview contains maps showing the
architecture over the study area. Most of the plates in the atlas use colours to emphasize the
location of the area, bathymetry, location of the seismic lines used in the atlas, and a general
information and to increase the clarity and understandability of the seismic data that are presented.
structural sketch of the margin/basin. Origin and processing of the seismic data used for this purpose
Simplified figure captions describe the main features of each plate. References are included for each
are also presented. One to several separated sheets also follow that depict the characteristics of the
study area. They are listed in the references chapter at the end of this document, in order to assist
MSC seismic markers over the study area. These markers are regrouped under the following themes:
readers who may require additional information.
MSC surfaces, MSC basinal units and, according to the study areas, clastics and/or salt
Observations that have been done over the 13 study areas presented in this atlas have been
tectonics.
compiled through a synthesis sheet (see appropriate map). This document illustrates: (1) some synthetic sketches. They illustrate under the form of dip-line drawings the most representative organization of the Messinian markers over each study area; (2) the present-day spatial extent of some specific MSC markers at the scale of the Mediterranean and Black seas: i) the deep basin Mobile Unit (MU, section 5.2.2 for description) and Upper Unit (UU, see section 5.2.3); ii) some clastics deposits (CU, see section 5.2.5); and iii) the Messinian thalwegs on some margins. Note that this mapping has not been performed for the central Mediterranean domain. The extent of these MSC basinal units results from a synthesis of all the study areas shown within this atlas. The spatial extent of the MSC markers that have been mapped corresponds to their present-day position (i.e. after Mediterranean basin geodynamic evolution during the Plio-Quaternary and possible tectonic or salt-related deformation, gravity-gliding or spreading processes, etc..) and not to the Messinian one. For example, as a result of salt tectonics (and basinward displacement of the salt), the mapped present day onlap limit of Mobile Unit MU generally does not reflect the initial Messinian pinch-out on the margin feet. However, considering the scale of the map, differences are
Figure C. – Relief map of the Mediterranean and Black sea area [Smith and Sandwell, 1997]. Black boxes show location of the 13 study areas illustrated in this atlas. The multi-site approach allows analyzing the impact of the MSC on margin segments and basins that have various structural, geodynamical and geological backgrounds. Algerian margin (1), Valencia through (2), gulf of Lions (3), Provençal (4) and Ligurian (5) margins, eastern Corsica basin (6), western Corsica (7), and western Sardinia (8) margins, Nile (9), Levant basin (10), Cyprus arc (11), Florence ridge (12), and western Black sea margin (13).
generally relatively small on passive margins. Out of the studied areas, distribution of the markers comes from a compilation of numerous data. Some Messinian thalwegs are mapped in blue whereas the limit onshore of the Messinian erosion has been drawn in red, mostly after the map from Clauzon et al. [1995]. For the eastern 9
Mediterranean, a pre-existing map was used [Sage and Letouzey, 1990]. The work by Moussat
-
the Bottom Surface (BS) observed in the basins and slopes at the base of MSC units, labelled
[1988] has been reinterpreted in terms of Upper and Mobile Units for the Tyrrhenian sea.
Bottom Erosion Surface (BES) [Maillard et al., 2006] when arguments for erosion processes
Reinterpretation of a great number of seismic lines permitted to draw the exact extents of the Upper
exist;
and Mobile Units in the western Mediterranean [works in progress: Maillard et al., for the South
-
Balearic domain and the Sardinia channel; Gorini and Maillard for the Alboran sea; Maillard and
units (CU, BU or UU, see section 5.2 for labelling explanation);
Gorini for western Sardinia]. Extrapolations along the Algerian and Tunisian margins are shown in
-
dashed lines. This document provides a new and more accurate map of the present-day MSC units
The above labelling is based on: (1) the spatial distribution of the surfaces with respect to the preMSC margin/basin morphology; (2) the relationship of the surfaces with the MSC units; and (3)
References to figures and numerotation
evidences or not for associated erosion. These characteristics are illustrated in figure B and
For the comfort of the reader, the explanatory note contains some references to figures, which are
described hereafter. The MES is only observed on the paleo-margins and is systematically overlain
referred as follow: -
by the Plio-Quaternary sequence. The BS/BES, TS/TES and IES are only observed in association
figures that are part of the notice have been labelled from “figure A” to “figure Y”. Two tables
with Messinian units, either in the deep basins or in the intermediate-depth basins (i.e. the Valencia
are part of the notice, they have been labelled “table A” and “table B”; -
and East Corsica offshore basins, which were shallower than the deep basins during the crisis).
figures that are part of the illustrative sheets start with the study area number (i.e. 1 to 13),
These surfaces merge together upslope into the MES, generally at (or close to) the pinch-out (onlap
followed by the figure number. As an example, figure 2.5 refers to the figure 5 from the study
point) of the deep basin Messinian units.
area 2 (Valencia through); -
some enlargement (zooms) of seismic line sections are show in the illustrative sheets. They have been numbered as previously, but are preceded by a “Z”. As an example, figure Z5.1
5.1.1.
refers to the zoom 5 from the study area n°5 (Ligurian margin).
records of deep erosion features in offshore areas [Ryan et al., 1978]. Among them, the MES (former “M horizon” [Ryan, 1973]) is the most striking feature, consisting in a widespread erosion surface
The Messinian seismic markers observed in each of the study area illustrated in this atlas are
generally well identified on the upper margins (eg. fig. 1.7). The MES has been correlated with
described below. Most of the time, depths and thicknesses are given in two-way travel time (sec twtt
several exploration boreholes in the Mediterranean sea. Boreholes located on the shelves reveals an
or msec twtt).
unconformity between pre-MSC and Pliocene deposits (or extremely incomplete successions) [e.g. Cravatte, 1974; Lanaja, 1987]. Subaerial erosion features (e.g. desiccation cracks and stromatolite
Messinian salinity crisis surfaces
layering, DSDP, Leg 42A; fossil meanders and fluvial terraces) [Stampfli and Höcker, 1989] have
Several surfaces have been evidenced on seismic data. In this atlas, we newly refer to these surfaces
been described from margin edges, suggesting fluvial erosion. The MES is thus commonly
as follows: -
Margin Erosion Surface (MES)
Evidences for a substantial drop of the sea-level during the MSC have been collected from numerous
5. Description of the Messinian markers in the offshore domain
5.1.
the Top Surface (TS) observed in the basins and slopes at the top of MSC units, labelled Top Erosion Surface (TES) [Maillard et al., 2006] when arguments for erosion processes exist.
extent in the deep Mediterranean.
4.3.
the Intermediate Erosion Surfaces (IES), observed in the basins and slopes within the MSC
interpreted as the result of subaerial erosion, essentially by river action and retrogressive erosion
the Margin Erosion Surface (MES) observed in the absence of MSC units, on the margins and
[Loget and Van Den Driessche, 2006]. Onshore, the MES is characterized by the presence of deep
slopes; 10
narrow incisions (“canyons”), which correspond to the entrenchment of streams in response to the
slope, the relatively smooth morphology of MES may result from wave-ravinement during the re-
huge sea-level drop [Chumakov, 1973; Clauzon, 1973].
flooding phase at the achievement of the crisis.
Offshore, on the seismic profiles, the MES generally forms a prominent reflection with a strong erosional character as usually indicated by the truncation of the pre-MSC reflections (figs D and E). The MES displays various morphologies, generally extremely rugged with deep incision beneath the inner and middle shelves (valley incisions > 0.5 sec twtt deep, eg. fig. 3.14) and becoming smoother basinward (eg. fig. 3.15). The MES is also commonly evidenced by a high angular discordance between pre-MSC and Plio-Quaternary deposits (downlaps and onlaps of the overlying prograding Plio-Quaternary reflections downslope and upslope respectively, and/or toplaps of underlying Figure E. – Above, southwest of Corsica, the MES (in red) has a strong erosional character illustrated by the truncation of some pre-MSC reflections (blue arrows). The MES is buried beneath the PlioQuaternary sequence. See area n°7 for more details and for profile location.
truncated reflections). The existence of the MES has been confirmed in the Black sea [Gillet, 2004; Gillet et al., 2007] (fig. F).
Figure F. – On the Roumanian shelf, the MES (in red) displays a set of deep canyons incising the Miocene sequence. The MES is buried beneath the PlioQuaternary sequence See area n°13, for more details and for profile location.
Figure D. – Beneath the gulf of Lions inner shelf, the MES (in red) has a strong erosional character illustrated by the truncation of some pre-MSC reflections (blue arrows). The MES is buried beneath the Plio-Quaternary sequence. See area n°3 for more details. Modified from Lofi et al. [2011]. Numerous investigations have enabled reconstructions of the detailed paleomorphologies of the MES
From a spatial point of view, the MES lies at the uppermost part of the margins and extends
at several margins, revealing the existence of Messinian paleo-fluvial networks: e.g. Egyptian margin
downslope to the onlap pinch-out of the deep basin Messinian units (eg. figs 2.3, 3.4, 8.3) . There, it
[Barber, 1981], gulf of Lions shelf (Guennoc et al. [2000] and fig. 3.5), Ebro margin and Valencia
passes laterally to the BS/BES, IES and TS/TES, each of these erosion surfaces being defined
trough [Stampfli and Höcker, 1989]. On large margins with thick sediment cover, it has however been
based on their relationship with downslope Messinian units. From a temporal point of view, the MES
proposed that sub-aquatic processes may have also contributed to the shaping of the MES at the
is a time equivalent of the entire pile of Messinian units and surfaces included between the BES/BS
beginning of the drawdown [Lofi et al., 2005]. Bache et al. [2009] also suggested that beneath the
and the TES/TS. 11
5.1.2.
Bottom Surface (BS) and Bottom Erosion Surface (BES)
The BS and the BES are considered as the same bounding surface marking the base of the MSC
In the Western Basin, the BS/BES passes basinward beneath the three Messinian units (UU, MU and
deposits. However, in some places, the base of the MSC deposits is a conformable surface, whereas
LU) and extends toward the centre of the basin, where it is conformable (fig. B). At the margin slopes,
in other places, it shows evidence of erosion. We thus use two different labellings:
evidence for erosion of the pre-MSC deposits is frequent on the seismic data. For instance, the
-
the bottom surface is labelled BS (Bottom Surface) when it corresponds to a concordant surface
erosional character of the BES is absolutely unequivocal in the intermediate-depth basins: it is clearly
(e.g. The deepest part of the gulf of Lions, area n°3) or to an angular unconformity with no
observed beneath UU in the Valencia basin (fig. H and area n°2) and beneath the Messinian bedded
evidence for truncation of the pre-MSC reflections (i.e. due for example to tectonics, Algerian
unit (BU, see section 5.2.4) of the eastern Corsica basin (area n°6). In those areas, the BES displays
margin, area n°1);
locally small discontinuous gully-type incisions.
-
the bottom surface is labelled BES (Bottom Erosion Surface) when clear erosion markers exist (eg. gullied morphology and/or evidences for truncation of underlying pre-MSC reflections). Often, differentiation between BS and BES is not easy.
On the Messinian middle/lower slopes, the BS/BES is absent except beneath the clastics (CU) infilling the Messinian thalwegs as illustrated in figure G (see also areas 3, 5 and 9). Generally, in these sectors of the margins, this surface shows evidences for erosion of the underlying deposits (truncated reflections, valley like morphology) and is thus labelled BES.
Figure H. – On the Valencia basin, an intermediate-depth basins, the BES (in orange) shows clear erosion. It is buried below the Messinian Upper Unit (UU) with gully-type incisions. See area n°2 for more details and for profile location. In the gulf of Lions, there is also locally evidence for erosion (truncations) quite far away basinward. In this area, the BES extends beneath UU, MU and for a while, beneath LU (Lofi et al. [2005] and fig. 3.5). However, it is often difficult to determine if the pre-MSC truncated reflections observed on the seismic lines are related to the Messinian event (creation of the BES) or instead reflect a pre-existing paleo-morphology predating the MSC. In any case, the BES becomes progressively conformable
Figure G. – On the Ligurian lower slope, the BES (in orange) displays valley like morphology, incising the pre-Messinian deposits. The BES is buried below the Messinian Upper Unit (UU) infilling the Messinian valley. See area n°5 for more details and for profile location.
(BS) with the underlying strata. In the deepest part of the Western Basin (fig. I), where observed, the lower limit of the MSC units is mostly conformable at the base of LU (BS). 12
canyons and accumulated beneath MU. On the Cyprus arc (area n°11) the BES/BS is also observed at the base of MU, and shows large scale deformation (fig. 11.9). Most of the time, no clear signs of erosion are observed (BS, Z11.6) although reflections below are locally clearly truncated (BES, fig. 11.9). In the Nile deep sea fan (area n°9), the BES is observed at the bottom of CU (see section 5.2.5). In this area, this surface is more complex because it is associated with several canyon incisions, which migrated spatially, making the BES a polyphased diachronous surface. In the Florence ridge, the BES is drawn with caution beneath a bedded unit identified locally beneath MU that may correspond to LU.
Figure I. – In the deep basin off the gulf of Lion, the BS (in orange) is a conformable surface at the bottom of the oldest MSC unit (LU). See area n°3 for more details and for profile location.
5.1.3.
Intermediate Erosion Surfaces (IES)
The IES represent some intermediate unconformities contained in the MSC depositional units. From a stratigraphical point of view, they were created after the BS/BES and before the TS/TES. One or several IES, clearly erosional with gullied morphologies, are well imaged within the thin Messinian deposits UU and BU of the intermediate-depth basins: the Valencia and East Corsica basins (see areas n°2 and 6). In the northern Ligurian margin (area n°5), an IES is also observed within the MSC upper unit pinching out on the lower slope (UU) as attested by the truncations of some of the underlying reflections (fig. K). In the deep part of the Western Basin and in the eastern Mediterranean basin, IES have not been observed on the seismic profiles.
Figure J. – In the Levant basin, the BES (in orange) shows locally clear erosion at the base of the MSC deposits. Here, it is buried below the Mobile Unit (MU). See area n°10 for more details and for profile location. Figure K. – On the Ligurian lower slope, an IES (in yellow) shows clear erosion (truncation of the underlying reflections) within the Messinian Upper Unit (UU) accumulated in a Messinian valley. See area n°5 for more details and for profile location.
In the eastern Mediterranean, the BS/BES generally extends beneath MU (fig. J). In the Levant basin (area n°10), this surface clearly shows evidence for erosion thanks to the truncation of the underlying reflections. It is thus labelled BES. In this area, the BES is related to MSC canyons systems [Bertoni and Cartwright, 2006]. Locally, this surface passes at the bottom of some deposits supplied by the 13
of the top MSC deposits is however frequent on the seismic data. As an example, an erosion is 5.1.4.
identified locally at the top of UU in the gulf of Lions (figs. Z3.4 and 3.7). In the Valencia intermediate-
Top Surface (TS) and Top Erosion Surface (TES)
depth basin (fig. 2.14) the erosion character of the TES is much more pronounced and absolutely
The TS and the TES correspond to the same bounding surface marking the top of the MSC deposits
unequivocal at the top of UU. It consists there of a very flat surface at the top of UU, with a sinuous
(either UU or CU/BU in the Western Basin or MU, in the Eastern Basin). In some places, the top of
central paleo-valley (fig. 2.7) and its tributaries extending toward the Provençal basin [Escutia and
the MSC deposits is a conformable surface, whereas in other places, it shows evidence for erosion.
Maldonado 1992; Maillard et al., 2006; see also area n°2]. In boreholes, the TES has been detected
We thus use two different labellings: -
at the top of the pinch-out of UU, at the edge of the Balearic abyssal plain (DSDP drilling site 372
the top surface is labelled TS (Top Surface) when it corresponds to a concordant surface (eg. fig.
[Cita et al., 1978]).
K). Locally, it may correspond to an angular unconformity with no evidence for truncation of the -
pre-MSC reflections (eg. downlap of the Lower Pliocene reflections);
As illustrated in figure M, in the East Corsica basin also, the TES shows large and deep incisions (up
the top surface is labelled TES (Top Erosion Surface) when clear erosion markers exist (eg.
to 0.4 sec twtt deep) at the flat top of a bedded unit (BU). In this intermediate-depth basin (see area
gullied morphology and/or evidences for truncation of underlying MSC reflections; fig. 2.5).
n°6), the TES draws a complex valley system composed of two networks. These networks merge downslope in the south to form one wider and deeper valley supposed to have flown into the deeper
On the Messinian middle/lower slopes, where the Messinian units (UU, MU and LU) are absent, the
Tyrrhenian basin (fig. 6.5). Unfortunately, the connection is not identified at the present time.
TS/TES is absent, except at the top of the clastics (CU) infilling the Messinian thalwegs (eg. fig. L and figs. 2.16, 3.10 and 5.4) [see also Barber, 1981; Guennoc et al., 2004].
Figure M. – In the eastern Corsican basin, the TES (in green) shows deep V-shaped incision in the Messinian deposits (BU). See area n°6 for more details and for profile location. Figure L. – On the gulf of Lion middle slope, the TES (in green) is observed at the top of the Messinian clastics (CU) infilling a Messinian valley (transversal cut). Modified from Lofi et al. [2005].
In the eastern Mediterranean basin, the TS/TES generally extends out at the top of MU where it has
In the western Mediterranean basin, the TS/TES passes basinward to the top of the most recent
a mostly erosional character [Ryan, 1978]. The TES is thus clearly imaged on the Levantine margin
Messinian unit (top of UU) and extends toward the center of the basin, where it is conformable (thus
(fig. N and figs 10.5 and 10.8) thanks to the areal truncation of intra-MU reflections (see area n°10).
labelled TS), if undisturbed by salt tectonics (figs. B and I). At the margin slopes, evidence for erosion
This unconformity has been interpreted as a subaerial exposure linked to a regression that occurred 14
during the last stages of deposition of the Messinian Mobile Unit [Bertoni and Cartwright, 2007a].
Gaullier, 1993; Dos Reis et al., 2005; Loncke et al., 2006]. Two other MSC units have been identified
However, unlike in the Western Basin (eg. Valencia basin), no evidence of valley like morphologies
in the western Mediterranean deep basin, one above the salt and one below [Montadert et al., 1970].
associated to the TES has been observed. The TES has also been clearly evidenced at the top of
These three seismic units are also called together “Messinian trilogy”. They also have for a long time
MU (fig. 11.4) on the Cyprus arc. In some other areas of the Eastern Basin, the location of the
been called “Lower Evaporites”, “Salt”, and “Upper Evaporites” [e.g. Ryan, 1973; Mauffret et al., 1978;
TS/TES limit is more uncertain, either because of the complexity of the area, the resolution of the
Réhault et al., 1985]. However, a “Lower Evaporite Unit” is also described onshore, but
seismic profiles or the lack of calibration wells. In the Nile deep sea fan (area n°9), for example, the
lithological/stratigraphical/geometrical correlations are not demonstrated. This ambiguous labelling is
TS/TES is observed either at the top of MU (fig. 9.15) or, with care, at the top of a bedded unit of
puzzling. In order to avoid misleading uses of these terms, we newly refer to the deep basin
unknown age and which could be interpreted as UU (fig. 9.8). In the Florence ridge, the TES is also
Messinian units as follow (see also fig. B):
drawn with caution either above CU (fig.12.8 and Z12.2) or above a bedded unit identified locally
-
the Lower Unit (LU) observed in the basins at the base of the MSC units, below MU;
above MU and that may correspond to UU.
-
the Mobile Unit (MU) observed in the basins;
-
the Upper Unit (UU) observed in the basins at the top of the MSC units, above MU.
The above labelling is based on: (1) the spatial distribution of the MSC units with respect to MU and (2) the relationship of the units with the MSC surfaces. These characteristics are illustrated in figure B. Two other MSC units are observed at a more local scale: -
the Complex Unit (CU) observed on the slope and at the margin foot;
-
the Bedded Unit (BU) observed on the slope and in intermediate-depth basins.
The relative age of the above two units cannot be established because BU is geometrically disconnected from the others Messinian units and CU shows a wide spatio-temporal variability from one area to another. No MSC depositional units are observed on the margins where only the MES is imaged. LU, MU, UU,
Figure N. – In the Levant basin (see area n°10), the TES (in green) has a strong erosional character illustrated by the truncation (blue arrows) of some reflections belonging to the MU unit.
BU and CU are only observed in the deep or intermediate-depth basins. Their characteristics are described hereafter.
5.2.
Messinian salinity crisis units 5.2.1.
Deep basin Messinian evaporites occupy most of the present-day Mediterranean domain (see
Lower Unit (LU)
synthesis map). The easiest MSC unit that can be evidenced on the seismic profiles is the Messinian
LU is the oldest MSC unit of the western Mediterranean Messinian trilogy and is bounded below by
salt (unit MU), thanks to its transparent facies and characteristic plastic deformation creating listric
the BS/BES (fig. B). LU has been observed and defined for the first time in the gulf of Lion area (area
normal faults and diapirs in the overlying Plio-Quaternary sedimentary cover [e.g. Le Cann, 1987;
n°3, Montadert et al. [1970]), where it corresponds to a group of continuous high amplitude 15
reflections and has only been observed in the deepest part of the basin, in onlap on the Miocene
In the Antalya basin (see area 12), a basinward thickening sequence of high amplitude reflections is
lower slopes (Réhault, pers. comm.) [Lofi et al., 2005] (fig. O).
observed below the Mobile Unit MU, and may correspond to LU. However, because of the low resolution of the seismic profiles and the lack of data density, those seismic reflections could also reflect pre-MSC deposits. At the present time, the age, lithology and depositional environment of LU are still speculative, as it has never been drilled. Some authors proposed that LU deposited in a fully subaqueous environment and may contain a large part of sediments eroded from the margins at the beginning of the drawdown and accumulated as giant turbidites above the abyssal plain [Lofi et al., 2005]. A chronostratigraphic correlation possibly exist between LU and the thick resedimented Messinian gypsum turbidites outcropping in the Apennine or in Sicily, also known as RLG (Resedimented Lower Gypsum) [Roveri et al., 2001, 2008a,b]. However, these outcrops are geometrically disconnected from the MSC
Figure O. – In the gulf of Lion, the LU (in purple) is characterized by low frequency sub-continuous reflections in onlap on the Miocene lower slope. See area n°3 for more details and for profile location.
deposits observed offshore, thus preventing any direct correlation.
The twtt thickness of LU spans from 0 (pinch out point on the Miocene slope) to up to 0.35 sec twtt
5.2.2.
(basinward). It onlaps the Miocene margin at a depth of ~4.5-5 sec twtt in the gulf of Lions (see area
This unit has been described both in the Western and Eastern Basins (eg. areas n°3, 4, 5 and 10). It
n°3), but this geometry is generally poorly imaged. In other places, LU has not been observed with
corresponds to the Messinian salt accumulated in the deep basins and onlaping the Miocene
certitude with the same seismic facies, either because this reflective unit does not exist or because
margins. MU is evidenced on the seismic profiles by a characteristic transparent acoustic facies (fig.
salt prevents acoustic waves to propagate deep enough. Its extension and thickness throughout the
I), interpreted as consisting dominantly of halite [Nely, 1994], and by an associated plastic
Western Basin is still thus poorly known. In the Algerian margin (see fig. P and area n°1), a reflective
deformation. Listric normal growth faults linked to the post-MSC salt tectonics are currently observed
unit located below MU is interpreted as the LU. However, its reflectivity is slightly lower than in the
at the lower slopes of the margins (fig. Q), passing progressively seaward to salt anticlines and
gulf of Lions and because of tectonics, no onlap of this unit on the Miocene lower slope has been
diapirs in the more distal areas [e.g. Le Cann, 1987; Gaullier and Bellaiche, 1996; Dos Reis et al.,
evidenced.
2005; Loncke et al., 2006; Gaullier et al., 2006] (see also fig. R). The root of the uppermost fault
Mobile Unit (MU)
indicates the location of the initial Messinian pinch-out of the salt (before salt tectonics) along the margin foot (eg. fig. Q; see also areas n° 4, 5, and 8).
MU shows characteristic plastic deformation in both the western and eastern Mediterranean basins. Figure P. – On the Algerian margin, the LU (in purple) is accumulated beneath MU and characterized by low frequency sub-continuous reflections. See area n°1 for more details and for profile location.
However, the seismic facies and thickness of this unit differs at the scale of the Mediterranean sea:
-
in the Western Basin, MU is < 0.5 sec twtt thick (i.e. ~1100 m thick using a mean internal velocity of 4.5 km/s) and shows essentially a reflection-free seismic facies (fig. I). It is bounded above and below by two other MSC units: UU and LU, respectively;
16
-
in the Eastern Basin, MU is up to 1 sec twtt thick (i.e. ~2100 m thick using a mean internal
In the Eastern Basin (see Levant basin, area n°10), refraction velocities analysis [Hübscher et al.,
velocity of 4.2 km/s) and shows a reflection-free seismic facies, with several internal reflections
2008] (fig. 10.5) evidenced that the MU shows reflection-free seismic facies with internal velocities of
(fig. S). Most of the time, MU is bounded above and below by two MSC surfaces that are
up to 4.5 km/sec. This transparent facies alternates with several discontinuous reflection packages,
clearly erosional (at least in the Levant basin (area n°10) and on the Cyprus arc (area n°11)):
with internal velocities below 4 km/sec. Those internal reflection packages delimitate up to six
the TES (fig. S) and the BES respectively (e.g. fig. J; Ryan [1978]; Tahchi et al. [2004]; Bertoni
depositional sequences that can be traced throughout the Levant basin [Netzeband et al., 2006b;
and Cartwright [2006, 2007b]; Maillard et al., [2011]). In the Eastern Basin, MU is thus directly
Bertoni and Cartwright 2006; Ottes et al., 2008]. They also suggest vertical changes in the evaporitic
recovered by the Plio-Quaternary, although, some seismic units resembling UU and LU are
facies of MU, with intercalated clastics and/or trapped fluids. Fluid escape structures have been
observed locally in the Nile and Florence areas (areas n°9 and 12).
frequently reported in the Eastern Basin, possibly linked to fluid resources located beneath or within MU [e.g. Loncke et al., 2004; Gradmann et al., 2005; Netzeband et al., 2006b; Huebscher et al., 2008] (see also Levant basin, area n°10). In the Levant basin lower slope, well data indicate the presence of thick halite intervals, intercalated with anhydrite, limestone and siliciclastic sediments in the upper and marginal part of the Mobile Unit [Bertoni and Cartwright, 2005]. However, this lithological information has a local character and cannot be directly extrapolated to the entire MU unit, as the wells only drilled part of the Messinian evaporites.
Figure Q. – In the Ligurian margin, MU (in yellow) shows characteristic transparent acoustic facies associated with listric normal growth faults linked to post-MSC salt tectonics. See area n°5 for more details and for profile location.
Figure R. – In the deepest part of the Provençal basin, MU (in yellow) shows characteristic transparent acoustic facies. Shortening and sediment loading created diapirs piercing through the Plio-Quaternary cover. See area n°4 for more details and for profile location.
Figure S. – In the Levant basin, MU (in yellow) shows alternating reflection-free and layered seismic facies. Internal reflections are sloping toward the NW and are eroded at the top by the TES. MU is directly overlain by the Plio-Quaternary sequence. See area n°10 for more details and for profile location. 17
5.2.3.
Provençal, Ligurian and western Sardinian margins (areas n°4, 5, 8), seismic facies can be either
Upper Unit (UU)
roughly bedded (UU1) or relatively well bedded (UU2). Some lateral facies variations within UU have
In the western Mediterranean basin, UU is the upper and most recent unit of the Messinian trilogy. In
also been evidenced while approaching the canyons outlets, where the facies often becomes less
the deepest areas, UU is identified by a group of parallel and relatively continuous reflections of
organized (see areas n°4 and 8). The strong internal reflectivity of UU, related to strong impedances
relatively high amplitude, deposited above MU and below the Plio-Quaternary (fig. I). In more
contrasts within the unit, supports that the unit consists of contrasted natures of sediments.
proximal areas, UU pinches out at the Miocene margin slopes above the BS/BES, and is often faulted due to post-MSC salt tectonics (figs. T and U). The top of UU correlates with a prominent seismic reflection usually referred to as the “M” reflection [Ryan and Hsü., 1973] and corresponding to the TS/TES in this atlas.
Figure U. – On the western Sardinia lower slope, UU (in green) is characterized by slight vertical variations in seismic facies, allowing to distinguish two sub-units (UU1 and UU2) separated by an erosion surface (IES). See area n°8 for more details and for profile location. The top of UU has been sampled during DSDP Leg XIII [Hsü et al., 1973] with the discovery of the
Figure T. – On the gulf of Lion lower slope, UU (in green) is identified by a group of parallel and relatively continuous reflections pinching out on the Miocene margin. UU lies above MU and is affected by faults due to salt tectonics. See area n°3 for more details and for profile location.
"pillar of Atlantis", made of layers of dolomitic marls and anhydrite. Stromatolites characterizing arid and shallow marine depositional environments (Sabkha) have also been observed, interbedded with marly levels locally rich brackish fauna. Such alternation can explain the internal reflectivity of UU.
The thickness of this unit spans from 0 (pinch out point on the Miocene slope) to up to 0.4 sec twtt
Because several different lithologies can generate the same seismic facies, the stratigraphical and
(i.e. 700 m thick using a mean internal velocity of 3.5 km/s). UU is mostly aggrading and onlaps the
depositional environment equivalency of two apparently identical seismic objects must be carefully
margin foots (fig. T). Such a geometry evidences the shoaling of the basin floor during the
discussed. Moreover, the vertical and lateral facies variability evidenced at the basin scale suggest
accumulation of UU. In some places, UU contains some erosion surfaces labelled IES (fig. G) that
that the nature of UU may vary from one point to another. We thus suspect that UU displays an
are observed at intermediate depths only (see Valencia through, area n°2 and Ligurian margin, area
important variability in terms of lithologies and depositional environments since it is recovered at river
n°5). Slight vertical variations in seismic facies are punctually observed in UU and two sub-units
mouths (increase of the clastic fraction), in the centre of the deep basins (possible increase of the
(labelled UU1 and UU2) have consequently been distinguished in some areas (fig. U). Indeed, on the
evaporitic fraction) or in an intermediate-depth basins (possible increase of the lacustrine fraction). 18
While a thick and widespread UU has been shown all around the northwestern Mediterranean (see
presence of a volcanic intrusion at the outlet of the Messinian Corsica basin (see eastern Corsica
Synthesis map), it is not the case in the deep eastern Mediterranean (e.g. fig. S and Ryan [1978]).
basin, area n°6). Moreover, BU has not been drilled in the East Corsican basin.
Indeed, in many areas, only the thick salt unit (MU) is visible on the seismic profiles (see also 5.2.5.
Levantine domain, area n°10 and Cyprus arc, area n°11). Deposits tens of meters thick, containing
Complex Units (CU)
gypsum have been drilled in the Cretan basin and on the crest of the Mediterranean ridge (Florence
Several Complex Units CU have been observed in the Mediterranean basin, commonly organized as
rise, Hsü et al. [1978]). However, those deposits could be too thin to be clearly observed on the
fan shaped accumulations at the margin foot (Figure V, see areas n°2 and 3). They display generally
seismic profiles or could have been subsequently eroded locally by a late phase of erosion during the
a chaotic seismic facies, with incoherent reflection configuration, more or less transparent. A careful
last stage of deposition in the Eastern Basin [Bertoni and Cartwright, 2007a]. In the Nile deep sea fan
examination of the seismic profiles however shows that the internal configuration does not only
(fig. 9.7) and in the Antalya basin (fig. 12.11), a bedded reflective unit is locally observed at the top of
consist of hyperbolic diffractions but contains a mix of seismic facies ranging from reflection free to
either MU or CU. This unit resembles the UU of the Western Basin, but may also be Pliocene in age,
sporadic high-amplitude semi-continuous reflections within the unit. In some places, CU can also
such as the bedded unit lying above MU in the Levantine basin (area n°10). Without borehole
contain be relatively continuous reflections. In order to avoid the multiplicity of labelling, we have
calibration, no definitive conclusion can be made regarding its origin.
chosen to gather these facies into a unique global term of “Complex Unit” and to describe them as
5.2.4.
distinct sub-units (CU1, CU2...) in case they depict significant differences. Thus, on the Algerian
Bedded Units (BU)
margin, a mostly chaotic unit (CU2) lays above a more bedded unit (CU1) that may correspond to a first stage of MSC deposition or to pre-MSC deposits.
Some Bedded Units BU have been observed in the Mediterranean basin. They are geometrically disconnected from the others Messinian units and their relative age thus cannot be established. These unit have consequently been labelled Bedded Unit (BU), with reference to their seismic facies, which consists most of the time of relatively continuous sub-parallel reflections (fig. M). Where described, BU are up to 0.2 sec twtt thick (i.e. 350 m thick using a mean internal velocity of 3.5 km/s). They are bracketed by very well expressed TES (fig. 6.11) and BES (fig. 7.11). In some places, BU also contain internal erosion surfaces labelled IES (fig. G).
Figure V. – In the Valencia basin, CU (in grey) is fan shaped and accumulated at the margin foot. See area n°2 for more details and for profile location.
Up to now, BU have only been observed in the East-Corsica basin (area n°6) and on the WestCorsica margin (area n°7) in MSC topographic lows that are located at intermediate depths, above the pinch out of MU (fig. 7.11). In the East Corsican intermediate-depth basin (area n°6), BU is characterized by a bedded seismic facies that can be divided into two sub-units based on a careful examination of the seismic facies and erosion surfaces [Aleria Group, 1978]. From a geometrical
CU are irregular in terms of lateral extent and overall thickness (up to 1 sec twtt thick). They are
point of view only, this unit strongly resembles the UU of the Valencia basin (in onlap on the Miocene
absent (or below the resolution of the seismic) on the upper slopes or on the margin shelves. On the
slopes and bounded above and below by the TES and the BES, Thinon et al. [2004]). The lateral
lower-middle slopes CU are mainly imaged above the BES, as infilling the Messinian thalwegs (fig.
equivalence is however not demonstrated at the present time because the eastern Corsican
W). In the Nile area, the internal configuration of CU is very complex CU because it is related to the
Messinian unit cannot be correlated with the rest of the western Mediterranean as a result of the
infilling of several canyon incisions which migrated spatially (fig. 9.6). At the transition with the basin, 19
CU display a complex relationship with the other Messinian units: CU are imaged either beneath MU
The products of the margin erosion have been observed at many margin feet: Ligurian and Provencal
(eg. gulf of Lions, fig. 3.14) or above MU (eg. Algerian margin, fig. 1.5; Provençal margin, Z4.3 ), and
margins [Savoye and Piper 1991], gulf of Lions [Lofi et al., 2005], Valencia basin [Maillard et al.,
a lateral facies change to UU and/or MU is locally inferred.
2006], Valencia seamount flanks [Mitchell and Lofi, 2008], western Sardinia [Sage et al., 2005], Provençal [Obone-Zué-Obame et al., 2011], Algerian margins [Déverchère et al., 2005; Gaullier et
Based on their external shape and spatial connection with the main Messinian thalwegs, CU are
al., 2006], Levant basin [Bertoni and Cartwright, 2007b]. The Nile system is however the best
interpreted as clastic deposits eroded from the margin during the drawdown phase and accumulated
documented case because of the high reservoir potential of these deposits [Rizzini et al., 1978;
at the MSC river mouth. Based on seismic attributes, the thin CU (up to 0.12 sec twtt thick) recovered
Barber, 1981; Ottes et al., 2008]. At the scale of the Mediterranean basin, CU are highly diachronous
locally at the base of MU in the Levant basin are also interpreted as MSC clastics, although these
and show a high variability in terms of seismic facies, spatial extent and stratigraphic position (see
deposits are very local, and not regionally correlated to other CUs (area n°10).
Synthesis map).
Where they have been sampled on the slopes, CU consist of sands and conglomerates intercalated with marly levels and overlain by early Pliocene deep marine sediments. They are therefore
5.3.
interpreted as Messinian fluviodeltaic deposits with stratified organisation when the facies is bedded
Limits of the offshore approach
Marine seismic data exploitations are submitted to some limitations summarized hereafter.
[Rizzini et al., 1978; Estocade, 1978; Stampfli and Höcker, 1989; Savoye and Piper, 1991]. Downslope, in the distal part accumulated below MU, CU may consist of subaqueous gravity-driven
The Messinian sequence of the abyssal plain has never been fully drilled for scientific purpose.
deposits resulting from an early erosion of the margin at the beginning of the drawdown [Lofi et al.,
Studies of the Messinian markers offshore are thus limited by the lack of lithological and
2005]. The presence of thick MSC deposits redeposited through gravitary processes into relatively
stratigraphical calibrations. In the absence of fully recovering deep boreholes, our knowledge of the
deep waters has also been evidenced in the Apennine foredeep [Roveri et al., 2001]. Transparency
nature and age of the deep evaporite sequence is weak, in particular concerning MU and LU (see
could in some cases be due to high gas contents as proposed along the Egyptian margin [Loncke et
section 5.2 for seismic unit descriptions). Divings can bring invaluable information (e.g. cirque Marcel
al., 2004].
on the Ligurian margin, Savoye and Piper [1991]) but they are limited because of sampling difficulties and because MSC deposits, generally overlain by the Plio-Quaternary deposits, rarely outcrop in the Mediterranean sea. Industrial boreholes could be very useful [Ottes et al., 2008] but data are generally not easily accessible to the scientific community. Thus, considerable progress will be done when this sequence is drilled integrally [eg. Rouchy, 2004; Gorini et al., 2006; Hübscher et al., 2007; Bassetti and Lofi, 2009]. Meanwhile, only the correlation of the seismic approach with information from IODP and DSDP drillings (that only partly sampled a few percent of the MSC units) can be envisaged [Hsü et al., 1973; Hsü et al., 1978; Kastens et al., 1987]. Regarding the geophysical data, offshore seismic studies are based on the recognition and interpretation of seismic facies and of the geometry of the depositional units and erosion surfaces, correlated where possible with borehole or core data. Because several different lithologies can lead
Figure W. – Beneath the gulf of Lions middle shelf, strike seismic reflection profile shows a paleoMessinian valley infilled with Complex Unit, CU (in grey). Modified from Lofi and Berné [2008].
to the same seismic facies, the stratigraphic and depositional environment equivalence of two a-priori 20
identical seismic objects must be carefully discussed. The seismic facies of one sedimentary unit
bedded seismic unit (BU) that strongly resembles the UU observed elsewhere [Thinon et al., 2004].
also depends on the seismic source used. It would thus be of interest to use systematically one
Their lateral equivalence however cannot be demonstrated at the present time because of the
unique and adapted seismic source allowing the direct comparison of the seismic facies. This would
presence of a volcanic intrusion at the outlet of the Messinian basin. Concerning the comparison
partly relieve ourselves from the data heterogeneity problem. Seismic data interpretation may also be
between the western and eastern Mediterranean basins, although a mobile unit has been evidenced
limited by data quality: various source types, analogical data, bad signal penetration in CU (Complex
in both basins, their synchronicity is not obvious because both basins are disconnected at the
Unit usually interpreted as Messinian clastics) or beneath MU (Mobile Unit mainly consisting of
present time. Some authors have thus suggested that it may exist a delay between the deposition of
halite), inadequate orientation of the seismic profiles, bubble pulses of air guns, remnants of
the MU in the Eastern and Western basins [Blanc, 2000; Ryan, 2008, 2009]. The knowledge of the
hyperbolas after inadequate migration, poor signal resolution or low signal-to-noise ratio may strongly
paleo-geography of the Mediterranean and Paratethys during the MSC is also crucial in order to
limit or even bias the interpretations. More specifically, evaporites have generally a high acoustic
recover the paleo-connections among the basins during higher sea-levels [Jolivet et al., 2006;
impedance and high velocity relative to overlying sediments: they produce pull-down and pull-up of
Clauzon et al., 2005; Popescu et al., 2006; Clauzon et al., 2008].
underlying units, and also frequency content decreases within the salt, making resolution lower than As a last point, land/sea correlations of the Messinian depositional units and erosion surfaces are
in the overburden. The high reflectivity of the evaporite layers also causes multiples and ringing
also a difficult challenge essentially because of both geometrical problems and scale integration
noise, distorting the image below them. Industry data are not usually targeted to image Messinian
limitations [eg. Cornée et al., 2008]. Firstly, correlations at the scale of a margin are difficult as no
evaporites, thus acquisition and processing parameters are not optimized for salt imaging (eg. AGC,
continuous record allows the connection between the abyssal plains, where deposition prevailed, and
over-under migration effects).
the margins, where erosion prevailed. Secondly, post-MSC tectonics locally produced a geometrical
The architectural complexity of one studied system, the lateral changes in seismic facies, the
disconnection of some deposits outcropping onshore from the rest of the Mediterranean (eg. Sicily).
deformations related to salt tectonics, volcanism or deep-seated tectonics are among the factors that
Thirdly, the vertical resolution is critical in seismic works. It concerns the minimum thickness of a bed,
make interpretation or reflection picking complex or equivocal at a local scale. For example, the
so that reflections from the top and the base of the bed can be distinguished. Resolution is
Messinian units found on the slope of the Ligurian margin (see area n°5) cannot be definitely
dependent on the data frequency content, which becomes more bandwidth-limited with depth of
correlated with the abyssal plain units, and especially with the Mobile Unit MU, because a set of salt-
penetration. For Messinian seismic marker imaging, the vertical resolution of seismic data used is
related listric faults strongly deforms the Messinian and post-Messinian units. In the Levant basin
therefore usually several meters or several tens of meters, which is much weaker than onshore
(see area n°10), the geometry of the MU is relatively close to its initial one and the MU seismic facies
resolution, where depositional units are sometimes few meters (or less) thick. Finally, note that
can be traced throughout the entire basin. However, further north and at the Cyprus arc (area n°11),
because of shallow bathymetries, it is generally difficult to acquire seismic profiles close to the
geometry and thickness of MU have been modified by subsequent crustal-scale tectonics and the
coastline, preventing us from an easy land-sea connection.
present-day geometry does not reflect the initial geometry. Correlation at a large scale is limited, mainly by the existence of topographic sills that disconnect the
6. Comparing the MSC markers at the scale of the Mediterranean area
different Mediterranean basins and sub-basins (ex. Western vs. Eastern basins, eastern Corsica, Tyrrhenian sea…). Thus, if it is relatively easy to correlate geometrically the Upper Unit (UU) at the
Seismic interpretation allows to study the spatial distribution of the MSC sedimentary units, their
scale of the Algero-Provençal basin (see Synthesis map), it is more difficult for sub-basins. This
internal geometries and spatial relationships. The multi-site data set presented in the atlas allowed to
problem is encountered for example in the eastern Corsica basin (study area n°6) containing a
solve some of the problems discussed above (Section 5.3) and to evidence the crucial influence of 21
the initial geological-morphological setting on the recording of the crisis [Lofi et al., 2011]. Indeed, the responses of the margin/basin to the MSC appear to be closely related to local and regional factors (morphology, dimensions and initial bathymetry of the area, type of pre-Messinian rocks, lithology and dimension of the drainage basins and of the continental shelves, proximity and height of aerial reliefs, tectonic context, subsidence…). These factors have played a key role on the spatial and temporal organisation of the Messinian erosions, the location, the amount and the nature of the eroded sediments and the modalities of sediment transport and sedimentation toward the basins. In this context, a careful observation of the MSC seismic markers at the scale of the entire Mediterranean area should allow discuss the paleo-environmental evolution of the basins and margins during the MSC and refine our knowledge of this event [see Lofi et al., 2011 for more
Figure X. – Ideal schematic section of the western Mediterranean basin illustrating the organisation of the Messinian salinity seismic markers in the offshore domain, in absence of salt tectonics. In the marginal basins, some evaporites accumulated at a first stage of the crisis, before the major sealevel drawdown [Clauzon et al., 1996]. Modified from Lofi et al. [2011].
details]. Indeed, one major result of the atlas is that, whatever the study area considered, we evidence a more or less complete association of characteristic seismic markers depicting, at a time, similarities and differences (spatial, temporal and geometrical). The MSC factors are thus superimposed to local trends, and should allow, in some cases, to differentiate/discuss their respective impacts. The aim of this section is to summarize the main characteristics of the surfaces and seismic units of the MSC within each of the main basins, namely the western Mediterranean basin, the eastern Mediterranean basin, and the Black sea basin. The general organisation of the MSC seismic markers in the western Mediterranean basin is shown in figure X. In the deepest part of the basin, in the absence of salt tectonics, the “trilogy” is generally concordant at the base and at the top with the Pre-MSC and the Plio-Pleistocene sequences, respectively (see e.g. gulf of Lions, area n°3 and Algerian margin, area n°1). Although a concordance does not necessarily mean that no erosion occurred, the observations suggest a permanent immersion of the deepest part of the abyssal plain during the “desiccation” phase.
Figure Y. – Ideal schematic section of the eastern Mediterranean sea basin illustrating the organisation of the Messinian salinity seismic markers in the offshore domain. Modified from Lofi et al. [2011].
Therefore, the deep Western Basin probably never emerged and appears as a continuous recorder of the entire MSC, with no hiatus. In more proximal areas, the deep basin trilogy is observed as a lateral onlap on margin foots. This peripheral onlap shows that, unlike in the more distal areas, the recording of the MSC is incomplete onshore and in the intermediate-depth basins. This onlap possibly reflects the progressive infilling of the abyssal plain by the Messinian deposits, as the
In the eastern Mediterranean basin (fig. Y), the three Messinian units (UU, MU, LU) have not been
subsidence did not compensate the extremely high sedimentation rate in the basin (i.e. more than
identified on the seismic profiles. For example, in the Levant basin (see area n°10) and in the Cyprus
1500 m in less than 300,000 years).
arc (see area n°11), only a thick Mobile Unit is visible, which is not bracketed by any LU and UU [e.g. 22
Bertoni and Cartwright, 2007a; Maillard et al., 2011]. However, further east in the Nile area (study
7. Conclusions and perspectives
area n°9) and in the Antalya basin (study area n°12), a bedded reflective continuous unit is locally
The Messinian salinity crisis (MSC) is a unique, outstanding event that have left imprints both
observed at the top of the MSC deposits. This unit resembles the UU observed in the Western Basin
onshore and offshore in the Mediterranean region, from far inside the neighbouring continents down
but may also correspond to Pliocene deposits. In the Antalya basin, MU lies sometimes above a
to the deepest parts of the sea. Although they have identified these imprints for long, offshore studies
basinward thickening sequence of high amplitude reflections that may correspond to either pre-MSC
dealing with the MSC failed until recently to provide a comprehensive and unified overview of the
deposits, LU, or MSC-clastics. In addition, the clear erosion surfaces found above MU along the
various markers left by the crisis and to favour comparisons between different areas in the
Levant margin [Bertoni and Cartwright, 2007a; Huebscher et al., 2008] attest for an important sea
Mediterranean.
level drop after salt deposition in the Eastern Basin.
Our purpose through the seismic atlas of the “deep parts” of the MSC is not to solve the debates on
In the Black sea, until now, none of these three seismic units (UU, MU, LU) have been observed on
the exact timing of the crisis, the importance and distribution of erosion, dissolution and deposition
the seismic profiles, but a major erosion surface (MES) has been described (see area n°13). Down
processes, and the factors responsible for its birth and death, which still remain today [e.g. Clauzon
slope, off the Bosphorus straight, the MES correlates with a Late Messinian shallow water unit drilled
et al., 1996, 2005; Rouchy and Caruso, 2006; Roveri et al., 2008; Govers, 2009; Govers et al., 2009].
during DSDP leg [Hsü and Giovanoli, 1979]. This shows that the Black sea, like the Mediterranean
Indeed, the MSC itself, the scenarii, the history and evolution of related ideas, have recently been
sea, suffered a desiccation period at the end of the Messinian [eg. Gillet et al., 2007].
developed and detailed, in a comprehensive and approachable way, in several publications [see e.g. Rouchy et al., 2006; Rouchy and Caruso, 2006; CIESM Consensus report, 2008; Ryan, 2009, 2011].
We conclude that major differences exist between the western, eastern Mediterranean, and Black
This atlas is built in order to fill two main gaps: (1) firstly, it proposes a new and consistent
sea MSC markers regarding the seismic facies, the number of depositional units, and their
terminology for Messinian markers (MSC surfaces and depositional units) identified from seismic
thicknesses (see Synthesis plate). Especially, whereas a clear deep basin trilogy (LU, MU, UU) is
reflection profiles in the entire offshore Mediterranean, resulting from an accurate observation and
observed in the Western Basin, only the Mobile Unit is clearly recovered on seismic data in the
picking of all the markers available: this new terminology is a key step in order to clarify the meaning
Eastern Basin. In addition, the Mobile Unit MU is generally characterized by a transparent facies in
of various descriptions of markers of the MSC reported both onshore and offshore; (2) secondly,
the Western Basin whereas it contains internal reflection packages in the Eastern Basin. These
using most recent seismic data as a mean to display the space and time organisation of these
observations suggest that the paleo-environmental changes or triggering factors during the crisis
markers, it provides a selection of high-quality, characteristic seismic lines that help identify and
were different in the two basins. Because the two Mediterranean basins are now disconnected, the
compare the various imprints of the MSC in the deep domain, the margin slopes and the perched
lateral correlation between the observed MU is not possible and the synchronicity between those two
basins.
units in both basins is far from obvious. Note that unfortunately, only the top part of the deep basin MSC sequence has been sampled during the ODP and DSDP legs [Hsü et al., 1973; Kastens et al.,
One of the inputs of this atlas is finally to highlight and provide further support to the major
1987]. Although the halite and potash salts encountered in Holes 134, 374 and 376 all belong to the
differences that indeed exist between the western, eastern Mediterranean, and Black sea basins
top deposits of MU, the latter was not fully penetrated due to safety concerns. The greatest part of
regarding the MSC markers (see Synthesis plate). The lack of 2 units (LU, UU) as well as the
the Messinian evaporites (around 90%) is therefore still unknown [Rouchy, 2004] and the lithology,
strikingly different facies of the Mobile Unit in the eastern Mediterranean sea reveal important
stratigraphy and depositional environments can only be studied indirectly. Seismic profiles remain the
changes during the crisis that are not understood yet and deserve further studies based on seismic
only mean until now to decipher the internal structure of the deep basin MSC sequence.
profiles but also on corings, deep drillings, onshore geological field investigation and modelling. 23
One other of the inputs of this atlas is that despite the above discrepancies, whatever the study area
In the future, as a support to the multi-site comparative study and in order to propose a coherent
considered, we evidence a more or less complete association of characteristic seismic markers
scenario of the MSC at the Mediterranean scale, several complementary approaches need to be put
showing, at a time, similarities and differences in space/time distribution and geometry (see
forward in our view, especially: (1) land-sea correlations of Messinian markers; (2) 3-D case studies
Synthesis plate). We related these differences to geological, structural and geodynamical settings:
at the scale of several drainage systems; (3) inter-site regional correlations with similar seismic
they result in contrasted sedimentary and morphological responses of the margins and basins that
acquisition systems, (4) backstripping of considered margins and basins [Steckler and Watts, 1980]
appear highly dependent on local triggering factors (e.g. paleo-bathymetry, paleo-topography,
in order to quantify post-Messinian tectonic motions and to restitute the initial MSC depth of all
morphology of the margin, lithology of the eroded terrains, dimension of the drainage areas,
markers, and (5) probably the most important step, chrono-stratigraphic calibrations requiring a
structural and tectonic settings, etc…), but also on superimposed regional triggering factors linked to
series of fully recovering deep boreholes to identify the nature and age of the deep basin depositional
the MSC s.s. (amplitude and modalities of the drawdown, location of the paleo-sills, erosional
units.
processes, isostatic rebound, climate changes, etc…). The MSC is such a complex event that cannot be fully understood based on observations from one single area. The multi-site comparative approach developed here should thus be considered as a new and powerful tool which could allow to extract and discriminate from the MSC sedimentary record the relative importance of these two superimposed types of factors (local and “Messinian”), and to discuss how the MSC shaped the offshore area at the scale of the Mediterranean basins. We emphasize the importance of the morphological and tectonic evolution of the study areas since the achievement of the MSC: obviously, the initial morphology of the margins and the paleo-topography/bathymetry of the subbasins (and their connections) play a key role, but this restoration also requires the quantification of the successive deformations that are quite contrasted in the Mediterranean. Accordingly, the Messinian events appear to provide several excellent stratigraphic markers of the geodynamic evolution, which has occurred over the last 5 Ma [e.g. Rubino and Clauzon, 2008].
24
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