Thesis
Reference
Carbonate coastal dunes: potential reservoir rocks?
FREBOURG, Grégory
Abstract
Eolianites are wind-driven supratidal and continental accumulations of carbonate dominated sand, lithified by carbonate cement. These bodies can reach huge spatial extents. Being formed by the landward continental accumulation of shallow-marine particles, they can be easily misinterpreted as subtidal high-energy deposits. Due to their granular composition, eolianites of important size may represent valuable hydrocarbon reservoirs. This study treats issues concerning their preservation potential, their recognition, their sequence-stratigraphic response and reservoir potentia. Their important reservoir potential is enforced by the reinterpretation as an eolianite of the main reservoir layer of present-day's largest gas field.
The recognition of these neglected deposits among the stratigraphic record may open the door to better understanding and knowledge of past littoral and shallow marine carbonate system and carbonate reservoirs.
FREBOURG, Grégory. Carbonate coastal dunes: potential reservoir rocks? . Thèse de doctorat : Univ. Genève, 2010, no. Sc. 4221
URN : urn:nbn:ch:unige-98242
DOI : 10.13097/archive-ouverte/unige:9824
Available at:
http://archive-ouverte.unige.ch/unige:9824
UNIVERSITÉ DE GENÈVE FACULTÉ DES SCIENCES
Département de Géologie et Paléontologie Professeur E. Davaud
Carbonate Coastal Dunes:
Potential Reservoir Rocks?
THÈSE
présentée à la Faculté des Sciences de l’Université de Genève
pour obtenir le grade de Docteur ès Sciences, mention Sciences de la Terre
par
Gregory FRÉBOURG de
Collonge-Bellerive (Genève)
Thèse N° 4221
GENÈVE
2010
Frébourg, G.:
Terre & Environnement, vol. 92, xii + 178 pp. (2010)
ISBN 2-940153-91-4 Carbonate coastal dunes: potential reservoir rocks?
Résumé
L
es éolianites sont des accumulations supratidales et continentales de sable majoritairement carbon- até qui peuvent avoir une grande extension spa- tiale. En raison de leur texture granulaire, ces dépôts éoliens pourraient représenter d’importants réservoirs d’hydrocarbures.La fréquence des éolianites au cours du Quaternaire a amené la communauté scientifi que à considérer que ce type de dépôt ne se formait qu’à la faveur de défl ation à grande échelle des marges continentales exposées lors des importantes variations glacio-eustatiques du niveau marin lors de périodes de Icehouse. La découverte ré- cente d’éolianites en période de Greenhouse a infi rmé cette hypothèse et laisse penser que ce type de dépôt est plus fréquent qu’on ne le supposait dans l’enregistrement sédimentaire pré-Quaternaire. Cependant, comme les éolianites résultent du transport en direction du continent de matériel issu d’une source marine littorale adjacente, leur identifi cation est rendue diffi cile par la similarité de leur composition pétrographique.
Pour être préservées dans l’enregistrement sédimentaire et représenter des réservoirs potentiels, les éolianites doi- vent résister aux transgressions marines sans perdre une fraction importante de leur volume initial. Les recherches bibliographiques et celles effectuées sur le terrain montrent que ces dépôts ont un potentiel de préservation important.
Les éolianites doivent ce potentiel à leur développement en milieu continental qui les expose à une lithifi cation précoce favorisée par la percolation d’eaux météoriques et à leur grande extension spatiale. La lithifi cation précoce les rend résistantes à l’érosion, tandis que leur grande taille requiert une érosion importante pour les faire disparaître.
De plus, leur exposition aux phénomènes diagénétiques subaériens contrôle l’évolution ultérieure des qualités réservoir des éolianites en limitant la compaction et en créant des barrières physiques ou hydrologiques.
Les éolianites, par leur environnement de dépôt, ne sont pas régies par les schémas classiques de la stra- tigraphie-séquentielle subaquatique. Leur composition pétrographique peut dépendre du moment de leur dépôt dans le cycle eustatique. Les régressions favorisent un fort taux de sédimentation et de préservation éolienne, les stagnations du niveau marin un taux moyen, alors que les transgressions montrent le taux le plus faible. Les périodes de Icehouse favorisent l’accumulation de dépôts éoliens par un climat généralement aride et semi-aride et des variations eustatiques de forte amplitude. Durant les périodes de Greenhouse, des conditions climatiques moins favorables et de faibles variations eustatiques sont
en partie compensées par une productivité carbonatée plus élevée.
Les éolianites montrent en carottes ou en lames-minces d’importantes similarités sédimentaires et pétrographiques avec les dépôts marins littoraux de haute-énergie. Leur différentiation est critique pour l’estimation correcte des volumes des corps sédimentaires, les reconstructions paléoenvironnementales et le séquençage-stratigraphique correct. A cause de la grande variabilité des comporte- ments aérodynamiques des particules carbonatées, le granoclassement inverse typique des pinstripe-lamination, seul critère diagnostique pour la reconnaissance des dépôts éoliens n’est souvent pas enregistré. La reconnaissance des éolianites doit alors se faire sur la base de critères sédimentologiques, diagénétiques et stratigraphiques convergents.
La découverte d’éolianites dans les séries pré-Quater- naires confi rment que leur potentiel de préservation est élevé et leur présence potentiellement sous-estimée. Dans le cadre de cette étude, deux niveaux considérés comme des shoals ont été réexaminés et réinterprétés en tant qu’éolianites : un dans le Crétacé Inférieur de la Forma- tion Meloussi de Tunisie Centrale, l’autre dans le Trias Inférieur de la Formation Mahil Inférieure, dans le Jebel Al Akhdhar, dans le Sultanat d’Oman, considéré comme analogue terrain de la plus grande formation gazifère connue, le Khuff. Dans les deux cas, ces dépôts granu- laires apparaissent en fi n de séquence régressive. Cette position stratigraphique suggère qu’il faut reconsidérer l’interprétation des corps sableux carbonatés carbonatés analogues.
Dans le cadre de ce travail, le niveau réservoir principal du plus grand champ de gaz connu (North Dome, eaux territoriales de l’Iran et du Qatar) a été réexaminé et s’avère être une éolianite. Cette réinterprétation confi rme le poten- tiel réservoir de ces dépôts. Leur indentifi cation correcte est cruciale. La non-reconnaissance du caractère éolien de ces dépôts peut conduire à la négligence de dépôts- réservoirs contemporains du système et au séquençage stratigraphique incorrect des puits. Les éolianites montrent une plus grande homogénéité de leur faciès au sein du dépôt que les systèmes marins littoraux de haute-énergie.
Cependant, elles peuvent être affectées par des paléosols ou des calcrêtes qui peuvent soit former des couvertures intéressantes soit des niveaux barrières cloisonnant le réservoir. Leur environnement de dépôt les place dans un contexte favorable à la genèse de réservoirs. Leur po- sition topographique dominante les expose à l’altération météorique pouvant augmenter leur potentiel réservoir
les systèmes subhorizontaux, plats ou crestaux. Suiv- ant la composition minéralogique des particules qui les composent et leur histoire diagénétique, les éolianites peuvent héberger différentes qualités d’hydrocarbures ou au contraire former des niveaux barrières dans des systèmes pétroliers.
L’identifi cation de ce type de dépôt souvent négligée au sein de bilans stratigraphiques contribue à une meilleure compréhension et connaissance des systèmes carbonatés littoraux fossiles et des réservoirs potentiels ou prouvés qu’ils renferment.
E
olianites are wind-driven supratidal and continental accumulations of carbonate dominated sand, lith- ifi ed by carbonate cement. These bodies can reach huge spatial extents. Due to their granular composition, eolianites of important size may represent valuable hy- drocarbon reservoirs.The occurrence of eolianites in the Quaternary record led scientists to assess that eolianites were restricted to Icehouse times, when their formation was favoured by margin defl ation during large scale, glacio-eustatic oscilla- tions of the sea-level. The recent discovery of Greenhouse eolianites discarded this hypothesis, leading to the pos- sibility of undiscovered, misinterpreted eolianites within the stratigraphic record. Eolianites being formed by the landward continental accumulation of shallow-marine particles, they can be easily misinterpreted as subtidal high-energy shoals.
To be found among the stratigraphic record and rep- resent potential reservoirs, the eolianites have to resist marine transgression without dramatic loss of their initial volume. Bibliographical and fi eld evidences show that eolianites have a high preservation potential. Eolianites owe this potential to their supratidal / continental depo- sitional setting that make them prone to undergo early meteoric diagenesis and to potentially important size.
Early diagenetic phenomena make them resistant to ero- sion, whereas their size requires important erosion rate to make them disappear. Moreover, subaerial diagenetic phenomena often hinder or prevent further loss of the eolianites’ poroperm qualities by inhibiting compaction and by creating physical or hydrological barriers.
Due to their supratidal / continental depositional set- ting, eolianites do not follow classical subaqueous se- quence stratigraphic rules. Eolianites’ composition can depend on their position in the eustatic cycle, providing eventual information on their eustatic depositional setting.
Lowstand regressions will favour high eolian deposition rate and preservation potential, highstands / stillstands medium, and transgressions show the lowest accumulation rate and preservation potential. Icehouse periods favour eolianite formation by conducive climatic conditions and high-amplitude sea-level variations. Greenhouse periods compensate their low amplitude sea-level variations by increased carbonate factory productivity.
Eolianites display in core or thin-sections sedimentary and petrographic features that are close or equivocal with high-energy marine littoral deposits. Their recognition is critical for correct volumetric estimations, paleoenvi-
ronmental reconstructions and stratigraphic sequencing.
Due to high-diversity of aerodynamic behaviour of most carbonate particles, the inverse grading of the diagnostic pinstripe lamination often cannot be recorded. Hence, the recognition of eolianites must be based on the base of converging sedimentological, diagenetic and strati- graphic clues.
Pre-Quaternary eolianites, if found in the stratigraphic record, will testify to good preservation potential and a potentially underestimated presence. In this study, two layers considered as subtidal shoals were reinterpreted as eolianites: one in the Lower Cretaceous of the Meloussi Formation in Central Tunisia, the other in the Lower Tri- assic of the Lower Mahil Formation of the Jabal Al Akh- dhar, Sultanate of Oman, a fi eld analogue of the biggest hydrocarbon bearing formation, the Khuff. In both cases these layers occur at the top of regressive sequences. This stratigraphic position states that high-energy deposits lo- cated at the end of regressive sequences might represented interesting targets for eolianite reinterpretation.
The main reservoir layer of present-day’s largest gas fi eld (North Dome Structure, offshore Iran and Qatar) was re-examined and reinterpreted as an eolianite. This confi rms the reservoir potential of these deposits. Their correct interpretation is crucial, for it may lead exploita- tion geoscientists to detect coeval reservoirs of the system, and correct well-to-well sequence stratigraphic bound- ing. Eolianites are expected to show more homogeneous facies throughout their body than shoal systems. They are often affected by pedogenetic overprints such as cal- cretes and paleosoils that may represent valuable seals or create extraction barriers. Their depositional settings place eolianites in a reservoir favourable context. Their high topographic position makes them prone to undergo reservoir properties enhancing diagenesis, the overlying growth or deposition of porous bodies during transgres- sions, and hydrocarbon migration in fl at, subhorizontal or crestal systems. Depending on their original mineralogy and diagenetic history, eolianites may bear different types of hydrocarbons or create reservoir barriers.
The recognition of these neglected deposits among the stratigraphic record may open the door to better un- derstanding and knowledge of past littoral and shallow marine carbonate system and carbonate reservoirs.
Abstract
Résumé ... III Abstract ... V Acknowledgements ... X
Chapter 1: Introduction ... 1
1.1The eolianites ... 1
1.2 Aim of this study ... 1
1.3 Previous studies within this project ... 3
1.3.1 Southern Australia ...3
1.3.2 Western Australia ...4
1.3.3 Bahamas Islands ...4
1.3.4 Chrissi Island ...4
1.3.5 Sardinia ...4
Chapter 2: Quaternary eolianites ... 5
2.1 Morocco ... 5
2.1.1 Observations ...7
2.1.2 Interpretations ...10
2.1.3 Summary ... 11
2.2 Oman... 11
2.2.1 The Al Jabin Unit ...12
2.3 Qatar ... 12
2.3.1. Observations ...12
2.3.2 Interpretation ...14
2.4 Tunisia... 14
2.4.1 Sidi Salem Formation ...15
Chapter 3: The Quaternary marine terraces of the Akamas Peninsula ... 17
3.1 Introduction... 17
3.2 Geographical and geological settings ... 17
3.3 Sedimentological observations ... 18
3.3.1 Sequence 1 ...18
3.3.2 Sequence 2 ...20
3.3.3 Sequence 3 ...21
3.3.4 Sequence 4 ...23
3.4 Petrography ... 23
3.4.2 Sequence 2 ...23
3.4.3 Sequence 3 ...25
3.4.4 Sequence 4 ...25
3.5 Interpretation... 25
3.5.1 Sequence 1 ...25
3.5.2 Sequence 2 ...27
3.5.3 Sequence 3 ...27
3.5.4 Sequence 4 ...27
3.6 Sequence stratigraphic interpretation ... 27
3.6.1 Sequence 1 ...27
3.6.2 Sequence 2 ...29
3.6.3 Sequence 3 ...29
3.6.4 Sequence 4 ...29
3.7 Conclusions... 30
Chapter 4: The preservation potential of eolianites ... 31
4.1 Introduction... 31
4.2 The absence of eolianites ... 31
4.3 Drowned eolianites ... 31
4.4 Observations ... 32
4.4.1 Hassi Jerbi (see section 2.4.1) ...32
4.4.2 Morocco (see section 2.1) ...33
4.4.3 Cyprus ...40
4.5 Discussion ... 40
4.5.1 Importance of early diagenesis ...41
4.5.2 Response to erosion and surface processes ...42
4.5.3 Meteoric phreatic infl uence ...43
4.5.4 Chronological infl uence ...44
4.6 Conclusions... 44
Chapter 5: Relationship between eolianites and eustacy ... 45
5.1 Eolianites in sequence stratigraphy ... 45
5.1.1 Siliciclastic vs. Carbonate eolian deposition dynamics ...45
5.2 Case studies ... 46
5.2.1 Cyprus ...46
5.2.2 Qatar ...48
5.2.3 Summary (Table 1) ...51
5.3 Icehouse or Greenhouse phenomenon ? ... 51
5.4 Refl exion on eustatic response of eolianites ... 52
5.4.1 Eolianites’ formation during Greenhouse periods ...52
5.4.2 Eolianites’ formation during Icehouse...52
Chapter 6 : Facies characteristics and diversity in carbonate eolianites ... 55
6.1 Introduction... 55
6.2 Importance of eolianites ... 57
6.3 Materials and methods ... 57
6.4 Results and discussion ... 59
6.4.1 Broad variety of facies ...59
6.4.2 Puzzling petrographic features ...59
6.4.3 Eolian or high-energy subtidal deposit? ...62
6.5 Some clues for eolianites recognition ... 62
6.6 Typical eolian features ... 67
6.7 Conclusions... 67
Chapter 7: Pre-Quaternary eolianites ... 68
7.1 Jebel Meloussi ... 68
7.1.1 Settings ...68
7.1.2 Description ...70
7.1.3 Interpretation ...70
7.1.5 Discussion ...70
7.2 Sayq Plateau... 73
7.2.1 Settings ...73
7.2.2 Methods ...75
7.2.3 Description ...75
7.2.4 Interpretation ...78
7.2.5 Discussion ...78
7.3 Conclusions... 79
Chapter 8 : Eolianites as reservoir rocks ... 80
8.1 Introduction... 80
8.2 Permian reservoir ... 80
8.2.1 Introduction ...80
8.2.2 Geological settings ...81
8.2.3 Methods and data ...81
8.2.4 Results ...83
8.2.5 Interpretation ...85
8.2.6 Implications ...88
8.2.7 Conclusions ...90
8.3 Eolianites as reservoir rocks ... 90
8.3.3 Environmental issues ...92
8.3.4 Topographic issue ...92
8.3.5 Sequence-stratigraphy ...93
8.3.6 Diagenesis ...95
8.4 Conclusions... 96
Chapter 9: Conclusions ... 98
9.1 Eolianites are good potential reservoirs ... 98
9.2 The preservation potential ... 98
9.3 The eolianites’ response to eustacy ... 98
9.4 The eolian facies diversity and their recognition ... 98
9.5 Pre-Quaternary eolianites ... 99
9.6 Eolianites as potential reservoirs. ... 99
Chapter 10 (Annex 1) :Catastrophic event recorded among Holocene eolianites (Sidi Salem Formation, SE Tunisia) ... 101
10.1 Introduction... 101
10.2 Geographical and Geological settings ... 104
10.3 Methods ... 104
10.4 Sedimentology ... 104
10.5 Petrography ... 106
10.6 Datings (Table 4) ... 108
10.7 Possible origins of the coarser grainstone layer ... 108
10.7.1 Eolian deposition ...109
10.7.2 Storm or Tsunami overwash? ... 111
10.7.3 The storm hypothesis ... 111
10.7.4 The tsunami hypothesis ... 111
10.8 Discussion ...111
10.9 Conclusions... 112
Chapter 11: References ... 113
Annex 2 : Chronostratigraphic chart ... 129
Annex 3 : Sample location ... 130
Annex 4 : Sample listing ... 145
Plates ... 159
Words fail when it comes to express my gratitude towards Pr. Eric Davaud. From the seats of the bachelor degree’s sedimentology lessons through my master dissertation and now my Ph.D., I’ve had the chance of having an admired, brilliant and enthusiastic teacher become a Master to me. I am honored to have been trusted by Pr. Davaud to work on this subject and to benefi ciate of his benevolent mentorship which extended beyond the scientifi c fi eld and helped me much more than one could imagine. On the fi eld or behind his desk, his advices were precious and always appreciated.
During numerous missions, I’ve had the chance to enjoy his humor and friendliness which turned all kind of situations into treasurable memories. From the bottom of my heart, Eric, thank you for everything.
I would like to thank Dr. Claude-Alain Hasler with whom I shared many great times in the fi eld or not, great scien- tifi c talk and less scientifi c ones, in scientifi c conditions or not, all with the same pleasure. His availability, scientifi c approach and humor made the time I worked with him wonderful times. I thank him for the many constructive discus- sions about eolianites which helped me improve this study. I also thank him for accepting being part of my jury and reviewing my manuscript.
I would like to thank Pr. André W. Droxler (Rice University, Houston, Texas, USA) for making me the honor and pleasure of reviewing my manuscript and being part of my jury. I also would like to thank him for his deep humanity and kindness and for the pleasant discussions we’ve had, were they scientifi c or not.
My deepest gratitude goes to Dr. Stéphan Jorry (IFREMER, Brest, France) who, before being part of my jury, has been an always available, kind and admired assistant I had during my bachelor years. His scientifi c sense, nuance, and open-mindedness were and still are a great inspiration to which I aspire. Was it during lessons, on the fi eld or later on, he always made to sure to communicate his passion of life in general, and geology in particular. Dr. Stéphan Jorry also has my co-authors and my gratitude for the review of the article issued in Chapter 8.2.
My gratitude goes to François Gishig and Pierrot Desjacques. If they delivered more than 600 thin sections for this study, they also were always friendly and eager to listen to and possibly solve all kinds of problems I could have experienced.
I would like to thank my friends and colleagues of the Maraîchers building, especially Aurèle Vuillemin, Chadia Volery, Jérôme Chablais, Matar N’Diaye, Nora Tuveri and Sylvain Rigaud, the master students (Rédha, Mica-Schmühl, Popo, Lolo) my friends of the bachelor class (Bret, Diana, Jones, Mathilde, Matt, Sab, Véro), and Petrus Lindh.
Big thanks to Dr. Pierre Le Guern, who is, with Dr. Stéphan Jorry, one of my “Geological Godfathers”. I’ve had the chance to have Pierre as an assistant during bachelor years, during which he fed my passion for discovery. For this Ph.D., I followed him in the eolianite project, and his observations were of invaluable help. His kindness, friendship and loyalty deserve all my gratitude.
My profound and warmest gratitude goes to my friends, for always having supported me during good or bad times and always being there for me, never sparing their love: Jérôme “l’Ours” Pontz, Pierre-Yves Frei, Nils Rusillon (with special thanks for helping me with the layout of the manuscript) and Laetitia Seitenfuss, Roelant Van der Lelij, Yorick- Yan Hossfeld and Géraldine Martinelli, Chloé Amberg and all the others.
It is with emotion that I thank Mary Kenny, my girlfriend, for her love and support, for making my life rich and full, for the memories we share and the projects we have. I also thank her children, Philippe, Louis, Olivia and Marc- Henry, for welcoming me in their family.
I naturally would like to thank my Family: my sister Fanny, my mother Maggy Cuénoud and Allen for all their love
much more than my affi nity for science. I am blessed to have as fi rst scientifi c memory the image of my smiling father bending to the side to take my young hand in his to go for a walk in the nature, to bring home tadpoles we would raise into frogs in the kitchen. This memory is part of the innumerable intense moments I shared with him; from running through the Corsican bush collecting (dead) insects or who-knows-what? for our collections or experiments to the darker moments of life when we had to hold on together. My Dad always supported me, and made possible for me to follow extended studies, sometimes at a cost I only realized later on. Babu, thank you for everything. Having you all is a true blessing. I love you all.
Finally, I would like to thank anybody who once taught me even the smallest thing (if such things exist), helped me or was once there for me.
Formal acknowledgements:
I would like to thank the Swiss National Science Foundation for granting this research (grant no. 200020-119777) from 2006 on, within the eolianite project supported since 1995.
My gratitude goes to the Augustin Lombard grant from the Geneva SPHN Society for its generous support.
On behalf of the people involved in this project, I would like to sincerely thank Prof. M. Aberkan of the University of Rabat for his kindness, his invaluable help for the outcrop localization and sharing of his knowledge of the Moroc- can Quaternary coastal deposits.
Chapter 5: Facies characteristic and diversity in eolianites:
On behalf of my co-authors, I would like to thank J. Titschack and an anonymous reviewer for the review and improvement of our manuscript.
Chapter 8: Eolianites as reservoir rocks
On behalf of my co-authors, I gratefully thank TOTAL S.A. and RIPI-NIOC for the authorization to publish this work, and TOTAL S.A. for the partial funding of this study and for granting the access to their facilities and material.
Our gratitude goes to Christopher Tiratsoo (JPG) for his comments and improvement of the original manuscript. We would like to thank Régis Lasnel (TOTAL S.A.) for operating the CT scanner and for the treatment of the tomographic data blocks.
Chapter 10 (Annex 1)
I would like to thank Mr. Sébastien Betrisey for his valuable help on the fi eld.
I dedicate this Ph.D. thesis to my Grandmother Erna "Nenny" Uber
In loving memory of Alexis Chauvet (15.10.1982 - 30.12.2007)
is not "Eureka!" but "That's funny..."
Isaac Asimov
IntroducƟ on
1.1 The eolianites
C
arbonate coastal dunes are landward depositing, wind driven, subaerial, mobile accumulations of carbonate dominated sand. They develop nearshore, with direct or very close spatial proximity with their intertidal or subtidal source of material. When these ac- cumulations become lithifi ed by carbonate cements, they take the name of coastal “eolianites” or “aeolianites” (sensu Brooke, 2001). Coastal eolianites are opposed to inland eolianites, which infi ll continental depressions (Goudie and Sperling, 1977; Abegg and Hanford, 2001) and result from large-scale defl ation of exposed carbonate margins (Abegg et al., 2001; Abegg and Hanford, 2001) or can be supplied be the erosion of neighbouring carbonate forma- tions (Pease et al., 1999; Radies et al., 2004).Coastal eolianites can form huge sand accumulations (Carew and Mylroie, 2001; Le Guern, 2005; among others), reaching several tens of metres in thickness, on tens or hundreds of kilometres of lateral extent (Fig. 1) (Aberkan, 1989; Cann et al., 1999; Huntley and Prescott, 2000; Radies et al., 2004; Bristow and Pucillo, 2006).
Such important sand bodies should logically draw at- tention upon them for their eventual economic potential as reservoir rocks among the stratigraphic record (Abegg et al., 2001). Numerous descriptions of Late-Tertiary and Quaternary coastal eolianites are found in the literature (Johnson 1968; McKee and Ward 1983; see Brooke 2001 for detailed Quaternary inventory; Fig. 2). The reasons of this rich inventory are to be found in the wide exten- sion and the ease of recognition of these deposits. They are easy to localize, since the coastline did not undergo drastic change from Quaternary to present-day, and often croup out with very good exposure conditions in coastal cliffs. Pleisto-Holocene eolianites show well preserved, distinctive sedimentological features at the outcrop, and are not affected by late or burial diagenetic phenomena that easily conceal the latter.
For long, and opposed to the Quaternary record, very scarce descriptions of carbonate eolian deposits are avail- able for pre-Quaternary times (Hanselman et al., 1974).
This paradox led Fairbridge and Johnson (1978) to assess that the formation of eolianites is linked to the rapid sea-level fl uctuations due to glacio-eustacy, thus making eolianites a possible exclusive Ice-house phenomenon.
The posterior discoveries of eolianites in Icehouse Mis- sissipian (Hunter, 1988 and 1993; Handford and Franka, 1991) and Pennsylvanian (Rice and Loope, 1991) marine formations reinforced this assumption.
The discovery of Mesozoic, Greenhouse eolianites (Kilibarda and Loope, 2001; Kindler and Davaud, 2001), showed that eolianites were not restricted to Icehouse periods, and might as well be more frequent than expected (Rice and Loope, 1991; White and Curran, 1998; Le Guern and Davaud, 2005). McKee and Ward (1983) already sus- pected that their pre-Quaternary scarcity could come from the diffi culty to distinguish coastal eolian deposits from high-energy shallow-marine deposits. This hypothesis was demonstrated by Le Guern and Davaud (2005).
The formation of coastal dunes relies on the existence of an active, nearshore carbonate production zone, pre- vailing onshore winds and favorable margin topography, such as a wide ramp or shallow bank, as suggested by Pye and Tsoar (1990), and a suffi cient tidal range to ex- pose the shoreface to defl ation. These conditions are not exceptional, and one could easily admit their continuous presence since the Paleozoic.
1.2 Aim of this study
The present study focuses on the potential reservoir interest of eolianites. The aim of this study is to focus on the following aspect of eolianites that are crucial in the hydrocarbon problematic:
The preservation potential
To be preserved among the stratigraphic record, a sedimentary body has to resist, at least partially, to erosion before its burial. It has to present the potential features that will allow it to stand abrasion, dissolution, mechani- cal constraints and biological destructive activity. The eolianites, by their supratidal deposition and the meta- or unstable composition of the carbonate grains, are exposed to early lithifi cation through meteoric and vadose dia- genesis. It might infer to these deposits the prerequisite conditions to resist marine transgression, even at the very early stage of the eolian system formation. This hypothesis was investigated and demonstrated through sedimentological, petrographical, sequence-stratigraphic and chronological evidences.
The eolianites’ response to eustacy
If the sequence-stratigraphy was defi ned and its bases are widely accepted for subaqueous deposition in marine basins, only few publications concerning the supratidal, eolian realm can be found comparatively (Havholm and Kocurek, 1994; Kocurek, 1998; Clemmensen et al., 2001;
Chapter 1: Introduction
Veiga et al., 2002, among others). A tentative refl ection concerning the sequence stratigraphy of carbonate eoli- anites was led by Le Guern (2005). A discussion about the formation of eolianites according to eustatic variations was issued from a bibliographical, theoretical approach and fi eld observations.
The eolian facies diversity and their recognition
The recognition of fossil carbonate dunes is critical for paleo-environmental reconstructions, as well as for correct stratigraphic sequencing (Smith et al., 2001), es- pecially at core and thin-section scale. The only reliable petrographic criterion for eolian deposition is the pinstripe lamination (Hunter, 1977; Clemmensen & Abrahamsen,
1983; Fryberger & Shenk, 1988; Eriksson and Simpson, 1998; Loope and Abegg, 2001; Le Guern & Davaud, 2005;
Grotzinger et al., 2005; Stolper, 2007). This criterion, issued from the observation of siliciclastic deposits is applicable to carbonate dunes but is often blurred by the great variability of shapes and hydro- and aerodynamic behaviour of the grains (Abegg et al., 2001; Jorry et al.
2006; Yordanova & Hohenegger, 2007). This study provides additional sedimentological, petrographical and diagenetic clues for the recognition carbonate eolian deposits.
Pre-Quaternary Eolianites
The eventual occurence of eolianites among the Pre- Quaternary stratigraphic record would prove that they
Fig. 1: (A): Satellite picture (Geo 1990) of the western Atlantic coast of Morocco showing lateral accretion of large Pleistocene dune belts (source:
NASA WorldWind). (B): Satellite picture (Geo 1990) of Edel Peninsula, Dirk Hartog Island and Shark Bay, Western Australia. Bright white areas
IntroducƟ on
are more common than expected and linked to favourable formation conditions rather than restricted to Icehouse periods. Carbonate eolian fossil strata were sought in Pre- Quaternary series. The attention was drawn in particular on Greenhouse periods, were they remain scarce, and on fi eld analogues of hydrocarbon bearing formations. Two examples are exposed, with the Lower-Cretaceous of central Tunisia, and the Lower-Triassic of the Sayq Plateau (Oman), best fi eld analog of the Khuff Formation.
Eolianites as reservoir rocks
If carbonate eolian layers are described in some hydro- carbon bearing formations, such as the St. Louis and Ste.
Geneviève Limestones of south-western Kansas (Loope &
Abegg 2001), they form barrier layers, due to later pore network occlusion. However, the potentially highly porous nature of eolianites, their non-limited chronologic reparti- tion as well as the frequent spatial proximity with potential source rocks make them prone to be great potential reser- voirs. This potential will be discussed and demonstrated with the discovery of a hydrocarbon bearing eolianite layer in the South Pars Field (offshore Fars, Iran).
1.3 Previous studies within this project
Along with novel material issued from several fi eld missions (Chapter 2), this study is based on numerous fi eld observations and samples collected during previ- ous works and missions within this project subsided by the FNSRS (Fig. 2). Thin-sections and fi eld observations from Australia, Bahamas, Chrissi Island and Sardinia was issued from P. Le Guern’s Ph.D. sample collection, and from several punctual fi eld sessions led by professors E.
Davaud and P. Kindler.
1.3.1 Southern Australia
The studied areas are located in the surroundings of Robe, Kingston, and Beachport, on the south-eastern shore of the South Australia state, between the Mont Gambier volcano and the Murray River.
Active coastal and fossil dune fi elds of the Coorong national park and the Lacepede bareer stretch themselves along the shore for over 400 kilometres, and 100 kilome- tres inland (Carr et al. 1999). These dunes are related to
Fig. 2: Quaternary eolianites (modifi ed from Brooke, 2001). The black dots show described Quaternary eolianites. Arrows point at locations studied before and during this study. (1): The Bahamas (previous missions). (2): Northern coast of Salé, Western Atlantic coast of Morocco (this study). (3): Sardinia, Italy (previous missions). (4): Southeastern coast of Tunisia: Jerba Island, Borj-El-Grine, Hassi Jerbi, Slob-El-Garbi and El Kettef harbour locations (this study). (5): Jebel Meloussi, central Tunisia (this study). (6): Chrissi Island, Crete (previous missions) (7): Akamas Peninsula, Cyprus Island (this study). (8): Qatar, two locations: Al-Fuwailat and Jabal Marmi (this study). (9): Al-Jabin Plateau, Western part of the Wahiba Sand Sea, Sultanate of Oman (this study). (10): Sayq Plateau, Al Jabal Al Akhdhar, Sultanate of Oman (this study). (11): Ningaloo Marine Park, Western Australia (previous missions). (12): Shark Bay surroundings: Edel Peninsula and Dirk Hartog Island, Western Australia (previous missions). (13): Coorong Natural Park, Southern Australia (previous missions). (14): Robe surroundings, Southern Australia (previ- ous missions).
sea-level highstands (Murray-Wallace et al., 1998; Mur- ray-Wallace et al., 2002; Banerjee et al., 2003;). Their age increase landwards (Sprigg, 1952, 1958; Murray-Wallace et al., 1998; Huntley and Prescott, 2001; Banerjee et al.
2003; Bristow and Pucillo 2006). The composition of the sand is mixed, with carbonate input from the Lacepede platform and siliciclastic input from the Murray River (Le Guern 2005 and references within; Bristow and Pucillo 2006). Inland detritism in this area is considered as low, averaging 5% of quartz (Short, 1988) but increases toward the Murray River estuary, with a 30% in the Lacepede bay and reaching 70% in the estuary (Short and Hesp, 1984). The tidal regime is considered as microtidal 0.4 m in average and a maximum of 1.1 m during the largest spring tides (Bristow and Pucillo, 2006). The wave action on the shore may be important when reinforced by cyclonic storms from the Antarctic Ocean (Le Guern, 2005). The dominant winds blow from the south.
1.3.2 Western Australia
Two main areas were studied in Western Australia:
the western part of Shark Bay, and the Cloates Point, in the Ningaloo marine park.
The Edel Peninsula and the island of Dirk Hartog form the western ridge of Shark Bay, 700 kilometres on the north of Perth, in Western Australia. They are made of Pleistocene eolianites and active dune fi elds. This Qua- ternary complex is 170 kilometres long and reaches 20 kilometres in width. This area is under the infl uence of south winds and the mean tide measured in Carnavon reaches 1.7 m. The main sediment production areas in this region are the Carnavon Ramp and the Shark Bay lagoon (James et al. 1999). Active dunes were sampled on the Edel Peninsula and on Dirk Hartog Island, and fossil eolianites were sampled in the Pleistocene cliffs of Edel Peninsula (Le Guern 2005 and references within).
The Ningaloo marine park is fringed by a reef located on the western margin of the continent, creating a 0.2 to 6 kilometres wide lagoon for an average depth of 2 metres (Le Guern, 2005). Tropical cyclones of February and March are the main precipitation source, and violent cyclones affect the area with a 25 year period, blowing winds up to 200 kilometres per hour, with precipitations reaching 200 millimetres in 24 hours (Le Guern, 2005). The main wind direction is south-west excepted in autumn when wind directions are very variable. The average wind velocity is 35 kilometres per hours and the mean tidal record is 55 centimetres, but can reach 2 metres during spring (Le Guern, 2005). The active dune fi elds of Cloates Point were sampled around 10 kilometres to the east of the point.
1.3.3 Bahamas Islands
Exception made for 20 metres elevation mid-Pleistocene beach deposit observed on Eleuthera Island (Hearty et al., 1999), all land above 7 metres elevation is formed by Quaternary highstand eolianites (Carew and Mylroie, 1995, 1997, 2001). The preferential occurrence of the islands is located on the windward side of most banks, the Bahamas being in the northeast trade winds belt some eolian ridges being nevertheless oriented along the seasonal westerlies (Carew and Mylroie, 1997). Since Bahamian eolianites are entirely carbonated, various samples from different missions and locations were used for comparison with mixed eolianites.
1.3.4 Chrissi Island
The island of Chrissi is located in the Lybian Sea, around 14 nautical miles southward the island of Crete.
Its dimensions are 5 kilometres on 1.5 kilometres. The dominant wind direction is north to south, and the tidal regime is low, averaging 40 centimetres.
The substratum is made of pre-Neogene igneous rocks, and Tyrrhenian lumachellic and beachrock terraces form part of the shore. The studied dune fi eld is located in the eastern part of the island. It is made of hummocky dunes and small blow-outs, this environment corresponding to stage 3 coastal dune morphology of Hesp (1988) (Le Guern, 2005). These active dunes lie over semi-lithifi ed, pinstripe laminated eolianites, dated 2400 ± 60 BP by Le Guern (2005).
1.3.5 Sardinia
Several localities were studied along the western coast of Sardinia, from north to south: Argentiara, Alghero, Is Arùtas, San Giovanni di Sinis, Torre di Corsari and Punto Manga. All of these are formed by Tyrrhenian (upper Pleistocene) subtidal and supratidal deposits. See Davaud et al. (1992), Le Guern (2005) and references within and Andreucci et al. (2009) for complete description of this well studied area.
Quaternary eolianites
T
his research is based on material collected during several fi eld missions as well as from previous work done within this project (Fig. 2). This section reports the geographical and geological settings of the Quaternary outcrops and active dune fi elds studied in this work. Pre-Quaternary eolianites are presented and described in Part 7. The fi eld studies led in the Akamas Peninsula (Cyprus) are reported in Chapter 3, for their complex stacking and intricate sequence stratigraphy cannot be summarized in this chapter.2.1 Morocco
The Atlantic coast between the city of Rabat and the village of Mehdia is bordered by a succession of several Pleistocene to Holocene littoral complexes formed on the Atlantic coast of the Gharb basin (Fig. 3, A, B). This dissymmetrical basin from Alpine origin is fi lled with Plio-Holocene sediments (Michard, 1976). It dips gently at the south and bends upwards steeply at its northern rim, as a result of remnant Hercynian faulting of the
Above: Fig. 3: (A): Composite satellite picture of the studied area of the northern Atlantic coast of Salé, Morocco (source: GoogleEarth). (B):
Simplifi ed sketch of (A). The four eolian systems are represented from the youngest in light yellow to the oldest in orange. The location of the studied outcrops is pointed by the white arrows.
Below:Fig. 4: (A): Picture facing north of the second eolian system (Eemian-Soltanian) displaying cliffs of more than 30 metres. (B): Picture facing south of the 35 metres high cliffs of the southern part of the Sidi Moussa creek.
Chapter 2: Quaternary eolianites
substratum. Most of its sediments have a fl uvial origin and were deposited by the Oued Sebou, the Gharb’s main drainage system.
The littoral belts display subtidal to supratidal, eolian deposits. These high-energy deposits can reach more than 50 metres in thickness (Fig. 4, A, B). The studied dune complexes form a succession of four parallel to the coastline and more or less consolidated ridges (Fig. 3, B).
According to Aberkan (1989) and Plaziat et al. (2006), the belts are stacked with their age decreasing seaward.
If seaward stacking was observed, vertical accretion and draping of older ridge is frequent, complicating the stra- tigraphy and the relationships between the different eolian episodes. The oldest observed deposits yielded an Amirian age whereas the youngest (consolidated) are Soltanian and Rharbian (Plaziat et al. 2006; Fig. 5, A,). The pres- ent day tidal range is mesotidal, ranging from 1.4 metre
Fig. 5: (A): Schematic cut through the eolian systems of the northern coast of Salé, with their absolute or relative ages. The dotted boxes show the location of the schematic section of the the following studied locations. (B): Schematic section of the stratigraphy of the Sidi Moussa cliff, second belt (Eemian-Soltanian). (C): Schematic section of the stratigraphy of the Sidi Bouknadel quarry, third belt (Tensiftian). (D): Schematic section of the stratigraphy of the Sidi Bou Taibi quarry, fourth belt (Amirian). The colour gradient for the stratas’ age is relative and is not
Quaternary eolianites
at neap tide more than 3 metres for spring tide near the Oued Sebou estuary (El Blidi and Fekhaoui, 2003). Field sedimentological evidences show that these parametres were close to present day conditions during Pleistocene, as the Gharb basin already reached its equilibrium profi le (Cirac, 1985; Aberkan, 1989).
2.1.1 Observations
Four of the fossil coastal littoral / eolian systems and the last active system were studied. The general belt-like morphology often hides complex and intricate sedimentary series due to the complex Quaternary sequence-stratigra- phy infl uenced by glacio-eustatic variations (Fig. 5, B, C, D). These belts are described in the following paragraphs, from the oldest to the most recent. The link between local and general chronostratigraphy is summarized in Annex 2.
4th belt
The oldest coastal belt was studied in the Sidi Taibi Quarry, located 150m inland along the national road Rabat- Tanger 13km after exiting the town of Salé (Fig. 6). This quarry is a small exploitation of calcarenite allowing easy access to fresh outcrop surfaces (Fig. 7; Plate 1, A). The exposed strata can be divided into two main units.
The first unit’s lowermost visible deposits consist in at least 1 metre of wavy bedding and current ripples Above: Fig. 6: Aerial view of the Sidi Bou Taibi Quarry, Amirian
(source: GoogleEarth).
Below: Fig. 7: Exploitation cut in the Sidi Bou Taibi Quarry. The base shows plane bed. It is overlain by large, landward dipping foresets, themselves capped by a thick, reddish, pulverulent paleosoil.
Right column: Fig. 8: Close up on the eolian foresets alternating vuggy, geode bearing layers and tight layers. Sidi Taibi Quarry.
overlain by sub-horizontal planar bedding on three me- tres. These deposits are conformably draped by up to 15 metres of cross bedded, high-angle plurimetric foresets (Fig. 7; Plate 1, A). These foresets show unusual dissolu- tion and reprecipitation features along the stratifi cation plane, alternating tightly cemented and vuggy to hollow, geodes bearing centimetric layers (Fig. 8; Plate 1, B). The large foresets are capped by a 30 centimetre thick red, arenaceous paleosoil (Plate 1, C).
The fi rst unit is cut by an important erosion surface that cuts down to several metres in the large foresets layer (Fig. 9, A) and is infi lled by the second unit’s deposits.
Cobbles to large boulders are deposited chaotically in the paleotopographic lows above the erosion surface (Fig. 9, A). Their arrangement is clast-supported, and sand infi l- trated between the blocks posterior to their deposition.
Fig. 9: (A): Erosion surface cutting through the large foreset layer, with large boulder deposited above (white arrow). Plane-bed caps the boulders.
(B): Barnacle encrutings of the boulders above the erosion surface. Hammer’s point for scale.
Fig. 10: Aerial view of the Sidi Bouknadel Quarry, Tensiftian (source:
GoogleEarth).
whole quarry is capped by three to four metres of pluri- metric landward-dipping foresets deposited above the fi rst unit’s paleosoil. Due to the geometry of the quarry’s cuts, genetic relationships with the abovementioned plane-bed layer cannot be proven. The terminal large foresets layer is affected by a red powdery paleosoil with solution pipes that can reach several metres in depth (Plate 1, C) and more than one metre in diameter.
3rd belt
This belt was studied in the Sidi Bou Knadel quarry, located 8 kilometres northwards after leaving the outskirts of Salé on the western side of the Rabat-Tanger road (Fig.
10). This quarry was exploited on the lee-side of the third belt (Plate 1, D).
Fig. 11: Picture of the northern wall of the Sidi Bouknadel Quarry showing the two eolian layers separated by a paleosoils and affected by plurimetric solution pits.
Quaternary eolianites
gastropods. Both under- and overlying units are made of grainstone showing high-angle, plurimetric, landward dipping foresets (Plate 1, E) with grainfl ow fi gures and pinstripe lamination. The whole quarry is affected by large, metres deep and up to metric diameter solution pipes (Fig. 11). The solution pipes show thick micritic sheaths around them (Plate 1, F).
2nd belt
The second belt mainly crops out along the coast (Fig.
12 A), forming large cliffs reaching more than 35 metres (Fig. 13; Plate 2, A). Several creeks cut this belt, allowing the observation of the exposed strata on more than 6 ki- lometres. The geometry of the outcrop and the important Fig. 12: (A): Aerial view of a portion of the second belt along the Atlantic coast (Salé). At this location, the second belt is eroded into high cliffs reaching more than 30 metres (source: GoogleEarth). (B): Aerial view of the Sidi Moussa Creek, second belt (source: GoogleEarth). (C): Aerial view of the Bou Touil Creek, second belt (source: GoogleEarth).
Fig. 13: Photographic panorama of the Sidi Moussa Creek. The left side of the picture is directed northwards, the right is directed southward.
lateral variation of the observed features did not allow to record one continuous stratigraphic log.
The oldest deposits can be observed at the south of the Bou Touil creek (Fig. 12 C; Plate 2, B, C). They consist in grainstone with high-angle, pluridecimetric, landward dipping foresets, themselves capped by a pulverulent, red- dish paleosoil. At this location, these deposits are draped by around two metres of trough-stratifi ed shell rudstones (Plate 2, C) and three to fi ve metres of very low-angle seaward dipping plane-bedded grainstone (Plate 2, B).
They laterally correspond to trough-stratifi ed grainstones at the southernmost creek of Sidi Moussa (Fig. 12, B; Plate 2, D). At this location, they are sealed by a 4 centimetres thick calcrete (Plate 2, E), overlain by up to two metres of lagoonal-lacustrine fossiliferous dark clays (Plate 3, A, Aberkan, 1989). Draping the abovementioned deposits over the whole area, are deposited of more or less cemented calcarenites displaying large, landward-dipping sometimes decametric foresets (Fig. 13; Plate 3, A, B, C). This unit makes most of the cliffs’ height, reaching more than 30 metres in thickness (Plate 3, C, D).
At the Sidi Moussa location, the large foreset unit overlying the lagoonal clays is cut by an important ero- sion surface (Plate 3, D, E). At its base is deposited a lag conglomerate containing blocks to boulders of the eroded unit, draped by four metres of plane bedded grainstone (Plate 3, E; Plate 4, A). At the Plage des Nations cliffs (Fig. 3, B), the plane bed unit is intensely bioturbated by Psilonichnus upsilon crab burrows (Plate 3, F). Chaotic blocks belonging to the eroded unit are deposited above the plane bed thicket (Plate 3, E). The plane-bedded unit cuts through the underlying deposits at the present-day altitude of fi ve to six metres (Plate 4, A). In some places, heavy minerals placers can be observed over the erosion surface (Plate 4, B), either as reworked pebbles or cobbles or loose, sometimes bioturbated sand (Plate 4, C). Large landward dipping, plurimetric foresets onlap the depres- sion created by the erosion surface and drapes the whole outcrop. This last unit is intercalated with up to seven reddish powdery paleosoil layers (Plate 3, C, D). The second belt is capped by a thin, patchy calcretized paleosoil. No solution pipes were observed in the second belt.
1st belt
The fi rst belt is well developed at the south of the town of Mehdia (Fig. 14;Plate 4, D). It makes an 11 kilometres long, up to 30 metres high ridge. It is intensely vegetated, and the few outcrops are road-cuts.
The lowermost stratigraphic outcrops can be found along the road near the fi sherman harbour of Mehdia (Plate 4, E, F). Very scarce exposures show intricate stacking of decimetric lunar (?) sigmoids and trough stratifi cations
large animal tracks (Plate 5, A, B). Some erratic rhizolites can be observed, as well as pinstripe-lamination.
Present day eolian system
An active eolian system is forming on both parts of the Oued Sebou estuary. The southern shore was studied in this thesis (Plate 5, C, D). The active dunes form a ridge that can reach up to more than 20 metres high (Plate 5, C). This system progrades inland (Plate 5, D) despites some attempts of the coastal authorities to decrease the inland sand input by excavations (Plate 5, C) or wooden palisades. The excavations led to the opposite results, defl ating more sand by perturbing the wind fl ow. Defl ated areas show outcrops of the underlying incipiently lithifi ed eolianites (Plate 5, C, D; “Dune Grise” of Aberkan, 1989).
These eolianites were dated 1000 ± 70 calibrated years BP on terrestrial gastropod shells found in some interdune areas (Plate 5, D).
2.1.2 Interpretations 4th belt
The Sidi Taibi quarry’s fi rst unit consists in a complete regressive depositional sequence showing deposits from shallow subtidal to supratidal and continental realms. Due to the stratigraphic continuity of the serie, the supratidal, eolian deposits, reaching up to 15 metres in thickness, are probably related to the highstand / stillstand deposition.
The erosion surface is linked to a marine transgression, as the massive lag conglomerate encrusted by barnacles testifi es to. Plaziat et al. (2006) gave this belt an Amirian Fig. 14: Aerial view of the fi rst belt and the present-day, active eolian system, 3 kilometres southwards of the town of Mehdia (source:
GoogleEarth).
Quaternary eolianites
3rd belt
The third belt displays clear eolian sedimentary fea- tures in the observed outcrops. Tensiftian age was at- tributed by Plaziat et al. (2006) to these units.
2nd belt
The very fi rst deposits of the second belt display clear eolian features. No age has been attributed to these ancient eolian deposits. The overlying succession is a complete shallowing-up sequence from high-energy subtidal to supratidal deposition. Plaziat et al. (2006) dated this suc- cession as Eemian. The lag conglomerate over the major erosion surface would then be the next transgressive system tract as the overlying regressive sequence from intertidal to supratidal deposits is dated Soltanian (MIS 5 Interglacial) by Plaziat et al. (2006). The eolianites are probably a Highstand / Stillstand System Tract, as the de- position is continuous from the shoreface to the eolian.
1st belt
The Mehdia harbour’s outcrop shows distinctive subtid- al features. The overlying strata are clearly eolian related, as the pinstripe lamination, rhizolites and animal tracks testify to. Plaziat et al. (2006) dated this ridge Soltanian (MIS 5 Interglacial).
Solution pipes
The large solution pipes affected the fourth belt and the third belt, but are not found in the second belt. These pedogenetic features would then be framed by the Ten-
siftian age of the third belt and the Eemian age of the second.
2.1.3 Summary
The Quaternary littoral belts are not gradually stacked seaward according to their decreasing age. They display a complex sedimentological stacking of regressive sequences and highstand and / or lowstand related eolian formations spilling over inland and draping anterior deposits.
The littoral belts of the northern coast of Salé give the opportunity to study the potential diagenetic imprints of transgressed eolianites.
The solution pits affecting the fourth and third belts are probably synchronous and their formation linked to peculiar climatic conditions, as this kind of alteration is not found elsewhere. To form such pedogenetic features, humid climatic conditions are needed (Herwitz, S.R., 1992;
Rodet, 1993; Marsico et al., 2003; Perica et al., 2004; Ker- shaw et al., 2005; Grimes, 2006; Bogatyrev et al., 2009)
2.2 Oman
The Wahiba Sands covers an area of 16’000 km2 (Radies et al., 2004; Preusser et al., 2005; Radies et al., 2005 ; Fig. 15). This active sand sea has a mixed siliciclastic / carbonate content supplied by several different sources, such as shore input, erosion of its eolianite substratum and sedimentary input from the surrounding wadis (Pease et al., 1999; Pease and Tchakerian, 2002). The sand sea is the most recent part of a fl uvio-eolian system that began
Fig. 15: (A): Satellite view of the Arabic Peninsula. The red box frames the area displayed on the right (B) (source: NASA WorldWind). (B):
Satellite view of the Wahiba Sand Sea (source: NASA WorldWind).
deposition during the Pleistocene in the Wahiba basin over the Barzaman Formation (Radies et al., 2004). The fossil deposits consist in 7 units defi ned by Radies et al. (2004) on core and outcrop studies: Al Jabin (MIS 6, carbonate eolianite), Al Batha (MIS 5e, fl uvial sandstone), Al Hibal (MIS 5d, calcareous eolian sandstone), Hawiyah (MIS 5b and 4, eolian sandstone), Qahid (Late MIS 3 to early MIS 1, eolian sandstone) and lacustrine sediments (early Holocene). The fossil eolian substratum forms the 2000 km2 Al Jabin Plateau (Fig. 15, B). The youngest eolian deposits (Qahid Unit) crops out in defl ated parts of the active sand sea (Plate 6, A)
2.2.1 The Al Jabin Unit
The studied outcrops belong to the Al Jabin Unit, the oldest eolianite with the highest proportion of carbonate sand (Radies et al., 2004). They are located 60 kilome- tres inland, (21°15’55’’ N / 58°22’11’’ E) at the western limit of the Al Jabin Plateau, where the Wadi Andam has cut in the Plateau’s eolian deposits several metres high cliffs on several kilometres of lateral extent (Plate 6, B).
Residual hills of the overlying eolian sandstone of the Al Hibal Unit were left by fl uvial drainage during more pluvial periods (Plate 6, C ; Maizels, 1987; Gardner, 1988;
Preusser et al., 2002; Radies et al., 2004). The exposed surfaces show typical eolian deposition features: stacked landward dipping high-angle metric foresets beds show- ing uniform thickness all along the outcrop and framed by paleosoil horizons (Plate 6, B, D, E, F), metric scale trough-stratifi cation (Plate 7, A), metric cross-stratifi ca- tion, pinstripe lamination, and rare, scattered rhizolites near the paleo-exposure surfaces (Plate 7, A, B, C, D).
Synsedimentary reworking of early lithifi ed eolian deposits
2.3 Qatar
Two fossil coastal systems were studied in Qatar, one on the North-western coast, the other further inland at the limit of the southern sand sea. These form remark- able topographic highs of 30 or 40 metres in altitude, as the Qatar’s highest point reaches only 103 metres above sea level.
2.3.1. Observations
The Northern Ridge
The fi rst system lies close to the shore near Al Fuwailat, at the north of the small town of Ain Synan. It makes one of the rare topographic highs of the eastern Qatari coast, reaching around 30 metres above sea-level. It is framed by the following coordinates: southernmost point:
26°02’20’’ N / 51°21’56’’ E, northernmost point: 26° 02’
58’’ N / 51° 21’ 32’’ E. Satellite photographs show a belt- like morphology, mainly parallel to the present-day coast, divided along two ridges (Fig. 16). Holail (1999) assigns to this ridge a Pleistocene age, and suggests a possible eolian origin for parts of these deposits, without giving detailed sedimentological argumentation.
The ridge can be subdivided into two distinct units.
The fi rst has a uniform thickness of 2.5 metres (Fig. 17, A). It begins with a lag conglomerate entirely composed of perforated pebbles of reworked dolomitic Eocene sub- stratum (Plate 8, A). This conglomerate is overlain by 1 metre of plane-bedded grainstone capped by a current- ripples horizon, itself overlain by around 50 centimetres Fig. 16: (A): Satellite view of the Northern Ridge eolianite (source: GoogleEarth). (B): Aerial photograph of the Northern Ridge eolianite, facing south-east (Courtesy of Isabelle Billeaud).
Quaternary eolianites
dolomite. It is cut by an erosive surface above which is deposited up to 1.5 metres of high-energy material (Plate 8, B). It consists of clean sands with planar bedding (Fig.
17, B) that can be contorted, decimetric foresets and at least one layer made of matrix-supported to clast-supported perforated Eocene pebbles and boulders of a laminated facies. The latter could correspond to the reworked base of the underlying sequence. This layer seems to be pinch- ing-out landward.
The second unit reaches 30 metres in thickness and consists in high-energy bio-oolitic grainstone. It can be divided in three subunits, each displaying similar features in different proportions but belonging to three distinct depositional phases. In normal contact with the fi rst unit lies an up to 3 metres thick layer of grainstone showing a dense rhizolitic network, ant-nesting features associ- ated with grass rootlets patches (“gingerbread facies”), pinstripe lamination, animal tracks (Plate 8, C, D, E) and diffi cult to discern landward dipping stratifi cation planes.
In some places and at the base of the layer can be found coarse pebbly material interfi ngering with the grainstone deposits with back-folded vegetation remnants (Plate 8, F). The second subunit reaches up to 25 metres. It lies conformably over the fi rst subunit and is composed of a stacking of several plurimetric, high-angle, landward dipping foresets beds (Plate 9, A). If this unit still shows
some rhizolites and “gingerbread” facies (Plate 9, B), these two features are overall less present that in the underlying layer. Impressive very early polygonal fracturing networks (Hasler et al. in review; Plate 9, C, D) and remarkable layers showing stratawise keystone vugs as described by Bain and Kindler (1994) in eolian realm can be observed (Plate 9, E). The third subunit drapes the whole system. It consists in several decimetric layers reaching more than a metre in total thickness (Plate 9, F). The eastern slope of the ridge is the structural slope of the plurimetric land- ward dipping foresets of the second unit. The high angle of repose of these foresets is confi rmed by the presence of grainfl ow lenses (Hunter 1977).
Jabal Marmi
The “Jabal Marmi”, was spotted from literature (Shinn 1973) and satellite pictures (Fig. 18). This hill lies 7 kilo- metres inland NW from the Sealine Beach Resort, on the eastern coast of Qatar. Its centre coordinates are 24°54’33’’
N / 51°28’7’’ E. This hectometres- to kilometre scale sys- tem forms a small hill that reaches 20 metres above the surrounding ground. The overall morphology displays a
“kettle backs” bumpy shape.
Like the Northern Ridge, the Jabal Marmi hill can be subdivided into two separated units. The fi rst unit’s deposits can be observed at some places at the base of the hill. These consist in shelly grainstones with some small fragments of the Eocene dolomitic substratum. At the northern extremity of the system, these deposits be- come plane-bedded, with a keystone-vugs bearing layer (Plate 10, A).
The second unit lays conformably over the fi rst. It can be subdivided into two subunits, the fi rst reaching up to 3 metres in height. It displays large low angle foresets (Plate 10, B) and pinstripe lamination. The second subunit lies directly above, reaching a thickness of ten to fi fteen metres Fig. 17: (A): Basal deposits underlying the Northern Ridge Eolianite.
The dotted line underlines the limit between the intertidal deposits and the conformably overlying eolianite. (B): Planar bedding.
(Plate 10, C). It shows large landward dipping, high-angle foresets, very few rhizolites and some scarce “gingerbread”
facies, and pinstripe lamination (Plate 10, D). Some key- stone-vuggy layers cut stratawise through this subunit.
The outcrop shows many reactivation surfaces (Plate 10, E), and foreset layers cutting through others. A third unit drapes the two abovementioned units (Plate 10, F).
At the base of the northern slopes of the hill, a con- glomerate drapes the fi rst unit. It contains dismantled bivalves and gastropod shells and cobbles to boulders of the dolomitic Eocene substratum and pinstripe laminated boulders reworked from the second unit.
2.3.2 Interpretation Northern ridge
The fi rst unit is interpreted as shallow-marine to intertidal deposition. The lag conglomerate at its base corresponds to the Transgressive System Tract, whereas the rest of the unit shows a clear regressive trend, with lagoonal and intertidal facies. This unit shows at the top overwash deposits, as the landward-thinning pebbles wedges and back-folded vegetation testifi es to. The second unit shows clear, distinctive eolian sedimentary features.
The fi rst subunit, conformably overlying the fi rst marine unit was probably deposited as the supratidal Highstand / Stillstand System Tract.The important vegetation and bioturbation indicates a possible decrease or stop in the sedimentary input.
The massive second subunit is probably due to the defl ation of the close coastal deposits during the fi rst
and sandsheets draping the main eolian body. The sedi- mentary input was locally lower than for the previous eolian stage, as no mature stoss-sides were observed in these deposits. It could correspond to the following step of the regression with the coast having shifted basinward and moved further away the sediment source.
Jabal Marmi
The fi rst unit is interpreted as intertidal, with the keystone vugs layer indicating the intertidal / supratidal realm boundary. The second unit shows distinctive eolian sedimentary features. The fi rst three metres of the sec- ond unit, being deposited conformably over the keystone vugs of the fi rst unit, are probably related to highstand / stillstand deposition. The overlying, thicker subunit’s deposition is probably linked to the following regression (post MIS 5e?). The conglomerate outcropping on the Northern slope of the hill has a marine origin from its fossil content. It yields a relative chronologic position due to its content reworking the underlying eolianites. It states that at the deposition time, the eolianites already formed a solid island. This conglomerate could be linked to the NS elongated Holocene spit found nearby (Shinn 1973). As stated by Bain and Kindler (1994), the stratawise keystone- vuggy layers record important precipitation events.
2.4 Tunisia
The studied area lies along the south-eastern coast of Tunisia, from the Jorf peninsula to the Lybian border, including the Island of Jerba (Fig. 19). Holocene dune belts of the Sidi-Salem Formation were studied as Fig. 18: (A): Satellite view of the Jabal Marmi (source: GoogleEarth). (B): Picture of the Jabal Marmi from its highest point facing north-east.
The scale is given by the car in the background.
Quaternary eolianites
2.4.1 Sidi Salem Formation
This formation is made of quartz bearing, oolitic, eoli- anites cemented by low-Mg calcitic pendant and meniscus cements (Jedoui, 2000; Fuchs and Klaus, 2001). It crops out in patches that can reach hectometric to kilometric sizes, from the Jorf Peninsula (south of the Gabès Gulf), on the north of Jerba Island (Sidi Salem town, locus typi- cus), in the Zarzis area and to the Libyan border (near the Harbour of El Kettef) lining the present day coastline (Fig.
20, A, B, C, D). In Lella Meriame and at the Sidi Salem location, the eolianites onlap Mio-Pliocene continental deposits, themselves capped by a Villafranchian calcrete (Plate 11, A) (Jedoui, 2000). They form small cliffs along the coast when present (Plate 11, B, C, D). At the Lella Meriame Location, they show at their base a bench close to present-day sea-level, locally draped by recent beachrock containing human artefacts (Paskoff and Sanlaville, 1983) (Plate 11, D). This bench has been related to the post- glacial hydro-isostatic rebound (Jedoui et al., 1998) and dated 2040 ± 35 by Morhange and Pirazzoli (2005). The Sidi Salem Formation can be found hundreds of metres inland draping Pleistocene littoral deposits (probably form the Rejiche Formation) near Borj El Grine (Plate 11, E). Fuchs and Klaus (2001) report that this eolian ridge can be observed down to 3 metres below the present-day sea-level in front of the barrier island sheltering the shal- low Bou Grara lagoon, offshore Hassi Jerbi (Plate 11, F).
14C datings ranging from 8660 BP to 6530 BP suggest an early Holocene age for the Sidi Salem Formation (Table 1). A peculiar coarse grainstone layer cutting through
the eolian deposits has been reported by Jedoui (2000) but not interpreted. The study of this layer is reported in Annex 1 (Frébourg et al., 2010).
The Sidi Salem Formation shows all the distinctive eolian features. High angle landward dipping foresets are found in every outcrop (Plate 12, A, B). Low angle inter- dune planar bedding is sometimes observed (Plate 12, B), as well as pinstripe lamination (Plate 12, B, C). Vegetation patches often associated with ant-nesting activity are scat- tered among some of the outcrops (Plate 12, D). Very early calcretes formed along the foresets planes (Plate 12, E), even before the root settlement, forcing them to develop above the calcretes (Plate 12, F). These eolianites do not show any paleosoils at their top or within them.
2.4.2 Rejiche Formation
Upper-Pleistocene carbonate eolian deposits were sampled in the Slob-El-Garbi quarry. This quarry was exploited into the upper, carbonate member of the Rejiche formation (Mahmoudi, 1986; Mahmoudi, 1988; Jedoui, 2000, Hasler et al., submitted) and shows remarkable three dimensional exposures (Plate 13, A, B, C, D). They are made of ooid-rich shallow-water to eolian sediments dated of isotopic stage 5e (Jedoui, 2000). These Pleisto- cene deposits form around the Bahiret-el-Bibane lagoon (south-eastern Tunisia) a several hundred metres wide and around 40 kilometres wide coastal belt, thus restricting its access to the sea.
The quarry cuts through the eolian deposits along verti- cal and horizontal planes, allowing a detailed observation of small to large scale eolian features. If the vertical cuts show mainly plurimetric, landward dipping high-angle foresets and rare root moulds (Plate 13, C), the horizontal section (Plate 13, D) displays a great variety of wind-driven depositional structures. Clear pinstripe-lamination can be observed (Plate 13, E), as well as grainfl ow scars (Plate 13, F), grainfl ow lenses (Plate 14, A), and homogeneous eolian facies described by Hasler et al. (in review, Plate 14, B). Major surfaces as well as smaller scale reactiva- tion surface cut through the outcrop (Plate 14, B, C).
Pedoturbation is also present, as nebkha ridge fracturing early lithifi ed eolian sand or nebkha patches sometimes reworking eolian endoclasts (Plate 14, D, E, F).
Fig. 19: Map of the region of Zarzis (modifi ed after Jedoui, 2000). The Sidi Salem Formation patches are represented in black.
Fig. 20: (A): Panorama picture facing west of the Sidi Salem Eolianite at the Lella Meriame outcrop. (B): Panorama picture south-east, towards the Tunisian-Libyan border of the Sidi Salem Eolianite at the El Kettef outcrop. (C): Panorama picture facing south of the Sidi Salem Eolianite at the Sidi Salem outcrop, Locus typicus. (D): Panorama picture facing south of the Sidi Salem Eolianite at the Marabout outcrop.
