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Abouessa Ashour 2013 ED413.Pdf Sedimentological and ichnological characteristics of Dur At Tallah siliciclastic rock sequence, and their significance in the depositional environment interpretation of tidal-fluvial system (Upper Eocene, Sirt Basin, Libya) Ashour Abouessa To cite this version: Ashour Abouessa. Sedimentological and ichnological characteristics of Dur At Tallah siliciclastic rock sequence, and their significance in the depositional environment interpretation of tidal-fluvial system (Upper Eocene, Sirt Basin, Libya). Earth Sciences. Université de Strasbourg, 2013. English. NNT : 2013STRAH010. tel-00985603 HAL Id: tel-00985603 https://tel.archives-ouvertes.fr/tel-00985603 Submitted on 30 Apr 2014 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. UNIVERSITÉ DE STRASBOURG ÉCOLE DOCTORALE DES SCIENCES DE LA TERRE ET DE L'ENVIRONNEMENT Institut de Physique du Globe de Strasbourg (UMR 7516) Thèse par: Ashour Abouessa Soutiendra le: 19 Décember 2013 Pour obtenir le grade de: Docteur de l’Université de Strasbourg Discipline: Sciences de la Terre Spécialité: Sédimentologie Caractéristiques sédimentologiques et ichnologiques de la séquence silicoclastique du Dur At Talah. Importance dans l'interprétation des séries tidales et fluviatiles (Eocène supérieur, le bassin de Syrte, Libye) Thèse dirigée par: M. Duringer Philippe Maître de conférences, EOST Université de Strasbourg Rapporturs : M. Jean-Noel. Proust Directeur de Recherche, CNRS UMR 6118, Rennes M. Jean-Luc Schneider Professeur, Université Bordeaux 1 M. Hubert Whitechurch Professeur, EOST Université de Strasbourg Autres membres du jury: M. Jean-Loup Rubino Ingénieur, Total Pau M. Jean Gerard Conseiller clastiques, Repsol M. Mathieu Schuster Chargé de Recherche, CNRS UMR 7516, Strasbourg Acknowledgements This PhD thesis is part of a project sponsored by Total Oil Company to study the Dur At Talah sequence. This project have granted two PhD thesis. The first was by J. Pelletier (2012) and this one is the second. Great appreciation and gratitude goes to Total for their financial support to this project. Total is also technically involved in the study through the scientific contribution by Mr. J-L Rubino, who has participated in one of the two expeditions to the study area. Similarly, I would like to thank the Libyan Petroleum Institute (LPI) and the National Oil Corporation, who allowed me to participate in this project and provided part of my personal expenses throughout the period of this study. Among others, colleagues from LPI how reserve special thanks are: M. Benissa, M. Harouda and M. Binrahauma. Special thanks go to Pr. Mustafa Salem for his role in initiating this project and in his technical and logistical assistance; thanks also extend to Dr: O. Hlal from Tripoli University. Unlimited gratitude goes to the advisors of this thesis, Dr. Philippe Duringer and Dr. Mathieu Schuster who have the greatest impact on this project. Unforgettable very kind help have been provided by Dr. Philippe during my stay in Strasbourg. And I will be more comfortable to extend my gratefulness to Drs. Ghienne and Whitechurch for their encouragements and scientific comments during the period of the study. Thanks also go to the colleagues in Strasbourg University for providing a worm working environments and unconditional help in some difficulties, especially those related to my poor French language. Pelletier, Alexis Nutz, Emillian and Pierre are among many others. Off course I would like to thank Betty Kiefer for her help in providing many remote literatures, and Anne for her help with the Adobe illustrator. Finally worm thanks go to my family especially my wife Malak Abuarkoub for securing the smooth working conditions along the period of the study. Abstract Dur At Talah sedimentary sequence, located at the southern side of the Sirt Basin in central Libya, is composed of 150 m thick of mainly siliciclastic rocks. The importance of this sequence is linked to the importance of the Sirt Basin as one of large hydrocarbon reservoirs in Libya. The sequence is also an excellent site for vertebrate fossils of Late Eocene, the age of the sequence. Previous studies, though very limited compared to the importance of this area, are focused on its paleontological content. Sedimentology received only scant attention before this project. This thesis is an outcrop based study in which the focus is given to the sedimentary and biogenic (trace fossils) structures, aiming at defining and interpreting depositional facies which building up the sequence. The study is mainly based on field data which are analyzed on the light of related published literature and on the comparison with modern sedimentary environments. Results of facies analysis have led to splitting the entire sequence into three genetically related intervals. The oldest, we called the New Idam Unit (around 80m), is composed of very fine sandstones to mudstones. New Idam Unit is unconformably overlain by the Sarir Unit (around 50m), composed of medium grained cross bedded sandstones (the lower 25-30 m) changes up to very coarse and microconglomeratic sandstone (the upper 20-30 m). Thus, the Sarir Unit is split into the lower Sarir Subunit and upper Sarir subunit. The New Idam Unit presents both classical and unusual sedimentary and biogenic indicators that attribute this unit to estuarine depositional environment. It starts with outer estuarine (the lower 35 m) and ends up with inner estuarine (the upper 45 m). Maximum flooding surface is located in between. Above this surface the fluvial indicators increase and tidal indicators decrease, thus providing clue for basinward (North) migration of the shoreline. The lower Sarir subunit which was previously interpreted as fluvial deposits, preserves multi- scale sedimentary structures that undoubtedly belong to tidal processes. This is especially evidenced at the lower part of the lower Sarir Subunit (LLS). Fluvial indications over dominates the tidal ones in the upper part of the lower Sarir (ULS). Due to this configuration the whole lower Sarir subunit is interpreted as shallow marine, deltaic, depositional system, occurred during sea level ¨normal¨ regression. This time, maximum flooding surface is located between the LLS and ULS. The lower Sarir subunit is terminated by subaerial unconformity, with evidences of subaerial exposure preserved at the top of the ULS. These are intruded by the upper Sarir subunit which presents clear evidences of strictly fluvial environment of deposition. The deposits of the upper Sarir subunit record the low stand system tract part of the Dur At Talah sequence. In addition to the outlined results, the sequential pattern of the depositional events is suggested for the entire sequence of Dur At Talah. This study provides a valuable information regarding the depositional and sequential aspects of the Sirt Basin during the late Eocene, it also provide an unique case study for the better understanding of the shallow marine tidal deposits. Key words: Dur At Talah, Late Eocene, Tidal indicators, Sirt Basin, sedimentary structures, trace fossils, burrows of crayfish, bioturbation Résumé La séquence sédimentaire du Dur At-Talah, située dans la partie sud du bassin de Syrte au centre de la Libye, est composée de 150 m d'épaisseur de roches principalement siliciclastiques (grès, siltites et argiles). L'importance de cette séquence est liée en partie à l'importance du bassin de Sirt qui est un des plus grands réservoirs d'hydrocarbures de Libye. La séquence est également un des plus important site de fossiles de vertébrés de l’Eocène supérieur, l'âge de la séquence. Les études antérieures, bien que très limitées par rapport à l'importance de cette zone, se sont concentrées principalement sur le contenu paléontologique de la séquence. L’étude sédimentologique n’avait été jusqu’à ce jour que très peu abordée. Cette thèse est un travail basé sur l’examen des affleurements et où l'accent est mis sur les structures sédimentaires et biogéniques (traces fossiles), visant à définir et à interpréter les faciès sédimentaires qui ont construit la séquence du Dur At Talah. L’ étude de la séquence a conduit à diviser la totalité de l’affleurement en trois intervalles génétiquement liés. Le plus ancien que nous avons appelé la « New Idam Unit » (environ 80 m d’épaisseur), est composé de grès très fins, de siltites et d’argiles. Cette unité (New Idam Unit) est recouverte en discordance par la « Sarir Unit » (environ 50 m d’épaisseur), composée de grès fins à moyens à faisceaux de litages entrecroisées dans sa partie inférieure (environ 25-30 m d’épaisseur) et de grès grossiers à microconglomératiques dans sa partie supérieure (environ 20- 30 m d’épaisseur). La « Sarir Unit » est ainsi divisée en « Sarir inférieur » et « Sarir supérieur ». La « New Idam Unit » présente des dépôts qui sont attribués à un milieu main estuarien. La série débute par des dépôts d’environ 35 m d’épaisseur typiques d’un estuaire externe. Les 45 mètres qui lui font suite passent progressivement à des dépôts tidaux caractéristiques d’un estuaire interne. La Surface d'inondation maximale se situe entre les deux intervalles. Au-dessus de cette surface les indicateurs fluviaux augmentent et les indicateurs de marées diminuent progressivement, offrant ainsi un indice pour la migration de la rive de rivage vers le bassin (vers le Nord). La sous-unité du Sarir inférieur qui avait été interprétée avant ce travail comme dépôts fluviatiles, préserve pourtant des structures sédimentaires multi-échelles qui sont le résultat incontestable de processus de marée. Ceci est particulièrement évident dans la partie inférieure de l’unité « Sarir inférieur » (la Lower Lower Sarir Unit ; LLS).
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