Article (Scientific journals)
Silurian deltaic progradation, Tassili n’Ajjer plateau, south-eastern Algeria: Sedimentology, ichnology and sequence stratigraphy
Djouder, Hocine; Lüning, Sebastian; Da Silva, Anne-Christine et al.
2018In Journal of African Earth Sciences, 142, p. 170-192
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Keywords :
Silurian; Sequence stratigraphy; Sedimentology; Ichnology; Black ‘hot’ shales; Tassili n’Ajjer plateau
Abstract :
[en] The economic potential for unconventional shale oil and gas production in the Silurian of the Berkine – Ghadames and Illizi basins (BGI) in south-eastern Algeria has been recently confirmed through exploration drilling. The aim of the present paper attempts a better understanding of the Intra-Tassilian depression within the entire Silurian of the Tassili n’Ajjer plateau. The continuous deposits of the Silurian are exposed at the southern margin of the prolific BGI basins, in the Tassili n’Ajjer plateau, offering the chance to understand the sedimentology, ichnology, and to present a detailed sequence stratigraphy framework for the region. The 410 m-thick clastic Silurian sedimentary strata are subdivided into three formations in the context of sequence stratigraphy, namely: (i) the Oued Imihrou Fm. (Llandoverian) overlain by (ii) the Atafaïtafa Fm. (late Llandoverian to Wenlockian), and (iii) the Oued Tifernine Fm. (late Wenlockian to Pridolian). These can be also distinguished across the entire investigated area and laterally traceable over kilometers. Clear cyclic stacking patterns are identified within the four studied sections showing progressively a general trend of thickening- and coarsening-upward, over a complete 2nd-order megasequence (SIL-1 MS). This transgressive-regressive succession suggests deltaic progradation, shallowing and basin infilling as evidenced by numerous diagnostic sedimentary features and trace fossils, largely from eastern- to western-Tassili plateau. Indeed, the wealth of outcrop data in the Silurian siliciclastic succession enables us to distinct thirteen facies (facies A-M), ranging from shallow- to marginal-marine facies, and in turn, grouped into six facies associations (FA1-FA6). The lowermost part of the succession, which is the most prolific sources of hydrocarbons in North Africa, consists of thick organic-rich graptolite-yielding black ‘hot’ shales and ‘lean’ shales with sparse bioturbation with small Thalassinoides belonging to the distal Cruziana ichnofacies. In contrast, the uppermost part of the Silurian deposits becomes progressively coarser and fluvial in response to the progradation of the North African Akakus deltaic system, during regional sea level fall and uplifting of the region. These progradational deposits exhibit well-preserved trace fossils with moderate to high degree of bioturbation, such as Skolithos or the so-called "Tigillites" pipe-rock, Cruziana isp., Rusophycus isp., Monocraterion isp., and Syringomorpha. The SIL-1 MS is bounded by a post-glacial latest Hirnantian unconformity on the basal (SB1), as confirmed by the moderately diverse early Silurian graptolite faunas, and by the Caledonian unconformity on the top (SB7). Each of the three formations of SIL-1 MS reveals two major 3rd-order progradational sequences, commonly delineated by discontinuity surfaces (in ascending order, SB1 to SB7), and in turn, these six sequences (i.e. Si-1 to Si-6) are subdivided into at least ten shorter-term cycles. The regional extent of each unconformity is directly linked to significant facies changes and to inflection points on the global sea level curve.
Research Center/Unit :
Sedimentary Petrology, University of Liège, B20, Quartier Agora, Allée du Six Août, 12, 4000 Liège, Belgium
CSIRO Exploration and Mining, Australian Resources Research Centre, P.O. Box 1130, Kensington, 6151 Perth, Australia.
Disciplines :
Earth sciences & physical geography
Author, co-author :
Djouder, Hocine  ;  Université de Liège - ULiège & CSIRO Exploration and Mining, Australian Resources Research Centre, Australia > Form. doct. sc. (géol. - paysage)
Lüning, Sebastian;  GALP Energia > Oil & Gas Exploration
Da Silva, Anne-Christine  ;  Université de Liège - ULiège > Département de géologie > Pétrologie sédimentaire
Abdallah, Hussein;  REPSOL > Oil & Gas Exploration
Boulvain, Frédéric  ;  Université de Liège - ULiège > Département de géologie > Pétrologie sédimentaire
Language :
English
Title :
Silurian deltaic progradation, Tassili n’Ajjer plateau, south-eastern Algeria: Sedimentology, ichnology and sequence stratigraphy
Publication date :
08 March 2018
Journal title :
Journal of African Earth Sciences
ISSN :
1464-343X
eISSN :
1879-1956
Publisher :
Elsevier, United Kingdom
Volume :
142
Pages :
170-192
Peer reviewed :
Peer Reviewed verified by ORBi
Name of the research project :
Silurian deltaic progradation rock record, under storm- and fluvial-influenced setting (Gondwana): Sedimentology, ichnology, sequence stratigraphy, clay mineralogy, and implications for thermal maturity evolution.
Funders :
ULiège ARD - Université de Liège. Administration Recherche et Développement
Commentary :
Highlights: * Sedimentary structures complexity and the trace fossils through the Silurian deposit. * A guide to recognizing storm- and fluvial-influenced sedimentation in a deltaic system. * Interpret the ancient depositional environment, the sedimentological and stratigraphic evolution.
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since 26 March 2018

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