Silurian deltaic progradation, Tassili n’Ajjer plateau, south-eastern Algeria: Sedimentology, ichnology and sequence stratigraphy

The economic potential for unconventional shale oil and gas production in the Silurian of the Berkinee Ghadames and Illizi basins (BGI) in south-eastern Algeria has been recently confirmed through explo-ration 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 Silu-rian 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 contextof 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 kilome-ters. Clear cyclic stacking patterns are identified within the four studied sections showing progressively ageneral trend of thickening- and coarsening-upward, over a complete 2nd-order megasequence (SIL-1MS). This transgressive-regressive succession suggests deltaic progradation, shallowing and basininfilling as evidenced by numerous diagnostic sedimentary features and trace fossils, largely fromeastern-to western-Tassili plateau. Indeed, the wealth of outcrop data in the Silurian siliciclastic suc-cession enables us to distinct thirteen facies (facies A-M), ranging from shallow-to marginal-marinefacies, 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 grap-tolite-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 depositsbecomes progressively coarser andfluvial in response to the progradation of the North Africanb Akakus deltaic system, during regional sea level fall and uplifting of the region. These progradational depositsexhibit well-preserved trace fossils with moderate to high degree of bioturbation, such as Skolithos or theso-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), asconfirmed by the moderately diverse early Silurian graptolite faunas, and by the Caledonian unconfor-mity on the top (SB7). Each of the three formations of SIL-1 MS reveals two major 3rd-order pro-gradational 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. Theregional extent of each unconformity is directly linked to significant facies changes and to inflectionpoints on the global sea level curve.