Université Abdou Moumouni de Niamey, Faculté des Sciences et Techniques, Laboratoire de Géologie: Eaux Souterraines et Géoressources, BP 10662, Niamey, Niger
Multi-channel seismic and well data from Agadem (Termit Basin), a Mesozoic–Cenozoic intra‐continental rift basin located in the West and Central African Rift System (WCARS), has been analyzed. Regional unconformities, including the top of acoustic basement, have been identified from seismic data.
The correlation of wells in the Agadem Block, as well as the seismic data interpretation, show that the lateral thickness variations of the layers is linked to the synsedimentary normal activation of the N140° to N150° trending fault system of the Termit Basin. The well logs correlation exhibits a high hydrocarbon potential, in the axial zone of the Agadem Block, which is characterized by a strong subsidence rate.
This study shows that the NW-SE-trending graben shaped Termit Basin exhibits a tilted block structuring, controlled by the synsedimentary reactivation of normal faults. Two major periods of structuring characterize the evolution of the Termit Basin:
- The first period, which was an extension stage, including the first rifting stage relayed by a thermal subsidence, occurred from early Cretaceous to Upper Cretaceous, during the south Atlantic opening.
- During the second period, which prevailed from Paleocene to Oligocene, the Termit basin was also affected by a second rifting event, followed by a thermal tectonic subsidence. The NW-SE trending faults was secondly reactivated during a NE-SW extensional regime.
In West Africa, particularly in the Taoudenni, Volta and Beli basins (southeastern border of the Gourma Basin), Neoproterozoic glaciogenic deposits have been assimilated to the Late Cryogenian «Tillites-Carbonates-Silexites» triad. Like those of the Taoudenni, Volta and Beli basins (southeast of the Gourma Basin), the Neoproterozoic formations of the Firgoun region on the southeastern border of the West African Craton exhibit characteristics of the Neoproterozoic triad. This study aims through sedimentological analysis and lithostratigraphic correlations to place the sedimentary sequences of the Firgoun area in their regional context. The Firgoun deposits, resting in fundamental unconformity on the birimian basement of the Niger Liptako, are essentially composed of quartzite sandstones, conglomerates, shales, silexites and more or less metamorphosed dolomitic limestones. The stratigraphic analysis reveals that the Firgoun deposits include 9 lithofacies from the bottom to the top: the lower lithofacies noted Fr1 to Fr3 have been attached to the base formation called «Firgoun Sandstone», and the lithofacies Fr4 to Fr9 related to the «Beli-Garous» formation. The summit lithofacies (Fr6 to Fr8) of this «Beli-Garous» formation, corresponding to a succession of interbedded diamictites associated with carbonate and silexites deposits, have been assimilated to the Neoproterozoic triad.
The present study concerns the south-centre of the Nigerien part of the Iullemmeden intracratonic Basin. A multidisciplinary approach involving sedimentological analysis of facies and determination of fossils has been implemented for the characterization and determination of the Maastrichtian to Paleogene paleoenvironments in the Garadaoua and Garadoumé areas. The stratigraphic series of the study area includes, from the bottom to top, the Formations of: Farin Doutchi, In Wagar, Garadaoua and Ader Doutchi (Ct1 or Ar). The Maastrichtian Formations of Farin Doutchi and In Wagar, as well as the Paleocene Formation of Garadaoua are fossiliferous. Indeed, the base of the Farin Doutchi Formation contains bivalves, foraminifera and sea urchins, while, In Wagar contains remains of vertebrates and casts of gastropods. In the Garadaoua Formation, presence of the benthic foraminifer Ranikothalia bermudezi species made it possible to assign to it an Upper Paleocene age. Moreover, the presence in the Garadaoua Formation of the sea urchin Linthia sudanensis species implies, during this periode, the pre-existence of an epicontinental sea in the Iullemmeden basin. Besides, in the Ader Doutchi Formation, the demonstration of α and γ oolites, as well as proto-oolites, testifies an agitated environment with at least two oolithization phases.
The Pan-African province of South Maradi represents the northern part of Benin-Nigerian Shield, corresponding to the Pan-African mobile zone located in the East of West African Craton. In this basement zone, the distribution of metamorphic formations is marked by gradual transitions: from schists to gneiss and then to migmatites. The schists outcrop in a NE-SW bands, alternating parallely with gneiss bands, limited by the shear zones. The alternation of schists and gneiss bands, associated to ductile shear zones and the existence of a gradual transition from schists to migmatite, raises questions about the geodynamic context of crustal evolution. The study of metamorphic paragenesis (aim of this study), allows to specify the geodynamic context that prevailed in this portion of the pan-African mobile zone of South Maradi. The methodological approach combines petrostructural analysis and determination of mineral assemblages by using the petrographic microscope. The chronology of the metamorphic mineral assemblages and their relationship with the deformation structures made it possible to highlight a prograde phase of metamorphic evolution (green schist facies- amphibolite facies, granulite facies) then retrograde, synchronous with a tangential tectonic and then transcurrent one. Further south in the contiguous Pan-African province of Nigeria, pressure elevation from 6.8 kbars to 9-10 kbars associated with a prograde metamorphism have been related to burial then to a crustal thickening by pile-ups.