The Continental terminal is the last continental detrital assemblage defined by Kilian in 1931. According to [23], it corresponds to the last episode of filling of the Iullemmeden basin, outcropping over more than 90,000 km² in Niger ([24]). It refers to quartz-kaolinitic detrital formations with siderolithic facies ([23]; [34]), characterized by iron mineralization represented by ferruginous oolites ([16]). Within this continental unit, [23] and then [24] distinguished three series, comprising, from base to summit: 1) the siderolithic series of Ader Doutchi or Ct1, 2) the clayey-sandy series with lignites or Ct², and 3) the Middle Niger clayey sandstone series or Ct³. However, in the study area, only Continental Terminal 3 (Ct³) is exposed. The latter consists of alternating clayey sandstone and ferruginous oolitic sandstone with hardened levels, more or less associated with termite tubes, resting on Infracambrian deposits and/or in major or fundamental unconformity on the Paleoproterozoic basement ([51]; [30]; [38]). Previous work on the analysis of Oligocene deformation markers in Ct³ is fragmentary. Consequently, a detailed structural analysis remains to be done. The overall objective of this study is to search for markers of Oligocene deformation in the Ct³ formation. Specifically, it aims to: 1) identify synkinematic deformation structures in the Ct³ deposits and 2) determine the geodynamic context responsible for the formation of these structures.

From a geological point of view, the Niamey region straddles two major geological units (Figure 1): to the west, the Paleoproterozoic Liptako basement (2300 to 2000 Ma, [45]), belonging to the Birr domain of West Africa, and to the east, the Iullemmeden sedimentary basin, which is a vast Paleo-Mesocenozoic syncline. The Niger Liptako corresponds to the northeastern edge of the Man Ridge (Birimian domain of the Man Ridge) ([6]; [7]; [15]; [45]; [48]; [47]; [46]; [49]). To the north, the Niger Liptako is bounded by the Gourma Basin and to the southeast by the Volta Basin (Figure 1).

The Precambrian formations mark the eastern edge of the West African Craton. They outcrop discontinuously from north to south in the regions of Firgoun (Firgoun sandstone), Gassa (Gassa sandstone), Niamey (Niamey sandstone), and Kirtachi-Tamou (Kirtachi sandstone) ([36]). In the Niamey region, the outcropping formations of the Iullemmeden Basin are represented by Oligocene-Miocene deposits of the Continental Terminal 3 ([4]; [11]; [31]) and Quaternary sediments ([38]).

Figure 2.Location of study areas on the Liptako geological map extract ([36]).

The geological map produced by [36] was used to locate the outcrops in the study area. The GPS coordinates of these outcrops were projected onto [36], then onto a Google Earth image of the Niamey region. [36] was superimposed onto the Google Earth image of the Niamey region. This enabled the geological map of the study area to be updated. The areas covered by this study are located on the right and left banks of the Niger River (Figure 2). These are the “Trois Sœurs” and Tondi-Gamey areas, respectively.

The methodological approach adopted is essentially based on field analyses and measurements of deformation structures. A total of 53 measurements were taken, including 37 in the Trois-Sœurs area and 16 in the Tondi-Gamey area. The results of these measurements are presented in Table 1 and Table 2. In the absence of any cross-correlation between the various directions of normal microfaults, the directions are classified into two groups, F1 (N120˚ to N160˚) and F2 (N50˚ to N80˚), based on their strike and dip. Then these measurements were processed using the Win-Tenseur program (version 5.8.9) to calculate the stress tensors (σ1, σ2, σ3) ([13]). These stress tensors (σ1, σ2, σ3) are defined by three principal stress axes: maximum stress (σ1), intermediate stress (σ2), and minimum stress (σ3), and automatically calculated by the Win-Tenseur program. The Wallace-Boot hypothesis is used to theoretically predict slip directions based on known stresses, and also serves as the basis for paleostress inversion in the analysis of slip on fault systems ([35]).

Table 1.Structural data of Trois Sœurs.

Table 2.Structural data of Tondi-Gamey.

The two phases of extensive deformation highlighted during this study are summarized in Figure 6. The first deformation phase D1a has an extension direction of N50˚, and the second deformation phase D1b has an extension direction of N170˚.

Figure 6. Summary showing the two extensive deformation phases D1a and D1b during the Oligocene to Miocene period.

The deposits of Continental terminal 3 (Ct³) in the Niamey region are affected by a phase of extensive synsedimentary deformation, marked by mirrors of normal microfaults oriented N110˚ to N140˚ and N50˚ to N80˚ with dips of 60˚ to 80˚ towards the SW or NE. These normal microfault planes are compatible with the respective extension directions N50˚ (NE-SW) and N170˚ (NNW-SSE). The N50˚ extension direction noted as D1a (Figure 6), highlighted in the Ct³ deposits in the Niamey region, is comparable to the N50˚ to N80˚ extension direction obtained by [38]. Comparable results were obtained in northeastern Tunisia by [5], [52], and [28]. These authors highlighted an extension in the N30˚ to N50˚ direction during the Oligocene, responsible for the formation of Oligocene grabens, which can be correlated with those observed in the Niamey region in this study. [5] and [12] suggest that during the Oligocene period, a distensive phase prevailed in Tunisia. A similar observation was made on the scale of the Arabian platform by [20]. The latter suggests that the E-W to NW-SE orientation of the grabens and associated normal faults is linked to regional extension in a N-S to NE-SW direction. By analogy, the N50˚ to N80˚ direction of the normal microfault mirrors, highlighted in the Tondi-Gamey sector, is compatible with the N170˚ (NNW-SSE) extension direction. However, this N170˚ extension direction, noted as D1b (Figure 6), was not highlighted by [38] in the Niamey region. In the Oligocene-Miocene basins of Somalia, the same NNW-SSE directions of normal faults were described by [1]. In the Termit basin in Niger, a phase of extensive deformation (NNW-SSE) identical to that obtained in the present study was highlighted by [22]. This author linked this phase of deformation to an Oligocene-Miocene (Neogene to present) extension. In Eritrea, an N170˚ extension direction was identified in the Antalo limestones (Adeilo sector) by [43]. This N170˚ extension direction correlates with the N160˚ deformation phase described by [29] in the basins of the northern margin of the Gulf of Aden (Yemen). These authors linked this N160˚ extension direction to the late Oligocene-Miocene deformation phase. In the eastern portion of the Gulf of Aden rift, at the conjugate margins of Oman and Socotra, two phases of extensive deformation, D1 (N20˚) and D2 (N150˚), were identified by [20]. Similarly, at the margin of Yemen, two phases of Oligocene-Miocene extensional deformation, D1 (N20˚) and D2 (N160˚), were also described by [29]. These two phases of deformation are comparable to the results obtained in the present study. To explain the prevalence of this Oligocene distension on an African scale, [9] suggests that the African plate became immobile relative to the mantle from the Oligocene onwards ([10];), whose mantle fluid circulations would have had a major impact on the development of Africa’s topography. These topographic bulges (the Hoggar, the Aïr, the Iforas, the Tibesti) formed in association with a major episode of volcanism in Africa since 35 Ma ([42]). This Oligocene distension on an African scale is associated with the opening of several basins, including the East African Rift, resulting from mantle dynamics. Indeed, the East African Rift is closely associated with magmatism and mantle fluid circulation during this Oligocene period ([19]; [8]; [3]; [37]). Furthermore, several authors ([29]; [20]; [38]) have interpreted this NE-SW (N50˚) extension direction as being the first phase of Oligocene deformation, resulting from mantle dynamics. Meanwhile, the overall NNW-SSE (N170˚) extension direction is linked to the second phase of Oligocene-Miocene deformation ([29]). These authors linked this D2 deformation phase to the westward propagation of the Gulf of Aden oceanic rift. According to [41], there were two episodes of extensive deformation during the Oligocene-Miocene within the general compressive regime caused by the collision between Africa and Eurasia. This event could be the origin of the tectonic structures characterizing the late D1b deformation phase with a N170˚ (NNW-SSW) direction, which was highlighted by the present study.

This study shows that the Oligocene-Miocene deposits of Ct³ recorded two major phases of extensive deformation in the tectonic history of West Africa during the Cenozoic era. The first extensive Oligocene phase D1a, oriented N50˚, appears to have been closely controlled by the mantle dynamics of the time, giving West Africa a dome and basin structure. This structure appears to be associated with ascending and descending mantle currents. In the study area, these descending mantle movements are contemporary with the last episode of filling of the Iullemmeden basin, promoting the reactivation of most of the NW-SE-oriented faults. The second phase of deformation D1b, oriented N170˚, also extensive, is thought to be linked to an extensive episode within the compressive regime that occurred during the collision between Africa and Europe.