This study uses field observations and new U-Pb ages of detrital zircon grains from three samples to question the stratigraphic position of the Firgoun and Niamey siliciclastic sediments, presumed to be Neoproterozoic in age. Sharing several lithological similarities with the Late Cryogenian ''Triad'' of the Taoudenni, Gourma, and Volta basins, the uppermost siliciclastic sediments of the Firgoun and Niamey areas were likely also deposited during this period. This is corroborated by matrix-supported diamictites with faceted or striated pebbles as well as by structures resembling cryoturbation processes. However, the detrital zircon U-Pb age record that we present here for the lowermost deposits of Firgoun and Niamey provides mainly Paleoproterozoic ages, and very few Archean ages, altogether in a range from 1822 AE 9 to 3392 AE 9 Ma. Therefore, the new data only show that the Firgoun and Niamey sediments were deposited before about 1800 Ma. Nevertheless, the U-Th-Pb zircon age data allows examining the possible provenance of the sediments. We show that the latter was likely in the westerly close vicinity of the studied areas. The Archean zircons are likely inherited, and possibly originating from a more westerly source. The nearby source of the Niamey and Firgoun sediments suggests that a high topographic relief was still existing in the south-central part of the West African Craton in the Mid Neoproterozoic.
In the Firgoun region located on the southwestern part of Niger, Proterozoic sedimentary deposits mark the southeastern edge of the West African Craton. The lowermost coarse-grained sandstones, structureless, are related to fluviatile deposits. They evolve vertically to alternating quartzitic sandstone beds and silty–clayey sandstone layers, interpreted as a shallow marine turbiditic sequence. The uppermost deposits have glacial features comparable with those found in Gourma and Taoudenni basins. These are diamictites interbeddeds–carbonates–silexites and cryoturbation features in slates, attributed to the association of ‘tillite–limestone–chert’ related to the ‘triad’. The Firgoun area deposits, as with their equivalents of Gourma and Béli basins, have recorded the Pan-African deformation episodes. In this paper we show that the studied deposits were firstly affected by an early distensive phase D1 and secondly by two Pan-African compressive episodes D2a and D2b. The distensive deformation episode is well recorded in the basal deposits (‘Sandstone of Firgoun Formation’). The deformation structures correspond to 70–80° N trending, syn-depositional normal faults. The plotting of the faults planes onto the stereographic diagram shows the prevailing of the extensional regime marked by a 140° N trending stretching. The first compressive deformation stage D2a is characterized in the basal deposits by isopachous folds and by anisopachous folds in the uppermost deposits. The combination of the satellite image and the plotting of the fold axial planes (30° N–45° NE to 50° N–50° NE) onto the stereographic diagram indicate a compressive regime with 120–140° N trending shortening. The last compressive deformation stage D2b is marked by thrust and reverse fault planes oriented 60–80° N, crosscutting all of the previous structures, mainly observed in the uppermost deposits (‘Béli–Garous Formation’). Their plotting onto the stereographic diagram reveals a 40° N shortening direction.
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