Geodynamic context of the Proterozoïc deposits of the Firgoun Region (eastern border of the West African Craton, West Niger)

Abstract

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.