Y a-t-il eu capture du Niger ?

Abstract

Résumé. - L'hypothèse d'une capture récente du Niger près de Taoussia est remise en cause pour trois raisons : l'improbabilité du creusement d'un défilé si important par le fleuve dans une roche très dure ; l'existence d'une vieille topographie cuirassée d'âge Plio-Pléistocène ( ?) très proche de l'actuelle ; enfin une nouvelle interprétation de l'évolution morphologique quaternaire de la Vallée.

Stabilization of large drainage basins over geological time scales: Cenozoic West Africa, hot spot swell growth, and the Niger River

“…The Niger River elbow region in between the inland delta and the Iullemmeden basin (Figure ) has been surveyed by Beaudet et al . [] and Michel []. Those authors have shown that the S2 ferricrete capping the mesas of the region (i.e., the relicts of the “fundamental topography” of Beaudet et al .…”

“…Those authors have shown that the S2 ferricrete capping the mesas of the region (i.e., the relicts of the “fundamental topography” of Beaudet et al . []) systematically dips toward the modern river and cement alluvial gravels in the vicinity of its course, arguing for its antiquity. Fieldwork also led to the same conclusion for the Tilemsi River, a tributary of the Niger draining the western flank of the Adrar des Ifoghas massif [ Beaudet et al , ] (Figure ).…”

“…The Niger River elbow region in between the inland delta and the Iullemmeden basin ( Figure 2) has been surveyed by Beaudet et al [1977aBeaudet et al [ , 1977bBeaudet et al [ , 1981a and Michel [1977]. Those authors have shown that the S2 ferricrete capping the mesas of the region (i.e., the relicts of the ''fundamental topography'' of Beaudet et al [1977a]) systematically dips toward the modern river and cement alluvial gravels in the vicinity of its course, arguing for its antiquity. Fieldwork also led to the same conclusion for the Tilemsi River, a tributary of the Niger draining the western flank of the Adrar des Ifoghas massif [Beaudet et al, 1981a] (Figure 2).…”

“…Since exploration by Chudeau [], the elbow between the High and Low Niger (Figure ) has been considered to result from a Quaternary capture of the inland delta, which would have been drained by river(s) flowing north or northwestward into the Taoudeni basin or the Hodh depression until capture by the Low Niger [ Chudeau , ; Furon , ; Urvoy , ]. But neither sediment accumulations nor paleo‐river courses exist that would support the existence of such rivers [ Beaudet et al ., ]. The delta was interpreted to result from damming of the High Niger River by Late Quaternary sand dunes and Urvoy [] and Tricart [] favored “very recent” aggradation‐driven overspill to explain its connection to the Low Niger.…”

“…The water gap allowing the lower High Niger to flow into the Senegal drainage would imply a loop in the river very near to the seashore, allowing back flow across the marginal upwarp. Such a configuration is unlikely given the very low relief and slope of S1 and the lack of evidence for paleo‐river course remnants in that area [ Beaudet et al ., ]. One must therefore consider that a mid‐Eocene High Niger used to flow northeastward to the sea (Figure f).…”