摘要
Studies undertaken at several sites in the Congolese basin and in the Chaillu massif are helping to reinterpret the sedimentological formations known as cover formations, which are widely exposed in Central Africa. This article presents the results of granulometric, morphoscopic, mineralogical and chemical analyzes obtained at three sites in the Chaillu. Gronulometric analysis indicates that the sands at the three sites are rated medium to high. The cumulative curves are essentially sigmoidal with regular and spreading slopes. This facies indicates that the deposition took place due to a banal and moderate variation in the competence of the carrier current. The morphoscopy of the quartz grains shows that they are mainly angular and shiny. These data reflect a relatively close source and transport by water. The synthesis of lithological, granulometric, petrographic and geochemical analyzes allowed us to highlight three episodes of deposits in the Mossendjo area: the first is characterized by a channel-type fluvial system marked by deposits from the hospital borehole and from the station;the second, which is alluvial, corresponds to the clogging of the channel. These two alluvial phases are overcome by a third, fluvial, channel-type episode, the end of which is marked by the establishment of the yellow formation. The vertical and lateral organization at the outcrop of this yellow formation has the same characteristics as those described earlier and in the sub-region where it was attributed to the wind-generated cover formation. This study thus highlights the existence of a wet phase during which two fluvial systems of non-contemporary channel types developed which overlap.
Studies undertaken at several sites in the Congolese basin and in the Chaillu massif are helping to reinterpret the sedimentological formations known as cover formations, which are widely exposed in Central Africa. This article presents the results of granulometric, morphoscopic, mineralogical and chemical analyzes obtained at three sites in the Chaillu. Gronulometric analysis indicates that the sands at the three sites are rated medium to high. The cumulative curves are essentially sigmoidal with regular and spreading slopes. This facies indicates that the deposition took place due to a banal and moderate variation in the competence of the carrier current. The morphoscopy of the quartz grains shows that they are mainly angular and shiny. These data reflect a relatively close source and transport by water. The synthesis of lithological, granulometric, petrographic and geochemical analyzes allowed us to highlight three episodes of deposits in the Mossendjo area: the first is characterized by a channel-type fluvial system marked by deposits from the hospital borehole and from the station;the second, which is alluvial, corresponds to the clogging of the channel. These two alluvial phases are overcome by a third, fluvial, channel-type episode, the end of which is marked by the establishment of the yellow formation. The vertical and lateral organization at the outcrop of this yellow formation has the same characteristics as those described earlier and in the sub-region where it was attributed to the wind-generated cover formation. This study thus highlights the existence of a wet phase during which two fluvial systems of non-contemporary channel types developed which overlap.
