Etude géologique du Sous-groupe de Mpioka dans le massif de Nionga. (Bas-congo, RDC)
The massif of Nionga is located in the province of Bas-Congo in the Democratic Republic of the Congo. The subgroup of Mpioka is lithostratigraphiquement to Group West-Congolien (566 Ma). And Zadinien, Mayumbien and West-Congolien together form what is called the supergroup West Congo (L. Tack et al., 2001). This work objectives were to determine: the petrographical types of rocks of this subgroup of Mpioka, the area source of debris, the mechanism and the physico-chemical conditions of the medium of filing. The interest of this study resided in the fact that the massif of Nionga is located in the Mayumbe ditch where manifestations lithologic and tectonic chain West-Congo are recorded. And into this ditch, extending south of Gabon to northern Angola from Congo-Brazzaville and the Democratic Republic of Congo, the massif of Nionga is the only sector where the subgroup of Mpioka outcrops in low vertical and lateral extension. As such, this massif was clear to understand, after the chemical stage schisto-limestone, the dynamics of the basin during the filing of the detrital episode of Mpioka in Bas-Congo. This study allowed us to conclude that the subgroup of Mpioka located in the Nionga massif is formed of clay and sandstone lithofacies, and that it is a rythmite deposited on a flat tidal where driveways come tides have alternately deposited sand (bench of sandstone and sandstone beds - average energy) and sludge (shoals of shales and clay beds - low energy). A medium scale, observed the intercalation of sandstone in shale and on a small scale, the alternation of the quartz-clay laminae. The slump structures resulting rupture and the transport of sediment layers thus producing intertidal sediments in the flats mixtures chaotic types and channels of rivers; rolled sludge have a zonation that would be the result of differences; in the texture, composition and colouring of rocks solid bedding would be due to a very rapid sedimentation in the flats intertidal. Oxidizing conditions from the middle of deposit are translated by the red hue of these rocks. And this colour is due to the presence of ferric iron (hematite) which is common in this massif. Apart from the neoformations chloritic, ironstone, and calcite, quartz sediments of the Mpioka subgroup come from erosion of metamorphic rocks, very rugged, Transmazonien and Kimezien. Correlated with the Pi of the subgroup of Mpioka term in Bungo basin (Angola), this sub-group flush in the massif of Nionga testifies to the existence of a shoal during the filing of Pc, Pe, Pk and Pm and a deflection crustal during the filing of Pi. This subgroup is also between two sedimentary gaps, and is transgressive on Schisto-limestone in the Mayumbe ditch where the level passed successively (from Pc to Pe) regression to a breach (of Pe to Pi) and finally to a regression (of Pi in Pm).
Mpioka subgroup, Nionga, DRC, Mpioka, RDC, Physical, chemical, mathematical & earth Sciences, Earth sciences & physical geography, Physique, chimie, mathématiques & sciences de la terre, Sciences de la terre & géographie physique
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