Evolution sédimentaire et tectonique du bassin protérozoïque supérieur de Comba (Congo) : Stratigraphie séquentielle du Supergroupe Ouest-Congolien et modèle d'amortissement sur décrochements dans le contexte de la tectogénèse panafricaine

Abstract

The Comba trough infilled by the Upper Proterozoïc West Congolian Supergroup trends N60°E and cuts across the Congo-Zaïre Archean craton. The trough is perpendicular to the Nyanga-Niari basin which itself represents the foreland of the Pan-African Mayombe belt. Moreover, the Comba trough was connected to other troughs extending further East beneath the Mesozoïc-Cenozoïc of the Congolian basin. The tectono-sedimentary evolution of the Comba basin was initiated between 950 and 700 Ma. The trough was formed by a NW-SE extension. It was filled up by periglacial fluvio-lacustrine sediments which correspond to the deltaïc formations of the “Bouenza” Group and the Upper Diamictite (“Tillite” supérieurerd), respectively. Then, a general climatic warm up produced a rise of the eustatic sea level which flooded the lake of Comba as well as the lakes and shelves of the Congo-Zaïre craton. This marine sedimentation is represented by the Schisto-calcaire Group which in terms of sequence stratigraphy we interpret as: a transgressive system tract (red marls of the lower part of the SCIa facies); a condensed section (pink dolomite of the upper part of the SCIa facies); a highstand system tract deposited along a carbonate ramp. The latter corresponds to a lateral facies variation and not a vertical facies succession as it was classically assumed. This highstand system tract was composed of: an external shelf (SCIb facies); a barrier (SCIc facies); a lagoon (SCII facies); a beach (“Kisantu Oolithe” of the SCIIIa facies and SCIIIb facies); a tidal flat (SCIIIc facies) with fluvial (SCIV facies) and tidal (Niari Breccia facies) channels. Between 700 and 600 Ma, a first compressive phase was responsible further west for the eastward Pan-African Mayombian thrusting. This phase was accompanied by the deposition of a thick molassic formation (Mpioka Formation) on the foreland belt and extending into the Comba basin. A late shortening phase of the Mayombian belt propagated into the Comba basin which was affected by strike-slip deformation along its southern edge. This strike-slip deformation induced important disruptions of the regional Mayombian tensor and a particular structural pattern of the Comba basin, characterized by strike-slip damping structures. Sections of these structures are interpreted as positive, negative, and dual “flowers”. This particular structural pattern provides an explanation of the Combian structures as they reflect local deviation of the Pan-African shortening axes. As a conclusion, the evolutionary stages of the Comba basin are discussed in the wider context of the Upper Proterozoïc of the Congo-Zaïre craton.