Early Cretaceous incised-valley fills, exposed in the eastern margin of the Congo basin (central south Atlantic rifted-margin): origin, depositional environments and sequence stratigraphy

Résumé : The Congo basin in south of republic of Congo displays well-exposed marginal incised-valley filled during the early post-rift stage of central south Atlantic rifting, which remain poorly studied. This current study focusses on morphology, sedimentological and stratigraphic architectural of these incised-valleys based on detailed field mapping and topography DEM analysis. Three NE-SW extending incised-valleys (known as Doumanga, Nkougni and Mboubissi incised-valley systems) were mapped in the study area, cutting into the Precambrian basement rocks (Mayombe belt) and separated each and other by ridge of basement rocks. These incised-valley systems exhibit dendritic-to-trellis morphology pattern and U-shape. Their location and morphology pattern are thought to be controlled by the basement tectonic features and lithology heterogeneity. They are interpreted as river incisions as results of base level fall in response to late rift tectonic uplift or early post-rift flexural uplift and subsequently back-filled by the Aptian nonmarine siliciclastic sediments (related to the upper member of the Chéla Formation (known as Vembo) as lake level rose in response of regional transgression in central south Atlantic. Detailed sedimentological analysis of well-exposed outcrops of Aptian infilling succession led to define distinct six main facies type (conglomerate, structureless to graded sandstone, laminated sandstone and silt, and organic-rich shale, containing abundant continental plant debris and coal), to infer the depositional processes from debris flows to hyperpycnal turbidity currents and finally to interpret the paleoenvironment of the Chéla Formation in the study area. The vertical and lateral variations of these facies record the evolution of flood-dominated lacustrine fan delta and related distal turbidites. The Aptian valley fill succession (ca. 60m-thick) form a onlapping transgressive depositional sequence bounded by Subaerial unconformity (correlated with regional extensive Pre-Chéla unconformity) and arranged internally in dominantly fining-upward vertical stacking patterns and minor coarsening-upward stacking pattern. This study provides new insights into the morphology pattern and origin of marginal valley-fills of the Congo basin and the paleogeography and sequence stratigraphy framework of the Aptian Chéla Formation (sag succession).