THE EARLY POST-RIFT INCISED-VALLEY FILLS ALONG THE EASTERN MARGIN OF THE CONGO BASIN (SOUTH ATLANTIC), APTIAN CHELA FORMATION

Sage Paterne Kebi-Tsoumou (1) , François Guillocheau (1) , Cécile Robin (1) , Julien Bailleul (2) , Vincent Delhaye-Prat (3)

1 GR - Géosciences Rennes
2 Institut Polytechnique LaSalle Beauvais
3 Total TEPNG

The Congo basin displays marginal incised-valley fills, which are for longtime missed of in many previous geological studies. We have mapped three of these incised-valley systems, informally named Doumanga, Nkougniand and Mboubissi incised-valley systems, which extend from the Mayombebelt in the northeast to the Lower Congo basin in the southwest. In the study area, the incised-valleys cut into the Precambrian metamorphic and magmatic rocks of the Mayombe Basement. Detailed field mapping reveals that the location and morphology pattern of these incised-valley systems were strongly controlled by both tectonic structures and lithology heterogeneities of the underlying basement. They are thought to be cut by rivers which incised into the Precambrian basement and acted as bypass systems prior to their infilling. These incised-valleys systems were filled with Aptian non-marine siliciclastic sediments of the Chéla Formation that is well characterized thanks to rich ostracod and pollen associations. Deposition occurred during the so-called sag phase subsequently to the Aptian regional transgression recorded in the central segment of the South Atlantic. The main sedimentary facies documented in the Chéla Formation are clast and matrix supported conglomerates, structureless and graded sandstones, cross-stratified sand and sandstones, horizontallaminated sand and silt, interbedded sand and silty shale, and organic rich silty shale and shale, which have been grouped into six facies associations interpreted as deposits of flood dominated fan-delta and subaqueous fan interfingering with organic rich lacustrine shales. These deposits are organized into a transgressive sequence bounded at its base by a major angular unconformity and characterized by recurring stacking of finingupward sedimentary packages. It can be suggested that these incised-paleovalleys were filled in the form of backstepping fluvio-lacustrine depositional environment subsequently to the rapid lake level rise during the Aptian period.

Domains: Tectonics; Stratigraphy

Sage Paterne Kebi-Tsoumou, François Guillocheau, Cécile Robin, Julien Bailleul, Vincent Delhaye-Prat. THE EARLY POST-RIFT INCISED-VALLEY FILLS ALONG THE EASTERN MARGIN OF THE CONGO BASIN (SOUTH ATLANTIC), APTIAN CHELA FORMATION. International Meeting of Sedimentology 2017, Oct 2017, Toulouse, France. pp.460. ⟨insu-01718034⟩