Cenozoic landscape evolution in and around the Congo Basin: Constraints from sediments and planation surfaces

Abstract

The Congo Cuvette (or Basin) is a large depression with an average elevation of 350 m, surrounded by topographic highs: The Central African Atlantic Swell (west), the Cameroon Highlands and the Ubangian Rise (north), the western flank of the East African Dome (east) and the Kalahari and Angolese Plateau (south). Our objective is to perform a geomorphological and stratigraphical study of the Congo Cuvette and its flanking relief to characterize the Cenozoic landscape evolution of Central Africa and associated vertical movements. Sedimentological and stratigraphical observations of both outcropping sediments (“Grès polymorphes” and “Sables ocres” Fm) and wells (Samba, Dekese, Mbandaka 1, Gilson 1) indicates a four stage history: (1) uppermost Cretaceous—lowermost Cenozoic lateritization; (2) Paleogene to Middle Eocene (66-47 Ma) deposition of a hot arid desert with aeolian dunes (“Grès polymorphes” Fm) flanking a large lake to the north (northern Central Cuvette); (3) Middle Eocene to Late Oligocene (45-23 Ma) base level fall and silicification/lateritization of the older deposits; (4) Neogene to Present-day (23-0 Ma) deposition of aeolian to alluvial thin deposits, covering previous planation surfaces (“Sables ocres” Fm). The geomorphological observations focus on the understanding of planation surfaces that are characteristic of much of Africa's landforms. Two main types of landforms are identified: (1) planation surfaces and (2) incised rivers and channels. Large planation surfaces (×104 to 2 × 106 km2) define etchplains and pediplains, and smaller surfaces (103 to × 104 km2) define pediments. Nine successive planation surfaces were mapped across the region. An age model is presented, based on the cross cutting relationships of these planation surfaces with dated volcanic and plutonic rocks, and on comparison with a well-dated sequence of weathering profiles and planation surfaces of West Africa. Paleo-topography is restored using slope changes of the pediments. The Congo Cuvette and most of the surrounding areas were near sea level at Paleocene-Middle Eocene times (65-46 Ma). Dates for the onset of uplift were determined as follows: Central African Atlantic Swell at 16 Ma and with increasing rate at 11.5 Ma; the Cameroon Highlands at 45 Ma including two episodes of accelerated uplift between 45-23 Ma and from 11 Ma; the Ubangian Rise, with moderate uplift, between 45-23 Ma to 11 Ma; the East African Dome at 45-23 Ma and with a significant increase at 11 Ma and a notable decrease in uplift since that time; the Angolese Plateau and the western part of the Kasai-Lunde-Kwango Plateau at 16 Ma and the Central Cuvette since 11 Ma. The present-day geomorphology of the Congo Basin reflects a complex differential uplift between the Central Cuvette (total uplift 200-300 m) and highlands (total uplift 700-1,800 m and probably more) of the surrounding areas. Subsidence of the Congo Basin terminated during the Middle Eocene times. Our results are inconsistent with dynamic topographic models that predict a collapse of the Congo Basin from 45 Ma on.