Formation and collapse of the Kalahari duricrust [‘African surface’] across the Congo Basin, with implications for changes in rates of cenozoic off-shore sedimentation

Abstract

Following Gondwana break-up and the separation of Africa from Antarctica, India and South America, significant climate fluctuations and epeirogenic uplift have influenced the formation of the Kalahari Plateau (also known as the ‘African Surface’) and the Congo Basin (also known as the ‘Cuvette centrale’), two iconic landscapes of central sub-Saharan Africa. Both landscapes are covered by an extensive Upper Cretaceous-Cenozoic sedimentary sequence with hard-caps (duricrusts) of calcrete and silcrete of the Kalahari Group. These terrestrial sediments preserve a record over more than 80 million years of the climatic and tectonic history of south-central Africa, yet they represent a major challenge to accurately date, and to correlate precisely, because of lack of suitable materials for radiometric dating techniques, and because episodes of bioturbation, weathering, and erosion have left a condensed sequence of less than 500 m. Based on recent drilling and field investigations across the Kalahari Plateau in Botswana, and eroded surfaces exposed along the southern margin of the Congo Basin, in southwest Democratic Republic of Congo, new regional correlations of the Kalahari Group reveal that a large part of the carapace of silcrete and calcrete, here named the ‘Kalahari Surface’, across the Congo Basin disintegrated during the Cenozoic, exposing a vast area with up to 500-600 m of underlying poorly consolidated Jurassic-Cretaceousred-beds to fluvial erosion. When linked to the off-shore sedimentation history of the Congo Fan along the Atlantic margin, the terrestrial observations suggest that accelerated erosion of more than 0.5 × 106 km3 of the red-beds and collapse of the Kalahari Surface across the Congo Basin closely match the sudden increase of sedimentation and the estimated accumulation of ca. 0.7 × 106 km3 of Oligocene to Recent marine sediments. Discharge of the proto-Congo river systems draining the Congo Basin through the Congo delta was possibly linked to increasingly humid conditions during the mid-Cenozoic and may, in turn, have had significant effect on the global ocean water chemistry.