Relationships between African landforms, regolith materials, and Cenozoic climate change: Implications for sedimentary source-to-sink systems

Abstract

This review of landform-regolith associations at a reference locality in Burkina Faso decodes the 50-million-year long paleoclimatic imprint on the African landscape. Stepwise morpho-pedogenesis formed a sequence of landform-regolith associations, offering insights into surface evolution processes at successive landscape stages. The landscape evolved in response to wet-to-dry climatic transitions at ca. 34(?), 24, 11, 6, and 2.9 Ma, with each shift leading to the formation of pediments that were weathered and cemented by iron duricrusts after subsequent dry-to-wet shifts at around 29, 18, 7, and 3.4 Ma. This evolution resulted in (i) sequential dispersion and recycling of Paleogene regolith on pediment surfaces, and (ii) regolith (i.e., clastic sediments) delivery to rivers during dry periods at 34(?)–29 Ma, 24–18 Ma, 11–7 Ma, 6–3.4 Ma, and 2.9–0 Ma. The time laps between regolith production in weathering profiles and its fluvial discharge can be >40 My, necessitating caution in paleoclimatic interpretations of the sink sedimentary record. Shifts in the nature of regolith delivery are anticipated at ∼24 Ma (end of extremely warm and wet climatic regimes) and at ∼6 Ma (settling of the modern latitudinal climatic zonation over Northern Africa). Very-slow and steady base-level lowering (<3.5 m/My since 45 Ma) responded to the very long-term sequence of eustatic sea-level fall since the Early Eocene. The morpho-pedogenetic evolution produced a landscape with its highest reliefs composed of weak, weathering-prone lithologies protected by iron-rich duricrusts. Regolith recycling implies that without detailed geomorphological, petrographic, and sedimentological field investigations, the geochronological characterization of the regolith can be misleading for paleoclimatic, morphogenetic and source-to-sink studies. This review provides an updated landscape evolution model for Northwestern Africa and highlights the potential of mapping landform-regolith associations as paleoclimatic markers, with broad applicability to Cenozoic continental surface dynamics. Landform-regolith mapping is proving a key tool to assess tropical geodiversity, which is shown to have increased since 45 Ma as a result of the diversification of the successive landform-regolith associations.