Chronology of Sedimentation and Landscape Evolution in the Okavango Rift Zone, a Developing Young Rift in Southern Africa.

The Kalahari Basin in southern Africa, shaped by subsidence and epeirogeny, features the Okavango Rift Zone (ORZ) as a significant structural element characterized by diffused extensional deformation forming a prominent depocenter. This study elucidates the Pleistocene landscape evolution of the ORZ by examining the chronology of sediment formation and filling this incipient rift and its surroundings. Modeling of cosmogenic nuclide concentrations in surficial eolian sand from distinct structural blocks around the ORZ provides insights into sand's residence time on the surface. Sand formation occurred from ∼2.2 to 1.1 Ma, coinciding with regional tectonic events. Notably, provenance analyses of sand within ORZ's lowermost block where large alluvial fans are found indicate different source rocks and depositional environments than those of the eolian sands found at a higher elevation. This suggests that the major phase of rift subsidence and the following incision of alluvial systems into the rift occurred after eolian dune formation. Luminescence dating reveals that deposition in alluvial fan settings in the incised landscape began not later than ∼250 ka, and that a lacustrine environment existed since at least ∼140 ka. The established chronological framework constrains the geomorphological effects of the different tectono‐climatic forces that shaped this nascent rifting area. It highlights two pronounced stages of landscape development, with the most recent major deformation event in the evolving rift probably occurring during the middle Pleistocene transition (1.2–0.75 Ma). This event is reflected as a striking change in the depositional environments due to the configurational changes accompanying rift progression. Plain Language Summary: Early rifting stages mark the beginning of the breakup of continents, making them crucial for understanding plate tectonics and the formation of extensional landscapes. These stages involve seismic activity and create unique environments, helping to assess geological hazards and study past habitats and biodiversity. Early stages of continental rifting in the Okavango Rift Zone, centered in northern Botswana, are described in this work from the perspective of sediment dynamics by constructing a time frame for their evolution. Two major types of sediment and their corresponding time scales are studied. The older sediments are eolian sands that were formed between ∼2.2 and 1.1 million years ago and lie today on elevated structural surfaces above the incised rift. Within the subsiding rift that was significantly geomorphologically modified not earlier than 1 million years ago, sediments were deposited by alluvial fans at least since 250 thousand years ago and were followed by a lacustrine environment with alternating hydrological conditions since at least 140 thousand years ago. Key Points: Nascent continental rifting stages are reflected through sedimentological variationsEolian sand that was formed before alluvial incision into the rift is preserved on elevated surfacesDepositional environments in the incised rift have shifted into alluvial‐lacustrine conditions around the Middle Pleistocene Transition [ABSTRACT FROM AUTHOR]