New chronological constraints on the history of the Kalahari Group from the Upper Ugab Valley, Namibia

The sedimentary fill of the Kalahari Basin, which extends across several countries in southern and central Africa, records landscape evolution processes and holds archeological evidence of early hominid occupation. Recent studies have demonstrated that the majority of the Kalahari Group sediments were deposited between the Pliocene and the recent present. However, due to limited access to natural outcrops in the flat Kalahari topography, the chronology of the sequence, mostly its earlier part, is not well constrained. Here, we set out to study the geologic history of the Ugab drainage system, located in northern Namibia adjacent to the western margin of the Kalahari Basin. The evolution of the upper Ugab drainage system and the adjacent region of the Kalahari Basin were modulated by similar climatic and geologic conditions and processes. Furthermore, the continental water divide at the Ugab catchment headwaters is pushed eastward, further than any ephemeral river draining to the Atlantic Ocean. This geometry raises the possibility of a local drainage divide migration and implies that the upper part of the Ugab drainage system was part of the Kalahari Basin until captured westward to the Atlantic Ocean. Thus, the lower part of the sedimentary section exposed in the upper Ugab Valley may be a remnant of the eroded Kalahari Group. We report new chronological constraints on Kalahari Group units from the “Base Camp” outcrop in the upper Ugab Valley, using cosmogenic 26 Al and 10 Be dating in quartz and U-Th dating of carbonate cements and tufa. The study site is comprised of a ∼20 m thick cemented conglomerate sequence overlain by 20-25 m of carbonate and siliceous duricrusts. We developed a novel multi-stage numerical model for interpreting cosmogenic 26 Al and 10 Be data, following key stages of landscape evolution through a Monte Carlo simulation. This model reconstruction indicates that fluvial deposition of at least 70-100 m of sediments in the upper Ugab Valley had commenced most likely during the Miocene. Following sediment accumulation, surface stability was maintained for a mean most probable duration of 8.86 +6.82 / -1.51 Myr, and then followed by a short (<∼300 ka) erosional phase that removed a minimum of 32-53 m of the valley fill. Fine-grained sediments were then deposited during the Middle Pleistocene on top of the erosional surface and cemented by calcite and dolomite at a low gradient valley floor. The beginning of the final incision through the duricrust layer and the subsequent emergence of ground water springs are indicated by tufa deposits dated to 58.3 +22.2 / -17.5 ka. Quartzite samples from the re-exposed top of the conglomerate show simple cosmogenic exposure ages that range between 15.8 and 39.4 ka. Though the model suggests a gradual exposure, with at least two different incision rates over time, the overall exposure time is within the Late Pleistocene. Both deposition, which probably occurred in the Miocene, and removal of basal coarse-grained conglomerate in the early Quaternary are indicative of high-energy fluvial systems suggesting greater relief and possibly wetter climatic conditions relative to present-day climate. The transition to low energy deposition of fine-grained duricrusts and back to high energy incision through both the sedimentary units, indicate a possible local drainage rearrangement and divide migration in the upper Ugab Valley. • The conglomerate terrace at the upper Ugab Valley most likely started accumulating during the Middle to Late Miocene. • Conglomerate deposition was followed by erosion, duricrust formation and recent incision. • Alluvial stratigraphy and processes at the upper Ugab Valley may be controlled by a local drainage divide migration. • Ugab Valley terraces may be remnant deposits of the eroded Kalahari Group.