GSA QUATERNARY GEOLOGY AND GEOMORPHOLOGY DIVISION FAROUK EL-BAZ AWARD FOR DESERT RESEARCH 2020: A NEW LOOK AT THE KALAHARI GROUP SEDIMENTS

The Kalahari Basin (KB), southern Africa, is one of the world’s largest cratonic basins (CB). It is believed that the sediments filling the KB (Kalahari Group Sediments; KGS) have been deposited at least since 65Ma. Considering this concept, one would expect cosmogenic burial dating, limited to ~5 million years, to provide ages for only the upper stratum of the KGS. However, dating results are surprising, indicating that the entire depth of the KGS is mostly younger than 5 million years, much younger than believed! For example, burial ages obtained from a 55 m section of KGS from the Mamatwan Mine, southern Kalahari, indicate that the majority of deposition occurred at ~1.1Ma. This Pleistocene sequence overlies the Archaean basement, forming a significant hiatus that permits the possibility of many Phanerozoic cycles of deposition and erosion no longer preserved in the sedimentary record. Results from other parts of the basin are similar.

Results also imply a less arid early-mid-Pleistocene environment than the present in southern Africa, and major reorganization of drainage systems. For example, our data establish the existence of a shallow water body that persisted for >450ka prior to 1.2Ma. Evidence from neighbouring archaeological excavations suggests an association of high-density hominin occupation with this water body.

Independently, the residence time of the Kalahari sand blanket, that covers large parts of the basin, is determined applying a cosmogenic isotope-based model. Results indicate that throughout the Kalahari, sand penetrated and covered the basin 1-2 Ma and that since 1Ma the Kalahari sand cover has been mostly stable.

Overall, the new examination of the KGS implies a dynamic landscape in which cycles of aggradation-erosion occurred, but only the last one is preserved in the sedimentary record. The dominance of aeolian activity in the last 1 Ma is the result of the overall aridification in southern Africa.