An Investigation of Lithology, Hydrogeology, Bioturbation, and Pedogenesis in a Borehole through the Cubango Megafan, Northern Namibia

The 260m thick Andoni Formation, deposited by an ancestor to the Cubango/Okavango River, forms the symmetrical, ultra-low gradient (similar to 0.017 degrees) Cubango Megafan, 350 km long and 300 km wide. As part of the thick, Cenozoic 'Kalahari succession' of northern Namibia, the formation consists of fine-grained, unsorted, siltto clay-rich sands; fine-grained, well-sorted and unconsolidated aquifer sands; and minor aquitard clays. Avulsive deposition under seasonal, semi-arid conditions in a grassland savanna environment was highly intermittent. Periodic flood events were separated by long intervals of nondeposition. Extensive post-depositional bioturbation and pedogenesis of each layer to depths of > 1m took place during subaerial exposure, destroying bedding and sediment sorting. Bioturbation consists of meniscate backfilled burrows likely produced by beetle larvae, soil bug nymphs, and other invertebrates in the A, B, and C horizons. As climate became drier, these features were overprinted by pedogenic calcrete and phreatic carbonate nodules. Unsorted clayand silt-rich sands suggest weak depositional currents, but aquifer sands point to wetter conditions with greater runoff. The pedogenic calcretes are indicative of limited annual rainfall (< 550 mm) and extremely stable land surfaces upon which no new sediment was deposited for centuries or even millennia.