Facies evidence of hydroclimatic regime shifts in tufa depositional sequences from the arid Naukluft Mountains, Namibia

The Naukluft Mountains on the eastern edge of the central Namib Desert contain numerous, largely inactive, fluvial tufas within headwater streams of the ephemeral Tsondab River which currently terminates in a vlei in the Namib sand sea. Extensive tufa barrage deposits have been mapped and described along the Brandfontein River, a small (6.5 km long) tributary of the Tsondab River. Here, a series of large barrages have developed along the river system which culminate in a large tufa cascade feature at the mountain front. Tufa facies at Brandfontein include cemented colluvial and fluvial gravels and boulders, moss tufas, as well as reed and root facies. The sequences provide evidence of several cut and fill phases which are interpreted as indicating alternating periods of tufa deposition and fluvial downcutting. The Brandfontein tufas are used to propose a three phase arid fluvial tufa deposition model in which erosive high magnitude, low frequency floods punctuate periods of quiescence and tufa deposition. Extensive downstream prograding ramped barrages and tufa cemented fluvial gravels are key components of this tufa deposition model, which illustrates the role of climate and topography in shaping tufa deposit morphologies.