『Abstract
　Low-grade carbonate-rich manganese ore of sedimentary origin in the giant Kalahari Manganese Field, South Africa, is upgraded to high-grade todorokite-manganomelane manganese ore by supergene alteration below the unconformity at the base of the Cenozoic Kalahari Formation. Incremental laser-heating ⁴⁰Ar/³⁹Ar dating of samples from the supergene altered manganese ore suggest that chemical weathering processes below the Kalahari unconformity peaked at around 27.9 Ma, 10.1 Ma and 5.2 Ma ago. Older ages are dominant in the upper part of the weathering profile, while younger ages are characteristic of the deeper part of the profile. Younger ages partially overprint older ages in the upper part of the weathering profile and demonstrate the downward progression of the weathering front by as little as 10 cm per million years. The oldest age obtained in the weathering profile, namely 42 Ma, is considered a minimum estimate for the onset of the post African I cycle of weathering and erosion that followed the break up of Gondwanaland and formation of the Cretaceous to early Cenozoic African land surface. The youngest ages, recorded at around 5 Ma, in turn, correspond well to the Pliocene transition from humid to arid climatic conditions in Southern Africa.

Keywords: Ar geochronology; Rate of chemical weathering; Supergene alteration; Kalahari Manganese Field; South Africa』

1. Introduction
2. Geological setting
3. Mineralogy and geochemistry
4. Geochronology
　4.1. Methodology
　4.2. Results
5. Discussion
　5.1. Timing of the post African I erosion cycle
　5.2. Rate of chemical weathering
　5.3. Transition to arid climatic conditions
6. Conclusions
Acknowledgments
References