An ⁴⁰Ar/³⁹Ar age of supergene K-Mn oxyhydroxides in a post-Gondwana soil profile on the Highveld of South Africa

Supergene ferromanganese deposits occur widely on the dolomites of the Transvaal Supergroup in the area between Krugersdorp and Lichtenburg in the western Gauteng and Northwest provinces. Most of the deposits occur along a Cretaceous to early Tertiary African land surface of deep weathering and erosion that developed on the subcontinent after the breakup of Gondwanaland some 180 million years ago. Other deposits occur immediately above the post-African I surface of erosion that developed in middle Tertiary times. The absolute age of these land surfaces is unknown. We have now directly dated the timing of formation of a pedogenic manganese crust composed of potassium-bearing cryptomelane from immediately below the post-African I surface of erosion at 12-15 million years using the ⁴⁰Ar/³⁹Ar technique. Similar crusts are also present in soil nodules immediately above this surface. The results imply a minimum age of 15 million years for the post-African I surface of erosion with abundant formation of manganese nodules and crusts in the soils under lateritic conditions in the period 12-15 million years ago.