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

Supergene ferromanganese deposits occur widely on the dolomites of the Tran svaal Supergroup in the area between Krugersdorp and Lichtenburg in the wes tern Gauteng and Northwest provinces. Most of the deposits occur along a Cr etaceous to early Tertiary African land surface of deep weathering and eros ion that developed on the subcontinent after the breakup of Gondwanaland so me 180 million years ago. Other deposits occur immediately above the post-A frican I surface of erosion that developed in middle Tertiary times. The ab solute age of these land surfaces is unknown. We have now directly dated th e timing of formation of a pedogenic manganese crust composed of potassium- bearing cryptomelane from immediately below the post-African I surface of e rosion at 12-15 million years using the Ar-40/Ar-39 technique. Similar crus ts are also present in soil nodules immediately above this surface. The res ults 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.