Proterozoic ⁴⁰Ar/³⁹Ar ages from cave deposits of the malapa, Sterkfontein and Dinaledi fossil sites, Cradle of Humankind, South Africa

We have explored the use of⁴⁰ Ar/³⁹Ar dating on Mn ooids and peloids, and fragments of indurated, extremely finegrained mudstone occurring in cave sediments in the Cradle of Humankind (CoH) UNESCO world heritage site, to obtain ages of fossil-bearing cave sediments. Samples analysed were from the Malapa site (envisaged as a testing ground because it was previously dated with high precision by the uranium-lead method and paleomagnetism), the floor of the Dinaledi Chamber in Rising Star Cave, as well as boreholes 2 and 3 of the Sterkfontein Cave. The ⁴⁰Ar/³⁹Ar apparent ages of the dated samples turned out to be unrelated to cave formation processes, rendering the Malapabased validation irrelevant. The results do, however, provide insight into aspects of the origin of the cave sediments and dated material. Most samples yielded disturbed ⁴⁰Ar/³⁹Ar spectra. The apparent pooled gas ages for the Mn ooids analysed from Malapa range between 476 ± 13 Ma and 1964 ± 13 Ma. Of 5 ooids from Sterkfontein, one yielded 1597 ± 13 Ma, while the rest range between I960 ± 35 Ma and 2081 ± 13 Ma, the latter being a plateau age. Peloidal Mn nodules from Dinaledi Chamber samples yielded apparent ages between 1975 ± 9 Ma and 2096 ± 13 Ma. Indurated mudstone fragments were analysed from Sterkfontein and for 3 out of 4 samples, the apparent pooled gas ages range from 1970 ± 14 Ma to 2059 ± 13 Ma (one yielded 1623 ± 15 Ma). We conclude that ooids, peloids and indurated mudstone fragments are derived from higher levels in the Transvaal Supergroup stratigraphy, trapped in karst features over time as these were eroded, and finally washed into the caves.