Field Evidence for Intrusive Speleothem Formation in the Cradle of Humankind, South Africa

The stratigraphic relationship between flowstones and clastic cave deposits in the Cradle of Humankind (Cradle) UNESCO World Heritage Site is central to establishing the ages of key hominin-bearing sequences. A recent petrographic synthesis suggested that intrusive flowstones do not occur in the Cradle, implying that all flowstones represent conformable depositional breaks and provide reliable chronological constraints. Our new field observations from the Gladysvale Cave challenge this interpretation and document a clear example of intrusive flowstone formation within fossil-bearing sandstone. The observed carbonate body penetrates the sandstone along bedding planes and fractures, incorporating angular clasts and forming irregular, cross-cutting, and vein-like geometries. These field relationships demonstrate the precipitation of calcite from infiltrating CaCO₃-rich fluids into pre-existing voids, which postdate clastic deposition and partial lithification. The intrusive flowstone displays multiple sub-packages with interfingering terminations, abundant small to large-sized cavities, and lateral offshoots that wrap around brecciated clasts. Such geometries are diagnostic of intrusion rather than surface accretion. The Gladysvale example provides definitive field evidence that intrusive flowstones occur within fossil-bearing deposits of the Cradle and can mimic the morphology of primary depositional flowstones. Their recognition is essential for correctly interpreting speleothem–clastic relationships and for refining the chronological framework of hominin-bearing cave systems.