Petrography and sedimentary sequence relationships of speleothems from the Cradle of Humankind, South Africa

Speleothems are ubiquitous sedimentary features of caves in the Cradle of Humankind (locally referred to as the Cradle). Dominantly composed of calcite/remnant aragonite, these deposits are often interbedded with clastic sediments, which host a rich collection of hominin and faunal materials. Research into flowstones (horizontally-bedded speleothems) laid the foundation for establishing a U-Pb based chronology for the formation history of the caves and the important fossils they preserve but little work has been done on the flowstones themselves. The relationship between dated flowstones and sediments is key to building the regional U-Pb chronology. Here we test whether speleothems from the Cradle represent a homogenous lithology and assess the presence/absence of diagenesis through petrography. To do this, we analyse flowstones (n = 63) and stalagmites (n = 4) from twelve major fossil-bearing caves, making this the largest and most comprehensive study on speleothems from this region. Five speleothem categories are identified based on macrofabrics: macro-layered, micro-layered, detritally-interbedded, rare (raft and microbially-influenced) and diagenetic. Based on these, we identify previously unrecognised heterogeneity in Cradle speleothems and discuss the impact of diagenesis on speleothem palaeoclimatic and chronological studies. Finally field observations, hand samples and petrographic analysis document alternating deposition between speleothem and clastic deposits, forming long conformable sequences, driven by external hydroclimatic changes. In our dataset, we do not find evidence of flowstone intrusion into existing clastic sedimentary sequences and argue that, unless exceptional circumstances can be documented in detail, flowstones represent breaks in clastic deposition and can provide robust bracketing ages for sediments and fossils.