Reconstructing the depositional history and age of fossil-bearing palaeokarst: A multidisciplinary example from the terminal Pliocene Aves Cave Complex, Bolt's farm, South Africa

The region ∼40 km north-west of Johannesburg, South Africa, known locally as the Cradle of Humankind, is of global significance as the caves preserve Plio-Pleistocene faunal and early hominin fossils. Despite a long history of research, there is still a need to contextualise and date the remarkable collection of fossils. An important but understudied palaeontological site, Bolt's Farm, may provide a key to addressing this as it preserves a series of >20 separate eroded palaeocave remnants occurring across a 1 km length of hillside. This is in contrast to highly concentrated deposits representing a single site, as is the case at the majority of the sites in the region. Historically, a lithostratigraphic approach to South African palaeocaves made reconstruction and comparison within, and between, deposits difficult or impossible. Here, we present a sequence stratigraphic approach and simple facies model for three palaeocave remnants at Bolt's Farm collectively termed the Aves Cave Complex (ACC), and a chronology based on combined uranium lead (U-Pb) dating, of basal and capping flowstones, and palaeomagnetic analysis. Results indicate that these currently discrete localities, formed together from a single entry dating to the end of the Gauss Normal Polarity Chron between 3.03 and 2.61 Ma, making ACC one of the oldest directly dated fossil deposits in the Cradle. The ACC contains the earliest occurrence of a key biochronological species, Metridiochoerus andrewsi, in the region. This work reinforces the model that clastic sedimentation and flowstone precipitation do not occur concurrently in Cradle caves; rather their mutually exclusive formation is driven by allocyclic changes in hydroclimate. This research contributes to understanding how Bolt's Farm developed the unprecedented high density of palaeokarst observed today, by offering the first evidence that currently discrete localities were once connected as a single cave system.