SEDIMENTOLOGY AND DEPOSITIONAL HISTORY OF THE RISING STAR CAVE SYSTEM: PROVIDING A FRAMEWORK FOR TAPHONOMIC AND GEOCHRONOLOGIC INVESTIGATIONS

In 2015, the discovery of over 1,500 fossil bones from a new species of human ancestor, Homo naledi, in the Rising Star cave system in the Cradle of Humankind, South Africa was announced. The location and abundance of bones of Homo naledi within a deep, inaccessible cave chamber was suggested to be the result of either deliberate body disposal, mass mortality, or a death trap scenario (not necessarily representing a single event). The confirmation of any scenario requires dating the age of the deposit, as well as additional sedimentological investigation of not only the main fossil-bearing chamber (the Dinaledi Chamber), but also of the other, linked cave chambers with fossiliferous deposits. Acquiring depositional ages for the fossils is difficult because of the challenging geologic context preserved in the Rising Star cave system. We present details of the sedimentary facies and flowstones that characterize different chambers within the cave system in order to provide a foundation for interpreting the geochronologic results. We have mapped and described multiple facies from chambers along the route from the surface to the Dinaledi chamber, and within other fossil-bearing chambers in other parts of the cave system. The Rising Star cave system is characterized by five repeated facies associations, each of which have several variants. These key facies associations include: well-laminated mudstone, mud-clast breccia, muddy sandstone, pebble conglomerate, and dolostone breccia. We also recognize at least six discrete flowstone episodes in the cave system. Facies relationships vary between chambers, as does the presence of flowstones and the faunal assemblages. Refined facies and flowstone relationships presented here from throughout the cave system are necessary for understanding the connectivity between chambers and the history of sediment reworking within and between chambers. In turn, this work provides the framework for interpreting the ongoing age dating in the cave system, which will not be presented here. These results also shed light on the sedimentary dynamics of the Rising Star cave system, which may help facilitate future discoveries and understanding throughout the Cradle of Humankind and elsewhere.