The stratigraphy of the Sima de los Huesos (Atapuerca, Spain) and implications for the origin of the fossil hominin accumulation

The Sima de Los Huesos (SH) is one of the many archaeo-palaeontological sites in the Sierra de Atapuerca. The characteristics of this palaeoanthropological site are unusual, due to the large size of the hominin accumulation and its location far from the karst entrances, at least today. In order to investigate the geological processes involved in the site formation and their relation to the hominin deposit, we have performed a study of the stratigraphical sequence and depositional history of SH. Analysis and correlation of the identified layers has allowed us to redefine the depositional structure of the site based on twelve lithostratigraphic units, which can be further grouped into five allostratigraphic units delimited by erosional boundaries. The results show that there was only one stratigraphic event of hominin accumulation. Cave bears and other carnivores occur with the hominin fossils in the same layer, but carnivores continued accumulating above the hominin deposit in an overlying layer. Our findings also argue against the hypothesis that the hominin or carnivore bones were transported to SH from a locus of primary accumulation remote from their present day location. The present study also reveals that, at the time of the hominin and carnivore fossil accumulations, the only possible access to the SH chamber was a deep vertical conduit, which was impossible to climb. (C) 2015 Elsevier Ltd and INQUA. All rights reserved.