DEPOSITIONAL ENVIRONMENT AND BONE DIAGENESIS OF THE MIO/PLIOCENE LANGEBAANWEG BONEBED, SOUTH AFRICA

Langebaanweg is a world renowned, richly fossiliferous Miocene/Pliocene bone assemblage on the West Coast of South Africa. Despite considerable palaeontological investigation of the biodiversity of the site, few studies have been conducted on the bone diagenesis. This study examines the petrography of fossil bones and teeth in relation to the host sediment matrix. The Langebaanweg bonebed is variably matrix- and clast-supported, containing abundant gravel-size bone and phosphorite clasts in a quartzose sand matrix with minor amounts of peloidal phosphate grains and kaolinitic clay. The bimodal size distribution of clasts and sand matrix indicates a mixture of beach, fluvial and aeolian sources, consistent with a riverine or aquifer-fed waterhole palaeoenvironment proximal to the sea. Both macro- and microfaunal bones exhibit a range of surface colours (cream/brown through dark orange/brown to black) indicative of secondary mineral precipitation (including manganese) and some show physical damage from trampling. Three phases of bone diagenesis are recognised; an early, rapid transformation of bone (bioapatite) to mineral carbonate hydroxylapatite, an apatite cementation phase and a single, late growth of infilling Mn-rich alteration material. Except for rare burnt bones, bones and teeth mostly preserve fine histological detail. Despite differences in secondary mineral burden and physical deformation (trampling), diagenesis is fairly uniform within the deposit. The excellent bone preservation, and the fact that the deposit has not been reworked is important for depositional models. The classic Palaeo-Berg River fluvial depositional model is challenged in this paper by an alternative possibility that the deposit was an aquifer-fed, near coastal waterhole.