Palaeoenvironmental significance of ‘bone-beds' in organic-rich mudstone successions: an example from the Upper Triassic of south-west Britain

A combined field-based/petrographic study to investigate the formation of Rhaetian 'bone-beds' from the Westbury Formation, Upper Triassic of south-west Britain, has been undertaken. 'Bone-beds' in the Westbury Formation have been found at the top of coarsening upward successions, on flooding surfaces which underlie organic-rich mudstones. These 'bone-beds' are frequently found either overlying or amalgamated with 'shell-beds', and often contain large (up to cobble-sized) intraclasts. This work indicates that they formed during a period of relative sea-level rise, when the sediment was being reworked (forming a transgressive lag) and a condensed section had developed. Phosphate authigenesis is thought to have occurred at the oxic/Fe-reduction interface, where the pore waters were dysoxic, enriched with dissolved phosphate and relatively acidic. 'Bone-bed' formation was terminated when siliciclastic mud prograded over the condensed section and 'normal' anoxic/sulfidic pore water conditions were established in the sediment.