PALEOENVIRONMENTAL SIGNIFICANCE OF BONE-BEDS IN ORGANIC-RICH MUDSTONESUCCESSIONS - 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 o f 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 'b one-beds' are frequently found either overlying or amalgamated with 's hell-beds', and often contain large (up to cobble-sized) intraclasts. This work indicates that they formed during a period of relative sea-l evel rise, when the sediment was being reworked (forming a transgressi ve 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 re latively acidic. 'Bone-bed' formation was terminated when siliciclasti c mud prograded over the condensed section and 'normal' anoxic/suifidi c pore water conditions were established in the sediment.