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

• Published in: Zoological journal of the Linnean Society Vol. 112; no. 1-2; pp. 285 - 308
• Main Author: MACQUAKER, J. H. S.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.09.1994
• Subjects: condensed section; iron; Marine; Phosphates; sea-level change; sequence stratigraphy; shales; British Isles
• ISSN: 0024-4082; 1096-3642
• DOI: 10.1111/j.1096-3642.1994.tb00321.x

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.