The development of hiatal surfaces in the Osmington Mills ironstone member of the upper jurassic ringstead formation of south Dorset, England

The Osmington Mills Ironstone Member, a Corallian hiatal-condensed bed, contains a varied, mainly in situ fauna. It developed over a relatively short time (<0.33 Ma) and is part of a transgressive systems tract. A sharp deepening in sea-level, which has not been previously identified, marks its base; its top, the Oxfordian-Kimmeridge boundary, is a recognized maximum flooding surface. gh it is the only Corallian bed in south Dorset that contains corals, it comprises ooidal ironstone in places. Replacement of corals by ooidal ironstone indicates how the local depositional environment changed and explains lateral variation in the unit. Hiatal surfaces in the unit formed in different ways. Firm grounds and increased bioturbation record surfaces developed over the shortest time. Coral masses that have been modified by biomechanical processes indicate hiatal surfaces formed over longer periods. Reworked bioclasts and mixed fossil assemblages mark longer hiatuses. Early near-surface diagenesis, resulting in formation of berthierine, apatite and siderite, indicates hiatal surfaces formed over the longest time. ental processes controlling bioturbation, destructive taphonomic patterns and early diagenetic mineralization interact with each other during the formation of hiatal surfaces. Integrated approaches to the identification and analysis of hiatal surfaces will help to unlock their potential use in sequence stratigraphy.