Sedimentological and geochemical controls on ooidal ironstone and ‘bone-bed’ formation and some comments on their sequence-stratigraphical significance

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Abstract
Ironstones and ‘bone-beds’ are both formed in shallow marine settings where the prevailing sediment accumulation rates are very low. A review of published work shows that both sediment types cap coarsening upward successions and are commonly developed in mudstonedominated settings. Both can be shown to contain similar sedimentary structures (e.g. rip-up clasts, disarticulated shell debris) and a diverse assemblage of ichnogenera. However, the early diagenetic assemblages within each sediment type are both different from each other and from the mudstones within which they are enclosed. In ooidal ironstones, the cement assemblage comprises glaucony, berthierine and siderite; in ‘bone-beds’ it comprises early apatite, glaucony and later calcite and pyrite; in the mudstones it is predominantly pyrite. It is argued that the physical environment in which the ironstones and ‘bone-beds’ formed was similar, whereas their diagenetic environments were rather different. In particular, the characteristic cement assemblage of ironstones was produced in suboxic pore waters which were subjected to intense Fe reduction, whereas the characteristic cement assemblage of ‘bone-beds’ was produced in suboxic pore waters where there was limited Fe reduction. As a consequence of these chemical and stratigraphical constraints it is believed that ‘bone-beds’ formed on marine flooding surfaces and ironstones formed either at sequence boundaries, major flooding surfaces or maximum flooding surfaces.