SEDIMENTARY ROCKS/Ironstones 105 Figure Model of mineralization for ooidal ironstones; (A), earliest phase; (B) and (C), middle phases; (D), latest phase (reproduced with kind permission of Kluwer Academic Publishers from Young TP (1993) Sedimentary iron ores In: Pattrick RAD and Polya DA (eds.) Mineralization in the British Isles, pp 446 489 London: Chapman & Hall, Figure 9.13) of transgression Similarly, the origin of early sideritic units can be related to a decrease in the role of sulphate reduction, a low sedimentation rate (less than 40 m Ma 1), and oxygenated and carbon-poor sediments (values of d13CPDB for various cements vary from À3 to À22%) The source and influx of iron is a subject of much controversy In 1856, Sorby proposed that extensive ferruginization occurred during later diagenesis, but this idea is no longer accepted, since most ferruginous ooids were formed within the depositional environment There are three proposals for the origin of iron enrichment: iron-rich exhalative fluids, supplying the sediment–water interface (some examples seem to be related to the episodic reactivation of faults involving exhalative hydrothermal or seep sources); mechanical accretion of lateritic terrestrial weathering products (e.g kaolinite and iron oxides) or lateritic soils to form ooids in a marine environment, with subsequent transformation to berthierine (this does not seem to be generally applicable since unaltered primary ooids with mixed iron oxide–kaolinite composition have not been found in marine ooidal ironstones); and