Haematite nodule horizons occur in alluvial-lacustrine deposits from the Mill Cove Formation (Silurian) of Dingle, Ireland and the Unidad Roja Superior (Permian) of the Spanish Pyrenees. These horizons, generally less than 1 m thick, are not associated with palaeosol profiles but are hosted by laminated or structureless, bioturbated mudstones to sandstones. The sequences contain weakly developed calcrete profiles. The haematite nodules are interpreted as having formed around fluctuating water-tables and are analogous to present-day groundwater ferricretes (laterites or plinthites). Fe2+, mobilised during periods of reducing groundwaters, was segregated into ferric oxide nodules during falls in the water-table or during influxes of oxygenated meteoric waters. The prevailing climate during deposition of both sequences was probably semi-arid and the presence of ferricrete-like horizons is not evidence of humid climates. The horizons represent early "diagenetic" effects, not related to prevailing soil moisture regime.
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