An unusual clay mineral assemblage from early Carboniferous (Arundian) palaeosols in South Wales contains a three component kaolinite/(chlorite-like material)/smectite mixed-layer clay. This is interpreted as the product of ferrolysis, a complex process involving dissolution of clays a d the subsequent production of interlayered varieties from the repetitive reduction and reoxidation of iron compounds in seasonally waterlogged, surface-water grey soils. Support for this interpretation comes from the distribution of iron species, iron concretions and vertic features in the palaeosol profile, which indicate formation under strongly seasonal, dry/hydromorphic conditions. The appearance of the ferrolytic pa- Iaeosol indicates a shi from a ‘dry’ to a ‘wet-type’ monsoonal regime in late Arundian times.

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