Abstract
Palaeosols may be modified in a variety of ways during burial diagenesis but significant alteration can take place during the earliest phases of burial caused by rising groundwaters. A calcrete palaeosol, the Heatherslade Geosol, from the Lower Carboniferous of South Wales, contains abundant pyrite which overprinted the original soil fabrics. The pyrite is interpreted as the product of anaerobic conditions caused by drowning during the early stages of a marine transgression. Unusual diamond‐shaped forms of pyrite occur which possibly represent pseudomorphs after gypsum.