Diagenetic origin of Cambrian marine red beds: Caerfai Bay Shales, Dyfed, Wales

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Unlike some other Lower Palaeozoic marine red beds, the Caerfai Bay Shales, a Lower Cambrian volcaniclastic red-bed horizon in Dyfed, Wales, show no evidence of detrital pigmentary haematite. These red mudstones contain illite, kaolinite, chlorite and smectite and the red colouration appears to have been produced entirely by diagenetic alteration of iron-silicates and volcanic ash. Associated thin medium- to coarse-grained tuffs show diagenetic features closely comparable with those seen in continental red beds, and including dissolution, replacement and authigenesis. Feldspars show evidence of extensive dissolution and replacement by clay (illite) and authigenic overgrowths of plagioclase are common. Euhedral ferromagnesian silicates (augite and olivine) have suffered dissolution and replacement by clay, anatase and haematite. Complete haematite pseudomorphs after augite are a common feature. Other authigenic phases include quartz and calcite. The diagenetic environment appears to have been an oxidizing one in which dissolution of detrital grains provided the requisite ions for the authigenic phases. Oxidizing conditions in the deposited sediment may have prevailed because of low-organic productivity and complete destruction of organic material prior to sediment burial.