Pedogenic dolomites from the oolite group (Lower Carboniferous) South Wales

Within the Lower Carboniferous Oolite Group of South Wales dolomites occur as 0.1 to 3 m thick stratiform bodies, sharply bounded against both underlying and overlying limestones. They contain rootlets and other plant remains and are often laterally equivalent to calcrete horizons. The dolomites are interbedded with thin coals and organic‐rich shales. In thin section a fine‐grained, xenotopic, slightly ferroan, inclusion‐rich ‘early’ dolomite forms a matrix to scattered moulds after ooids, shells, and evaporite minerals. The moulds are filled with ferroan calcite, early dolomite, and ‘late’ strongly ferroan saddle dolomite. The early dolomite is highly subzoned under cathodoluminescence whilst the late dolomite is nonluminescent. Whole rock analyses have δ 18 O = −5.95%, to −0.80%, and δ 13 C = +0.50%, to +2.80%. The early dolomite is believed to have precipitated from low sulphate, low Eh fluids within the waterlogged soil of a schizohaline coastal swamp. The late dolomite may also have formed within the pedogenic environment as a precipitate from highly evolved swamp waters, heavily enriched in organically‐derived compounds. Alternatively the later dolomite may have a hydrothermal origin as is more commonly associated with saddle dolomite.