Carbonate spheroids in Permo-Triassic sandstones of the Sellafield area, Cumbria

SUMMARY Spheroidal structures composed typically of radial-fibrous dolomite or calcite are described from the St Bees Sandstone and Calder Sandstone formations of the Triassic of the Sellafield area, Cumbria. Etching of the spheroids in some sandstones, probably by present-day groundwaters, has revealed intricate internal structures composed of ferruginous clay, forming near-concentric bands and radial spokes. The spheroids are interpreted as early diagenetic groundwater precipitates. They typically occur in non-compacted fabrics where cementation prior to burial compaction has preserved a generally uncompacted grain fabric. It is unclear whether they are inorganic nodules or biologically mediated precipitates.