Early diagenetic septarian concretions from the Oxfordian (Jurassic) Corallian Group, Osmington Mills, Dorset, UK

Early diagenetic septarian concretions nucleated around large oyster shells are common in the Oxfordian (Late Jurassic) Nothe Grit Formation of the Corallian Group at Osmington Mills, Dorset, UK. The host sediment is dominated by quartz, feldspar, mica and clay minerals along with abundant bioclasts. The concretions are predominantly composed of non-ferroan calcite, post-dated by ferroan calcite, along with minor pyrite, glauconite and silica (chalcedony and quartz) cements. Radial septarian fractures are infilled by non-ferroan and ferroan calcite cements. The first phase of calcite cementation occurred prior to significant compaction; septarian fracture generation occurred following burial and localised over-pressuring of the sediment.