Petrography and geochemistry of septarian carbonate concretions from the Boom Clay Formation (Oligocene, Belgium)

The septarian carbonate concretions from the Boom Clay (Belgium) consist ma inly of authigenic minerals such as micrite (less than or equal to 70% bulk volume) and pyrite framboids (similar to 3%). These mineral phases occur b etween detrital grains and fossils. The septarian cracks are lined with cal cite, which is sometimes covered with pyrite. The preservation of delicate sedimentological features in the concretion matrix (hardly compacted faecal pellets, burrows and uncrushed shells) points to an early origin of the co ncretions. Systematic geochemical variations from concretion centre to edge suggest that growth continued during shallow burial. The delta(13)C values (-17.5 to -20.5%) of the concretionary carbonate show that bacterial sulph ate-reduction processes were dominant. Sulphate-reduction-derived HCO3- was diluted by marine-related HCO3-, derived from dissolved bioclasts. A sligh t enrichment in delta(13)C during growth is caused by the decreasing influe nce of sulphate reduction because of the progressive closure of the diagene tic system due to shallow-burial compaction. The delta(18)O values (-0.5 to +1.0%) of the concretionary carbonate point to a marine origin. The slight ly O-18-depleted signature with respect to time-equivalent marine-derived c arbonate relates to the incorporation of an O-18-depleted component, origin ating from sulphate and organic matter. The slight decrease in delta(18)O d uring growth relates to an increasing influence of this component and to a decreasing influence of seawater-derived oxygen during early diagenesis.