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

The septarian carbonate concretions from the Boom Clay (Belgium) consist mainly of authigenic minerals such as micrite (≤ 70% bulk volume) and pyrite framboids (∼ 3%). These mineral phases occur between detrital grains and fossils. The septarian cracks are lined with calcite, 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 concretions. Systematic geochemical variations from concretion centre to edge suggest that growth continued during shallow burial. The δ¹³C values (-17.5 to -20.5%o) of the concretionary carbonate show that bacterial sulphate-reduction processes were dominant. Sulphate-reduction-derived HCO₃^- was diluted by marine-related HCO₃^-, derived from dissolved bioclasts. A slight enrichment in δ¹³C during growth is caused by the decreasing influence of sulphate reduction because of the progressive closure of the diagenetic system due to shallow-burial compaction. The δ¹⁸O values (-0.5 to +1.0%o) of the concretionary carbonate point to a marine origin. The slightly ¹⁸O-depleted signature with respect to time-equivalent marine-derived carbonate relates to the incorporation of an ¹⁸O-depleted component, originating from sulphate and organic matter. The slight decrease in δ¹⁸O during growth relates to an increasing influence of this component and to a decreasing influence of seawater-derived oxygen during early diagenesis.