Carbonate concretions in the Oligocene Boom Clay in Northern Belgium grew as a result of bacterially mediated geochemical processes. Concretion distribution patterns are controlled by glacioeustatic controls on water depth and related sediment detrital carbonate contents. Bacterial remains are present throughout these early diagenetic concretions and can be observed by SEM analysis of etched carbonates. The oxygen isotopic composition (delta 18 O approximately 0 per mil) of the calcite in the concretions indicates early diagenetic growth close to the sediment-water interface. Negative carbon-isotope values (delta 13 C -17 to -22 per mil) from concretion carbonates, as well as pyrite delta 34 S values of -15 to -45 per mil, suggest precipitation resulting from bacterial sulfate reduction during shallow burial diagenesis. Additionally, organic components in the concretions are characteristic of the remains of sulfate-reducing bacteria.