Marine low-magnesium calcite concretions are widespread in many siliciclastic and mixed carbonate–siliciclastic shelf and basinal settings. The process of concretion formation is generally well established and involves microbial influence (mostly sulfate reduction to oxidize organic material at or just below the seafloor). The microbes produce interstitial fluids that are conducive to abundant, and apparently rapid, precipitation of calcite cements. Pervasive cementation generates well-indurated beds or isolated flattened “pods” that are commonly confined to specific stratigraphic horizons. Stratabound concretions can be important as fluid-flow barriers during subsequent burial and compaction. Thin-section and scanning electron microscopy of Cenozoic and Mesozoic concretions has...