Chemical analyses of acid-soluble material and XRD study of both concretionary and disseminated diagenetic carbonates in fine-grained sediments provide evidence of time and origin of growth.Siderite occurs both as segregations (nodules and continuous sheets) and as disseminated cement in freshwater/brackish mudstones. Substitution by Mg and Ca in siderite is very variable but a later generation of siderite cement with higher Mg/Ca ratio is identified in one mudstone and towards the margins of a sectioned nodule. A change in pore-fluid composition is implied which is probably related to diagenetic modification of silicates.“Bullion” concretions in carbonaceous marine shales began growth very early around centres of decaying organic matter. Either dolomite or calcite is interspersed in these complex zoned bodies with pyrite derived by bacterial reduction of seawater sulphate. Gypsum and in one concretion later dolomite formed by reaction of carbonate and pyrite in an oxidising groundwater regime.