Controls on the Precipitation of Early Diagenetic Calcite, Dolomite and Siderite Concretions in Complex Depositional Sequences

abstract Carbonate concretions occur commonly in organic-rich clastic sediments of ancient coastal floodplains and deltas. They consist of calcite, dolomite, or siderite or combinations of these. The influence of marine, mixed or non-marine depositional waters is seen in early cements; solute sources from decomposition of unstable detrital minerals become more important in later diagenesis. The mineralogy and chemistry of early cements are dependent on dissolved sulphate concentrations and processes which affect them (notably depositional water composition, microbial sulphate reduction and mixing with waters of different origin). Calcite precipitates from marine-influenced pore waters rich in dissolved sulphate, dolomite from similar waters depleted in sulphate and siderite from meteorically-derived pore waters. Systematic compositional variations with depth of precipitation are preserved both within and between siderite concretions. Earliest cements have higher Mn/Fe, Fe/Mg and δ13C values: all decline in later cements. These trends are linked with solute supply for cations and microbial metabolism for bicarbonate.