Geochemistry of Early Carbonate Cements in the Cardium Formation, Central Alberta

Abstract

ABSTRACT Authigenic minerals and diagenetic textures in the marine conglomerates and sandstones of the Cretaceous Cardium Formation in the northeast Pembina area preserve a complex sequence of diagenetic events. Textural relationships observed in thin section and under the scanning electron microscope were used to determine the relative timing of diagenetic events in these marine sediments. Paragenetic sequences are similar for the conglomerate, sandstone, and siderite nodules in the enclosing shales. Extensive cementation by siderite and calcite occurred early in the diagenetic history, before any significant compaction. The geometry and distribution of lithofacies in the Cardium may have influenced the diagenesis and internal stratigraphy of the conglomerate. Overlying shale matrix conglomerate may have trapped upward-migrating fluids increased in buoyancy by dissolved CO2 produced by decay of organic matter in the enclosing shales. These fluids could have infiltrated the upper portion of open matrix conglomerate, causing cementation. Petrographic evidence shows alternating precipitation of siderite and pyrite, implying fluctuating activities of dissolved carbonate and hydrogen sulfide, probably caused by bacterially mediated processes during early diagenesis. Early sequences of siderite and pyrite precipitation can be related to zones of iron reduction, sulfate reduction, and, possibly, decarboxylation during diagenesis. Carbon and oxygen isotopic data indicate a systematic change in the isotopic compositions of calcite and siderite from -25 to -3013C and 018O, for cements early in the paragenetic sequence, to 0 to -513C and -15%18O for cements which are interpreted to occur later in the paragenetic sequence. This observation is consiste t with the influence of meteoric water on later stages of cement deposition in the Cardium, a unit considered to be marine and deposited well offshore on a shallow marine shelf. The intrusion of meteoric water far offshore may be related to changes in sea level. Many variables must be accounted for, but it is feasible, using the estimated regional dip of the Cardium at the time of deposition (0.005°), for a 1-m drop in sea level to push the freshwater--seawater interface seaward on the order of 100 km.