Siderite Textures in Cardium Formation, Ferrier Field, Alberta: ABSTRACT

Siderite in the Upper Cretaceous Cardium Formation of Alberta, Canada, is unusual not only because it is the dominant carbonate cement, but because it is more abundant in Cardium offshore sands and conglomerates than in similar Cretaceous stratigraphic and depositional units such as the Shannon, Sussex, and Gallup Sandstones or the Viking Formation. Core and thin sections have been studied to understand the occurrence of siderite in the Ferrier field (T38-42, R6-10W5). On the basis of texture, the siderite in the Cardium can be divided into two groups. Siderite associated with End_Page 714------------------------------ shales and bioturbated siltstones is characterized by a nodular habit, fine-grained spherulitic texture, and calcite-filled fractures, in contrast to the coarse lozenge-shaped or anhedral siderite that rims voids and replaces pebbles and sand in conglomerates and sandstones. Modular siderite is believed to form early in diagenesis, just below the water interface. Coated clay particles transport ferric oxides to the site of deposition, where they are precipitated in a colloidal gel; organic debris provides a source of carbonate ions and establishes reducing conditions. To insure low sulfide concentration, rapid sedimentation excludes marine sulphate ions which might otherwise be reduced by anaerobic bacteria to form HS- or H2S (the environment is abiotic). Based on its replacive nature, the coarse siderite is interpreted to be late diagenetic. The source for iron and carbonate ions may be linked to mudstone diagenesis, or to remobilization of early siderite. End_of_Article - Last_Page 715------------