Upper Cretaceous siderite concretions of Abu Tartur, Egypt: depositional environment and growth mechanism

Iron carbonate (siderite) occurs mainly as concretions with subordinate disseminated siderite rhombs. The siderite concretions are restricted to a single horizon, they generally represent the same genetic type. This horizon forms a characteristic features of the Late Cretaceous black shale—glauconite transition at Abu Tartur Plateau, Egypt. The concretions are mostly discoidal or ellipsoidal, rounded, oval and elongated and flattened in shape. They are composed mostly of siderite (up to 87%), with subordinate detrital sediments. The internal structure of the concretions is usually homogeneous. High organic matter content (up to 3.4%) and abundance of euhedral and framboidal pyrite spheres (up to 5%) in the hosting shale, suggest an euxinic depositional environment for these sediments. The concretionary horizon contains less contents of organic matter, organic detritus and pyrite. Subordinate disseminated euhedral siderite rhombs occur within both the black shale and glauconite hosts. The occurrence of euhedral siderite within both the black shale and glauconite suggests that the two siderite forms (concretions and disseminated rhombs) grew due to methanogenic conditions. These conditions developed within the sulfate reduction zone, which resulted in dissolved bicarbonate and Fe²⁺ in the pore water. These conditions were caused by a rapid consumption of sulfate via the severe oxidation of organic remains and formation of pyrite. This is followed by predominance of methanic conditions leading to deposition of the two forms of the studied siderite.