Early diagenetic siderite as an indicator of depositional environment in the Triassic Rewan Group, Southern Bowen Basin, eastern Australia

Early concretionary and non-concretionary siderites are common in subsurface Triassic sandstones and mudrocks of the Rewan Group, southern Bowen Basin. A detailed petrological and stable isotopic study was carried out on these siderites in order to provide information on the depositional environment of the host rocks. The siderites are extremely pure, containing 85-97 mol% FeCO3, and are commonly enriched in manganese. delta(13)C (PDB) values are highly variable, ranging from -18 . 4 to +2 . 9 parts per thousand, whereas delta(18)O (PDB) values are very consistent, ranging from -14 . 0 to -10 . 2 parts per thousand (mean= - 11 . 9 +/- 1 .parts per thousand). The elemental and oxygen isotopic composition of the siderites indicates that only meteoric porewaters were involved in siderite formation, implying that host rocks accumulated in totally non-marine environments. The carbon isotopic composition of the siderites is interpreted to reflect mixing of bicarbonate/carbon dioxide generated by methane oxidation and methanogenesis. Very low delta(13)C values demonstrate that, contrary to current views, highly C-13-depleted siderite can be produced at shallow burial depths in anoxic non-marine sediments.