ENVIRONMENTAL IMPLICATIONS OF EARLY DIAGENETIC SIDERITE IN THE SHINARUMP MEMBER OF THE CHINLE FORMATION, CHOCOLATE CLIFFS, UTAH AND ARIZONA

Along the Utah-Arizona border, the Triassic Shinarump Member of the Chinle Formation contains distinctive iron-oxide concretions, bands, and pseudomorphs that indicate siderite (FeCO₃) precipitated within fluvial sandstones and siltstones soon after their deposition. Large (50cm diameter), discoidal concretions are present in laminated, floodplain mudstones. Although these concretions are dominantly cemented by iron oxide, thin sections contain up to 46% siderite. Rhombic, iron-oxide pseudomorphs after siderite cements are disseminated in the sandstone, and indicate that gaining streams received groundwater flow that infiltrated through adjacent, heavily vegetated wetlands. Iron-reducing microbes metabolized organic matter in the floodplain mud and used the ferric iron within grain coatings as their electron acceptor; the ferrous iron they released became the source of iron for siderite precipitation in both the mud and the channel sand.

Based on clay mineralogy, previous workers have interpreted the Shinarump paleoclimate as wet; our observations support and strengthen that interpretation. “Rattlestones” (iron-oxide concretions with heavily rinded perimeters and copious void space in their cores) are present in the Shinarump. These are similar to concretions described from Quaternary sediments in The Netherlands and in the Cretaceous Dakota Formation in eastern Nebraska and constitute evidence for an early origin for the siderite: erosion of floodplain sediments along stream banks released sideritic mud balls into the channel. Microbes that colonized redox boundaries along mud ball perimeters oxidized the siderite and generated the iron-oxide rinds. Fractures that intersect the rattlestones also have thick iron-oxide coatings. Oxidation post-dated these fractures, and the fractures coincide with the trends of the dominant joints in the western Colorado Plateau (NNW), thereby indicating that oxidation of the sideritic mud balls took place relatively recently, during the late Cenozoic. The Shinarump sandstones contain prominent Liesegang banding. True Liesegang bands accompany and alternate with thick (up to 2 cm) rinds containing dense iron oxide cement. These rinds are also microbial precipitates, and joints controlled their late Cenozoic oxidation.