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

Along the UT-AZ border, the Triassic Shinarump Member (Chinle Fm) contains distinctive iron-oxide concretions, bands, and pseudomorphs indicating siderite (FeCO₃) precipitated within fluvial sandstone and siltstone soon after deposition. Based on clay mineralogy, others have interpreted the Shinarump paleoclimate as wet. Because siderite develops in methanic, ever-wet sediments and soils (persistent, high water table), our observations support and strengthen this interpretation. Siderite can precipitate during late diagenesis, long after lithification (Jurassic Navajo Sandstone). “Rattlestones” (iron-oxide concretions with heavily rinded perimeters and copious void space in their cores) form intraformational lags at the bases of Shinarump fining-up sequences. These are similar to concretions described from Quaternary sediments in The Netherlands and in the Cretaceous Dakota Formation in eastern Nebraska and constitute clear evidence of an early origin for siderite within the Shinarump. Fractures within rattlestones have thick iron-oxide coatings. The orientations of these fractures coincide with trends of dominant joints (NNW), thereby indicating that oxidation of siderite (and precipitation of iron oxide on fractures inside rattlestones) took place post-lithification, during the late Cenozoic. Like the Dakota Fm, the Shinarump contains abundant rhombic, iron-oxide pseudomorphs after siderite, as well as prominent Liesegang banding. True Liesegang bands accompany thick (up to 2 cm) rinds containing dense iron-oxide cement. Rocks displaying complex patterns of iron-oxide cements are locally quarried as “Kanab Wonderstone”. Bands show two basic patterns: 1) centripetal (in which oxidation of siderite-rich zones began at joints and bedding planes and proceeded toward centers of isolated blocks); 2) centrifugal (in which oxidation of dispersed siderite began within restricted, oxygen-rich flow paths and expanded radially and down-gradient).

Early diagenetic siderite and pedogenic calcite should be mutually exclusive because they form within two distinct climatic regimes (Ludvigson et al., in press). Future work should include search for clues of early siderite precipitation above the Shinarump, in stratigraphically higher portions of the Chinle.