PALEOENVIRONMENTAL IMPLICATIONS OF IRON DIAGENSIS IN THE TRIASSIC SHINARUMP MEMBER OF THE CHINLE FORMATION

The Shinarump Member of the Chinle Formation in southwestern Utah and northwestern Arizona contains several types of iron accumulations, ranging from 1 cm to 50 cm in diameter, and composed of iron oxide, siderite, and pyrite. Iron oxide is present in different depositional facies, but is most common in channel sandstone bodies as dispersed, rhombic pseudomorphs and as small, discoidal concretions within intraformational conglomerates. Large discoidal, septarian concretions occur in thinly laminated mudstone overbank facies. Several different lines of evidence from the concretions and surrounding sediments indicate that the ferrous carbonate mineral siderite was the precursor mineral for current iron oxide cements. Early diagenetic siderite characteristically forms in conditions that are consistently water-logged, organic-rich and methanic (like those found in present-day swamps and peat bogs). It is typically difficult to find preserved siderite because it quickly dissolves in oxygenated pore waters. We have found little preserved siderite, but physical evidence such as rhomb-shaped iron oxide pseudomorphs indicate its previous distribution. Accumulation of iron oxide in rinds around NNW-SSE trending joints indicate siderite was oxidized long after the Triassic and more likely during Basin and Range deformation (Miocene to Recent). Rattle stones are intraformational clasts that comprise iron-poor, mudstone centers surrounded by iron-oxide cemented rinds. Iron oxide pebbles, are also found in intraformational conglomerates. In thin-section, these pebbles resemble iron-cemented concretions and commonly have sharp edges, suggesting they reworked early oxidized concretions. Septarian concretions display varying fracture networks, iron oxide cement, and calcite fracture fills. Rattle stones, iron oxide pebbles, and septarian in the Shinarump provide valuable clues about over bank deposits and floodplain hydrology. All had precursors composed of early diagenetic siderite. Some were oxidized at shallow depth, during the Triassic (iron-oxide pebbles and septarian concretions), and others (rattle stones and dispersed cements were oxidized long after lithification).