Date of this Version
Burgess, D.T. 2014.Early and late iron diagensis in the upper Triassic Shinarump Member of the Chinle Formation (Utah and Arizona)(MS Thesis). University of Nebraska, Lincoln, NE
The fluvial Shinarump Member of the Chinle Formation in southwestern Utah and northwestern Arizona contains several distinct types of diagenetic iron accumulations. They range to more than 50 cm in diameter, and are dominantly composed of iron-oxide cement, but siderite, rhodochrosite and pyrite cements are common. Iron-oxide cements occur in all facies, but are most abundant in channel sandstone bodies. There, iron-oxide-cement occurs as wonderstone fabric (rinds & staining), dispersed rhombic pseudomorphs, and as discoidal concretions within intraformational conglomerates. Evidence from Shinarump sediment indicates that the ferrous carbonate mineral siderite (FeCO3) was the precursor mineral for current iron-oxide cements. Early diagenetic siderite typically forms in environments that are consistently water-logged, organic-rich, sulfate-poor, and methanic. Those environments are typical of freshwater swamps and bogs. It is difficult to find preserved siderite in outcrop because it quickly alters to iron-oxide in oxygenated pore-waters. Rhomb-shaped iron oxide pseudomorphs in sandstones and mm-scale spheroids defined by displaced silt grains within the iron-oxide rinds that surround intraformational clasts indicate pre-existing siderite. Discoidal septarian concretions occur in a thinly laminated mudstone facies above channel sands displaying complex fracture networks, iron-oxide cement, and ferrous iron-carbonate cements. Box-work concretions are cemented with iron oxide and developed thick rinds along NNW-SSE trending joints. Joint controlled precipitation of iron-oxide indicates early-diagenetic siderite was not oxidized until the onset of Basin and Range deformation (Miocene). Rinded clasts are iron-oxide cemented intraformational mudstone clasts that comprise dense iron-rich rinds surrounding iron-poor centers. Rinds developed after transport of sideritic mud clasts when siderite dissolved and diffusing ferrous iron was oxidized as rinds along fractures inside and along the perimeters of these clasts. Rinds surrounding fractures formed after Basin and Range faulting. Iron-oxide-cemented clasts are found in intraformational conglomerates. In thin-section, these pebbles resemble iron-cemented concretions and commonly have sharp edges, suggesting that they represent reworked Shinarump sediments. Box-works, rinded pebbles, cemented channel sands, and septarian concretions indicate that early diagenetic siderite is present.
Advisor: David B. Loope