ABSTRACT

Iron analyses of 47 samples of red and drab, very fine-grained sandstone, well-sorted siltstone, poorly sorted siltstone and silty claystone from the Red Peak and Crow Mountain members of the Chugwater (Triassic) Formation in Wyoming indicate that the iron oxide content varies with texture and with rock type. In both red and drab rocks the percent of total iron (Fe), Fe₂O₃, FeO and TiO₂ increases with decreasing gram size.

Iron in the Chugwater Formation occurs: 1. in grains of iron oxide minerals (hematite, ilmenite, magnetite, goethite); 2. in rare authigenic minerals (pyrite, siderite); 3. in grain coatings (hematite); 4. in hematite pigment on matrix material; and 5. in the clay minerals (illite, chlorite). Very minor amounts of iron may also be present in other minerals (dolomite?, etc.).

There is approximately a 1:1 increase in Fe₂O₃ as the total iron (Fe) content increases. The curve for the FeO:Fe ratio is more gentle than the Fe₂O₃:Fe ratio, and apparently the FeO:Fe curve is asymptotic (leveling at <1.90 percent FeO) for silty claystone. The slope of the curve for TiO₂:Fe is more gentle than the slope of the curve for FeO:Fe, but the general resemblance of the two curves suggests that TiO₂ may be mainly present as ilmenite. Seventy-two percent of the samples have approximately 70 percent or more of the total iron (Fe) as Fe³⁺.

Calculations of normative minerals indicate: 1. magnetite and ilmenite (wt. percent) increase with decreasing grain size; 2. leucoxene occurs principally in drab, very fine-grained sandstone; and 3. there is a large amount of hematite in red, poorly sorted siltstone and silty claystone compared with the amount of hematite in drab rocks of this grain size.

Studies of drab rocks suggest the following sequence of events. 1. The rocks were once uniformly red. 2. Hematite (Fe₂O₃) was reduced to FeO. 3. FeO was removed in a carbonated solution. Nearly equal amounts of FeO in red and drab pairs of the same rock type probably reflect the maximum Fe²⁺ saturation in the bleaching solution.

A post-depositional origin is proposed for the red pigment of the Red Peak and Crow Mountain Members. Formation and redistribution of ferric oxide probably occurred for the most part during diagenesis. The original material of the two members probably was deposited essentially as drab sediment.