Petrographic, elemental, and isotope data suggest that siderite in the Ivishalk Sandstone is not entirely an early diagenetic phase. Previous workers have used siderite cement in the Triassic Ivishak Sandstone as an example of early diagenetic siderite from a fresh-water depositional environment. Abundant and ubiquitous siderite cement includes: porefilling, siderite replacement of framework chert grains, and wheat seed morphologies that include a later diagenetic phase. Petrographic data show that some siderite cement follows compaction. Burial history curves indicate that the Ivishalk was not buried to more than 2,000 ft. before it was uplifted and exposed in the Cretaceous. It was subsequently buried to its present burial depths of 8,000-1 0,000 ft. A siderite phase following compaction is probably post-Triassic in age. Elemental and isotopic analyses indicate distinct generations of siderite cement. Siderite compositions fall into a non-marine field with high Fe and low Mg and Ca concentrations. Zoned siderite shows lower Fe concentrations. Siderite cement in mudstones contain higher Fe concentrations than in sandstones and conglomerates. The isotopic compositions of siderite varies widely with [delta][sup 13]C and [delta][sup 18]O of -1 3.7 to +1 7.4 PDB and -8.8 to 0.0 PDB, respectively. The [delta][sup 13]C and [delta][sup 18]O of sideritemore » is more enriched in the mudstones (0.0 to +1 7.4 and -5.0 to 0.0, respectively) than in the sand- stones and conglomerates (-1 3.7 to -1.6 and -8.8 to -5.0, respectively). Siderite in mudstones document precipitation under methanogenic conditions, whereas siderite in sandstones and conglomerates record precipitation in the suboxic environment.« less