Siderite in the Ivishak Sandstone, Prudhoe Bay, Alaska: Is It an Indicator of an Early Burial Environment?
N. T. Harun
Petrographic, elemental, and isotope data suggest that siderite in the Ivishak 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: pore-filling, 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 Ivishak 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 8,000-10,000 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 ^dgr13C and ^dgr18O of -13.7 to +17.4 PDB and -8.8 to 0.0 PDB, respectively. The ^dgr13C and ^dgr18O of siderite is more enriched in the mudstones (0.0 to +17.4 and -5.0 to 0.0, respectively) than in the sand- stones and conglomerates (-13.7 to -1.6 and -8.8 to -5.0, respectively). Siderite in mudstones document precipitation under methanogenic conditions, whereas s derite in sandstones and conglomerates record precipitation in the suboxic environment.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California