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Mozley, P.S., 1988

Petrography and diagenesis of the Sag River and Shublik Formations in the National Petroleum Reserve, Alaska, and topics in siderite geochemistry

Bibliographic Reference

Mozley, P.S., 1988, Petrography and diagenesis of the Sag River and Shublik Formations in the National Petroleum Reserve, Alaska, and topics in siderite geochemistry: University of California, Santa Barbara, Ph.D. dissertation, 252 p., illust.


The Sag River and Shublik Formations (Triassic) consist of organic-rich clastic and carbonate rocks that were deposited in a marine shelf environment. Early diagenetic alterations probably began near the sediment-water interface, and are consistent with those known to occur in the zones of sulfate reduction and methanogenesis. Late alterations may have been influenced by a high geothermal gradient and/or an incursion of meteoric water in the Early Cretaceous, and continued through deep burial and hydrocarbon generation in the basin. Carbonate cements (calcite, siderite, dolomite, and ankerite) formed throughout the diagenetic history, and record a number of significant changes in pore water chemistry through time. Diagenetic siderite commonly exhibits strong variation in fluorescence under exposure to ultraviolet (UV) light, but the controls on this fluorescence are poorly understood. Compositional analysis of siderites from different localities indicates that siderite fluorescence is related to the amount of CaCO3 present in the siderite lattice. Siderites containing more than around 10% CaCO3 are always fluorescent, whereas those containing less than 10% are usually non-fluorescent. Nevertheless, the possibility that the fluorescence is related to inclusions of fluorescent organic matter can not be ruled out. Examination of siderite concretions under exposure to UV light and in back-scattered electron imaging mode reveals complex micron-scale compositional zonation. The nature of this zonation indicates that concretion growth is a considerably more complex process than is generally believed. Failure to recognize such zonation may result in an erroneous interpretation of pore water chemical evolution during concretion growth. In general, early diagenetic marine siderite has a higher Mg/Ca ratio, and is considerably less pure than fresh-water siderite. Fresh-water siderite is relatively pure and often contains more Mn than marine siderite. These compositional differences are the result of differences in the activity of Fe++, Mg++, Ca++, and Mn++ in fresh and marine pore waters.

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