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A variety of diagenetic hematite and manganese oxide deposits occur within well-exposed Jurassic eolian and related deposits of southeastern Utah. Hematite concretions (millimeters to tens of meters in size) and strata-bound layers occur in the permeable Navajo, Page, and Entrada sandstones. Localized manganese oxide deposits without significant iron oxide occur in the overlying rocks covering the Summerville-Tidwell interval. Field, lab, and numerical modeling studies indicate the diagenetic deposits are related to the Moab fault. Fluid inclusion studies show salinities of fault fluids range from 0 to 19.7 NaCl equivalent weight percent. The δ18O (SMOW) and δ13C (PDB) values of cements and veins range from 7 to 27‰ and -12 to +5‰, respectively. The δ87Sr (SMOW) values of these cements and veins range from 0.210 to 2.977‰, values substantially more radiogenic than Pennsylvanian seawater. Saline brines formed from solution of Pennsylvanian salts by meteoric water and are interpreted to have flowed up the Moab fault and outward into adjacent permeable rocks. These brines are reducing from interaction with hydrocarbon, methane, organic acids, or hydrogen sulfide, and thus remove iron, manganese, and 87Sr, and bleach the sandstones near the fault. The isotopic evidence suggests multiple episodes of fluid flow up the Moab fault system. When saline, reduced brines mixed with shallow oxygenated groundwater, iron and manganese oxides were precipitated as cements to form concretions and tabular deposits in the porous sandstones. Multiple episodes of iron oxide mineralization and concretionary geometries are evident and can be explained as the result of permeability heterogeneities in the host rock, presence of favorable nucleii for precipitation, a self-organization process, or the influence of microbes.
This study emphasizes the nature of the reducing fluid that mobilized iron and its relation to the movement of saline or hydrocarbon fluids along the Moab fault system to precipitate hematite and manganese oxides as a result of oxidation of the fluid. The preservation of diagenetic concretionary iron and manganese oxides offers an excellent insight into permeability contrasts of sandstone units, an understanding of mixing fluid compositions, and solute transport along a major fault system.
Marjorie A. Chan is professor of geology at the University of Utah, where she joined the faculty in 1982. She received her B.S. degree from the University of California-Davis and her Ph.D. from the University of Wisconsin-Madison. Her current research focuses on sedimentology and stratigraphy in Precambrian through Pleistocene rocks of the Wasatch front in Utah and in Mesozoic deposits of the Colorado Plateau.William T. Parry is professor of geology and geophysics at the University of Utah. Former positions include associate professor of geosciences at Texas Tech University, Lubbock, Texas, and exploitation engineer for Shell Oil Company, Midland, Texas. He received B.S. and M.S degrees and a Ph.D. in geological engineering from the University of Utah. His research interests are geochemistry and mineralogy related to faults and ore deposits.
John Bowman received his B.S. degree from the College of William and Mary (1969), his M.S. degree from Ohio State University (1971), and his Ph.D. from the University of Michigan (1977). He has been a faculty member at the University of Utah since 1977, where he specializes in petrology and geochemistry. His research focuses on using stable isotopes to evaluate fluid flow and fluid-rock interaction in a variety of crustal hydrothermal environments, including groundwater systems.