AAPG Bulletin, V. 84, No. 9 (September 2000), P. 1281-1310.

Diagenetic Hematite and Manganese Oxides and Fault-Related Fluid Flow in Jurassic Sandstones, Southeastern Utah¹

Marjorie A. Chan, W. T. Parry, and J. R. Bowman²

©Copyright 2000. The American Association of Petroleum Geologists. All rights reserved.
¹Manuscript received April 16, 1999; revised manuscript received March 6, 2000; final acceptance March 15, 2000.
²Department of Geology and Geophysics, 135 S. 1460 E., University of Utah, Salt Lake City, UT 84112-0111; e-mail: machan@mines.utah.edu
We gratefully acknowledge the helpful input of AAPG reviewers Earle McBride, Peter Huntoon, and M. C. Erskine. Suggestions made by Craig B. Forster and D. Kip Solomon are gratefully acknowledged. We thank J. W. Valley and M. J. Spicuzza for the oxygen isotope analyses of hematite. This work was funded through a University of Utah Mineral Leasing Fund grant. Chan is grateful to Fran Barnes for sharing his ideas and for his expertise in locating field sites.

ABSTRACT

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 d¹⁸O (SMOW) and d¹³C (PDB) values of cements and veins range from 7 to 27o/oo and -12 to +5o/oo, respectively. The d⁸⁷Sr (SMOW) values of these cements and veins range from 0.210 to 2.977o/oo, 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.