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: [email protected]
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.
