Chemical bleaching indicates
episodes of fluid flow in
deformation bands in sandstone
W. T. Parry, Marjorie A. Chan, and Brenda Beitler
ABSTRACT
Jurassic sandstones on the Colorado Plateau have been variably
bleached through interaction with hydrocarbon-bearing solutions
or other reducing agents. Deformation bands in the Navajo Sandstone have a variety of colors in comparison with the host rock color
that indicate the timing of bleaching relative to deformation-band
formation. White deformation bands in red sandstone indicate that
deformation bands were likely permeable at an early dilatant stage in
their development history. Field characteristics, petrography, bulk
rock chemistry, clay mineralogy, and geochemical modeling show
that bleached deformation bands experienced an episode of chemical reduction where fluids removed some iron and left the remaining iron as pyrite and magnetite. Mass-balance calculations show
that as much as 10 kg of chemically reducing fluid per 100 g of rock
(1500 pore volumes of fluid) are necessary to remove 0.1 wt.% iron
from a deformation band. These large pore volumes suggest that
moving, reducing solutions regionally bleached the sandstone white,
and bleached deformation bands resulted where deformation bands
provided localized fluid access to unbleached, red sandstone during
an initial dilatant stage. Alternatively, access of reducing soil solutions may be provided by gravity-driven, unsaturated flow in arid to
semiarid vadose zones. Color and chemical composition is a valuable
index to the pathway and timing of hydrocarbon movement through
both host rocks and deformation bands.
INTRODUCTION
Flat-lying, red Mesozoic sedimentary beds of the Colorado Plateau
region of southern Utah are interrupted by Laramide (Late Cretaceous to middle Eocene) structures and by widespread bleaching
of previously red sandstone. The Laramide structures of the Utah
study area are the Kaibab, Circle Cliffs, Monument, and San Rafael
swell uplifts, together with the great monoclines on the eastern
Copyright #2004. The American Association of Petroleum Geologists. All rights reserved.
Manuscript received April 3, 2003; provisional acceptance July 7, 2003; revised manuscript received
September 8, 2003; final acceptance September 9, 2003.
AAPG Bulletin, v. 88, no. 2 (February 2004), pp. 175 – 191
175
AUTHORS
W. T. Parry Department of Geology and
Geophysics, 135 S. 1460 E. University of Utah,
Salt Lake City, Utah 84112-0111;
[email protected]
William T. Parry is professor emeritus 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 his B.S.
and M.S. degrees and his Ph.D. in geological
engineering from the University of Utah. His
research interests are geochemistry and mineralogy related to faults and ore deposits.
Marjorie A. Chan Department of Geology and Geophysics, 135 S. 1460 E. University
of Utah, Salt Lake City, Utah 84112-0111
Marjorie A. Chan is professor of geology at the
University of Utah, where she joined the faculty
in 1982 and currently serves as department
chair. 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 Mesozoic deposits of the
Colorado Plateau.
Brenda Beitler Department of Geology
and Geophysics, 135 S. 1460 E. University of
Utah, Salt Lake City, Utah 84112-0111
Brenda Beitler is currently a Ph.D. candidate in
the Department of Geology and Geophysics
at the University of Utah. She earned a B.S.
(1998) degree and an M.S. (2000) degree in
earth sciences from the University of California,
Santa Cruz. Her thesis research focuses on color
variations and iron mineralization resulting
from micro- to regional-scale fluid flow in the
Navajo Sandstone.
ACKNOWLEDGEMENTS
The Petroleum Research Fund of the American
Chemical Society provided financial support
for this study. We gratefully acknowledge the
United States National Park Service for permission to conduct scientific research under Permit
Number CARE-2001-SCI-0002. Dennis Netoff
assisted with deformation band localities.
margins of the uplifts: the East Kaibab, Waterpocket,
Comb Ridge, and San Rafael Reef (Figure 1A). Deformation bands are small-scale structural features, which
occur in many sandstone petroleum reservoirs and are
important in evaluating tectonics and kinematics of
these Laramide structures.
The purpose of this paper is, first, to describe the
field evidence for fluid flow along the seemingly lowpermeability deformation bands; second, to estimate
the quantity of fluid responsible for the color changes;
and third, to determine the timing of fluid flow with
respect to deformation-band formation. This analysis
of fluid flow associated with deformation bands has
valuable applications to understanding petrophysical
controls on fault-zone deformation and hydrocarbon
migration associated with tectonic activity.
Stratigraphy
The Jurassic system on the Colorado Plateau in southern Utah contains three prominent and well-exposed
eolian units. The oldest is the fine-grained Wingate
Sandstone (Figure 2) (Blakey et al., 1988; Blakey,
1994). Above the Wingate Sandstone, the Lower Jurassic Navajo Sandstone and its related equivalents, the
Nugget Sandstone of northern Utah and Wyoming and
the Aztec Sandstone of Nevada, form the largest eolian
dune sea deposit in North America (Blakey et al., 1988;
Peterson and Turner-Peterson, 1989; Blakey, 1994; Peterson, 1994). In the study area, the Navajo Sandstone
(Figure 2) is a well-sorted, fine-grained quartz arenite,
dominated by large-scale eolian cross-stratification (as
much as a few tens of meters). The Navajo Sandstone
is an aquifer throughout regions of the Colorado Plateau. The homogeneity of the well-sorted sandstone and
its preserved porosity and permeability likely allowed
large volumes of fluids to flush through the unit (Hood
and Patterson, 1984; Howells 1990; Spangler et al.,
1996). The Middle Jurassic Entrada Sandstone (Figure
2) is the third, fine-grained sandstone, dune-sea deposit
(Kocurek, 1981; Kocurek and Dott, 1983).
Several thin-bedded, siltstone-dominated units are
relatively low-permeability aquitards that separate the
higher permeability eolian units.
Structure
Laramide structures of the Colorado Plateau include
the East Kaibab, Waterpocket, San Rafael, and Comb
176
Ridge monoclines (Figure 1A). These large and long
uninterrupted folds described by Davis (1999) link to
regional faults at depth. The Waterpocket fold, which
is 140 km (85 mi) long, comprises the east limb of the
Circle Cliffs uplift and is 8 km (5 mi) east of the crest
of the uplift. Structural relief on the Waterpocket fold
is 1666 m (5500 ft). The monoclinal fold strikes 320j
at right angles to the east-west direction of Laramide
compression, and strata dip 15 to 80j east-northeast.
The Navajo Sandstone here contains abundant cataclastic deformation bands along the exposures of the
Waterpocket fold.
Sandstone Bleaching
Modern dune deposits are known to redden soon after
deposition and during early diagenesis as iron is released from detrital minerals and oxidized to form
hematite coatings and cements soon after deposition
and during early diagenesis (Walker, 1975; Walker
et al., 1978, 1981; Turner, 1980). The Jurassic, eolian
sandstones were likely ubiquitously red soon after deposition in oxidizing environments and early burial.
However, large areas of Navajo, Entrada, and Wingate
sandstones show extensive bleaching because of reduction and removal of iron (Beitler et al., 2003, in press).
The sandstones are now bleached from inferred contact
with hydrocarbons or other chemical reducing agents.
Where bleached (chemically reduced) sandstone transitions to red (oxidized) sandstone, the oxidation-reduction fronts commonly cut across stratigraphic elements
(Surdam et al., 1993). The most extensive bleaching is
associated with the Laramide age structural uplifts and
monoclines (Beitler et al., 2003, in press).
Geologists have long noted that red sandstones
invaded by hydrocarbons are bleached (Moulton, 1926;
Todd, 1963; Levandowski et al., 1973; Dixon et al.,
1989; Surdam et al., 1993). The bleaching patterns
have suggested the paths of fluid flow and hydrocarbon
migration (Garden et al., 2001). Bleached selvages on
bitumen veins and bleached Navajo Sandstone along
the Moab fault are caused by reducing fluids (e.g.,
Foxford et al. 1996; Chan et al., 2000). In the normally
red Entrada Sandstone, a localized (near CH in Figure
1A), bleached, gray, 9-m-thick tar-saturated sandstone
is flanked both above and below by approximately 5
and 12 m, respectively, of bleached yellowish orange
to grayish orange sandstones (Chan et al., 2000). Permian White Rim Sandstone was bleached by invasion
of oil (Huntoon et al., 1999). The Nugget Sandstone,
Chemical Bleaching Indicates Episodes of Fluid Flow in Deformation Bands
Figure 1. (A) Index map of
Utah showing Laramide uplifts
indicated with structural contours (in thousands of feet) on
the Kaibab (Permian) and location of photographs and samples in the study area. SR =
Sooner Rock, VF = Valley of Fire,
CH = Court House Rock, CR =
Capital Reef, BF = Bullfrog
Marina. Laramide uplifts and
monoclines: S = San Rafael swell
and San Rafael monocline, M =
Monument uplift and Comb
Ridge monocline, CR = Circle
Cliffs uplift and Waterpocket
monocline, K = Kaibab uplift and
East Kaibab monocline. Structural contour interval is 2000 ft
(610 m) (modified from Hunt,
1956). (B) Detailed map
of individual deformation-band
sample locations in the Waterpocket fold area. Topographic
contour interval 250 ft (76 m).
Parry et al.
177
Figure 2. Stratigraphy in the southern
Utah study area modified from Hintze
(1988). Three major eolian units (bold)
are bleached and contain deformation
bands. The Chinle and Moenkopi that
are both aquitards underlie the Wingate
Sandstone. Major Triassic unconformities
(Tr-1 and Tr-2) and Jurassic unconformities (J-0 through J-5) are indicated (Pipiringos, and O’Sullivan, 1978).
a Navajo equivalent, is bleached to white and pink in
the Painter oil field in the Wyoming thrust belt (Lamb,
1980). The brilliant red sandstone, sandy shale, and
shale of the Triassic Chugwater of Wyoming and southern Montana is bleached along crests of minor folds
and on flanks of the main mountain uplift, where
bleaching is associated with oil saturation (Moulton,
1926). The Permian Lyons Sandstone in the Denver
basin, Colorado, is gray when associated with producing oil fields and red in nonpetroliferous areas (Levandowski et al., 1973).
Bleaching by petroleum is also demonstrated in
laboratory pyrolysis of water-rock-oil mixtures of Shebl
and Surdam (1996). The color changed in the hematitic
Pennsylvanian Tensleep Sandstone dune facies from
original bright or dark red to light pink, white, gray, or
dark gray. However, although empirical, experimental, and theoretical evidence shows that hydrocarbon
bleaches red sandstone, other bleaching agents are
possible, such as hydrogen sulfide, organic acids, and
methane.
178
Deformation Bands
Deformation bands are abundant tectonic structures
in Mesozoic sandstone units of the western United
States. Aydin et al. (in press) summarize deformationband kinematics and micromechanics, and Davis (1999)
presents a comprehensive treatment of kinematics,
micromechanics, and distribution of deformation bands
on the Colorado Plateau. Deformation of the sandstone begins with single deformation bands that can
coalesce into a zone of deformation bands, and then, a
slip plane or fault forms (Antonellini and Aydin, 1994;
Roznovsky and Aydin, 2001). A deformation band is a
thin (0.5 – 2-mm [0.02 –0.08-in.]-thick), tabular structure commonly with large lateral continuity compared
to thickness. Deformation bands form as a response to
localization of strain into narrow, tabular bands. Two
types of deformation bands are recognized: (1) sheardeformation bands (the focus of this paper) and (2)
bands in which volumetric deformation (either compaction or dilatancy) predominates (Aydin et al., in press).
Chemical Bleaching Indicates Episodes of Fluid Flow in Deformation Bands
Macroscopic shear offset of a few millimeters to a few
centimeters is diagnostic of shear bands. Grain fracturing, crushing, grain-size reduction, and comminution are the micromechanical processes that form shear
bands. Grain-size distribution is broadened, and sorting
is decreased with shearing. Shear bands associated with
grain fracturing and grain-size reduction are called
cataclastic deformation bands (Davis, 1999). Cataclasis, grain crushing, collapse of pore space, and reduction of mean pore size are highly localized (Antonellini
et al., 1994; Antonellini and Aydin, 1994; Aydin et al.,
in press). Positive dilatancy accompanies the initial stage
of formation of cataclastic deformation bands, but continued deformation soon results in volume loss (Davis,
1999).
Cataclastic deformation bands have been shown
to decrease permeability in eolian sandstones. Laboratory measurements, numerical simulations, and field
observations consistently indicate that cataclastic deformation-band porosity and permeability evolve from
initial elastic compaction to a dilatant stage, then to
grain crushing and compaction (Antonellini et al.,
1994; Antonellini and Pollard, 1995; Davis, 1999; Main
et al., 2000, 2001; Mair et al., 2000; Du Bernard et al.,
2002a). The dilatant stage increases porosity and permeability. The initial, dilatant stage of deformationband development is preserved in the tip region of
some deformation bands that show an increase in porosity from 10% in the host rock to 19% in the tip region
(Antonellini et al., 1994). The latter stage of grain
crushing and compaction reduces porosity to one order
of magnitude less than host rock and reduces permeability to three orders of magnitude less than the host
(Antonellini et al., 1994; Antonellini and Aydin, 1994;
Matthai et al., 1998; Taylor and Pollard, 2000; Shipton
et al., 2002). The intensity of cataclasis and the clay
content control the amount of permeability reduction
(Antonellini and Aydin, 1994). Low initial porosity and
low confining pressure promote the formation of dilatant bands with no cataclasis and, hence, relatively
higher permeability (Du Bernard et al., 2002b).
Deformation processes that form cataclastic deformation bands sharply reduce porosity and permeability and form barriers to saturated flow (except in
initial, dilatant stages of formation), but changes in
pore-size distribution should enhance unsaturated
flow relative to host sandstone (Sigda et al., 1999;
Sigda and Wilson, 2003). Laboratory measurements of
Sigda and Wilson (2003) show that saturated hydraulic
conductivity is three orders of magnitude less in
cataclastic deformation bands than in undeformed
sand. However, for unsaturated flow similar to arid
and semiarid vadose zones, deformation-band hydraulic conductivity may be greater by two to six orders of
magnitude. Under gravity-driven flow conditions, moisture and solute transport may be two to six orders of
magnitude larger in a deformation band than in host
sandstone. Deformation bands act as capillary barriers
to nonwetting fluids such as oil and capillary conduits to
wetting aqueous fluids (Sigda et al., 1999).
METHODS
Chemical changes in the deformation bands are evaluated using petrologic examination, whole rock chemistry, clay mineralogy, calculated modal mineralogy,
and geochemical modeling. Samples of deformation
bands and their host rock Navajo Sandstone were collected in outcrop in the Waterpocket fold (locations
in Figure 1A, B). Samples of host rock and deformation band were submitted to a commercial laboratory
for whole-rock analyses by a combination of analytical
techniques. Clay minerals were extracted from 10-g
samples of host rock and deformation band by mild
hand grinding in a mortar and then peptizing in water.
The less-than-2-mm size fraction was separated by centrifuging. A centrifuged slurry was smeared on glass
slides for x-ray diffraction analyses. Thin sections were
stained for K-feldspar and examined in the petrographic microscope, and polished sections were examined
under reflected light. Modal mineralogical composition of host rock and deformation-band samples was
calculated using a linear, least-squares fit of whole-rock
chemistry, mineral composition, and mineral mass abundance described by Parry et al. (1980). Geochemical
phase diagrams and reaction paths were calculated using
the Geochemist’s Workbench (Bethke, 1998).
DEFORMATION BANDS IN OUTCROP
Deformation bands exhibit a variety of colors in comparison with their host sandstone. Each of the six color
combinations is significant in terms of interaction of
fluid with the deformation band. Red deformation
bands in normally colored red sandstone (Figure 3A)
suggest that bleaching solutions were not present. Red
deformation bands in bleached sandstone (Figure 3B)
could result from either formation of the deformation
Parry et al.
179
Figure 3. Deformation bands that occur in Jurassic sandstone outcrops from localities in Utah and southeastern Nevada. The color
of the deformation bands indicates the nature of solution interaction. (A) Red deformation band in red Entrada Sandstone at Sooner
Rock, Grand Staircase-Escalante National Monument (west of the Circle Cliffs). Bleaching solutions were probably not present. (B)
Red deformation bands in bleached Aztec Sandstone at Valley of Fire State Park, southeastern Nevada. The deformation band formed
before bleaching and remained impermeable to bleaching solutions, or the band was colored by secondary mineralization. (C) White
deformation bands in bleached Entrada Sandstone at Court House Rock, Moab, Utah. The bands formed after the sandstone was
bleached. (D) Bleached Aztec Sandstone with some white deformation bands and some red deformation bands, Valley of Fire State
Park, Nevada. (E) Bleaching along a deformation band in Entrada Sandstone, Dance Hall Rock, Utah. Bleaching solutions entered the
deformation band after formation. (F) Assemblage of bleached deformation bands in Entrada Sandstone, near Bullfrog Marina, Utah.
Bleaching solutions entered the deformation bands after formation.
180
Chemical Bleaching Indicates Episodes of Fluid Flow in Deformation Bands
band before bleaching, to retain the original red color
and remain impermeable to bleaching solutions, or
formation of the deformation band, followed by secondary mineralization by iron-coloring minerals. White
deformation bands also occur in bleached sandstone,
suggesting that the deformation bands formed after
the sandstone was bleached (Figure 3C). White deformation bands in red host sandstone indicate iron
depletion or chemical reduction of the iron from fluid
interaction in the deformation band (Figures 3E, F; 4).
Bleaching is also observed on the selvages of white
deformation bands and at deformation-band intersections, implying fluid movement along the selvages
and intersections. Concretions of iron and carbonate
minerals that formed preferentially along deformation
bands also suggest fluid interaction in a deformation
band.
The lighter color of bleached deformation bands has
been ascribed to comminution of the sandstone coated
with hematite to expose more white quartz surfaces.
However, in our experiments, red sandstones ground
in a motorized laboratory mortar and pestle to less
than 10 mm retains much of its color. Only modest
changes in hue, lightness, and saturation were observed.
For example, Figure 5 shows that moderate reddish
brown to dark yellowish orange host rocks become light
brown to moderate yellowish brown when ground. In
contrast, deformation band color is typically changed
from reddish brown to very light gray (Figure 4). Thus,
the lighter, neutral color of the deformation bands is
more likely caused by reduction and/or removal of iron
and not merely a function of grain comminution.
The focus of this mineralogical and geochemical
study is white deformation bands in red Jurassic Navajo
Sandstone from the southern part of the Waterpocket
fold illustrated by seven sandstone-deformation-band
pairs (Figure 4). The samples were collected from the
Surprise Canyon to Headquarters Canyon area of Capitol Reef National Park (Figure 1B). All seven of the
deformation bands are shear bands of the cataclastic
type as defined by Davis (1999). Shear offset of sample
DB-8 is indicated by subhorizontal slickensides, and
shear offset of sample DB-6 is shown in Figure 4D by
offset of bleached eolian foreset beds shown as white
lines. Strike and dip of deformation bands are DB-2,
48j, 50jS, DB-4, 100j, 60jS, DB-5, 80j, 90jS, DB-7,
40j, 50jS, and DB-8, 60j, 60jS. Orientations of the
deformations bands are consistent with extension accompanying Laramide regional shortening during formation of the Waterpocket monocline as described by
Davis (1999).
PETROGRAPHY
The Navajo host sandstone ranges in color from dark
reddish brown and pale reddish brown to pale red
(Figure 4; Table 1). The deformation bands are very
light gray, grayish pink to white. Host rock near the
deformation bands is bleached in a thin selvage (Figure
6A). The Jurassic Navajo Sandstone, host rock for the
deformation bands, is a fine-grained, well-sorted, wellrounded arenite. Most of the sand grains are quartz, but
K-feldspar grains (3.17 – 7.50%) are scattered throughout. The sand grains are mostly 0.1 – 0.25 mm (0.004 –
0.010 in.) in size and are commonly coated with 2 –
4-mm-thick illite and hematite (Figure 6B). Diagenetic
illite also occasionally fills pore space, and diagenetic
kaolinite commonly occurs as pore fillings (Figure 6B).
Calcite cement is sporadic.
Deformation bands included in this study are all
the cataclastic type, with broken, angular quartz clasts
surrounded by fine-grained (<4 mm) quartz matrix
(Figure 6C). A few of the larger quartz clasts have
preserved, discontinuous illite coatings that were present in the host rock. Kaolinite cannot be recognized
petrographically in the fine-grained matrix. K-feldspar
forms ribbons and irregular, elongate masses that
occur as shadows of larger quartz clasts (Figure 6D).
K-feldspar rims on quartz grains are uncommon.
K-feldspar also fills pore spaces in the deformationband selvage. Calcite occurs as finely disseminated,
veinlike concentrations in the cataclasite. The calcite
appears to crosscut the finely comminuted quartz in
the cataclasite.
Iron has been redistributed in the deformation
bands. Most of the deformation bands contain less iron
than the host rock, but even in the most iron-depleted
deformation band, some iron remains. Some iron in the
deformation bands is in the form of pyrite, which is
not present in the host rock and hematite. In a few
cases, iron oxides occur in individual quartz grains,
where the iron minerals are protected from solution
activity. The iron coloration of the host rock occurs
primarily as hematite coatings along with illite on the
sand grains. The hematite in a deformation band does
not coat the crushed sand grains, but instead occurs as
discreet masses of hematite, possibly a result of oxidation of magnetite or pyrite.
In reflected light, larger masses of hematite appear
to fill pore spaces in the host rock in addition to the
hematite coloration coating the sand grains. Pyrite is
observed in five of the seven deformation bands studied
as irregular- to triangular-shaped grains (Figure 6E).
Parry et al.
181
182
Chemical Bleaching Indicates Episodes of Fluid Flow in Deformation Bands
Figure 4. Shear deformation bands of the cataclastic type in red Navajo Sandstone, Capitol Reef National Park, Utah. Each band is bleached because of chemical reduction and
removal of iron by bleaching solutions following formation of the deformation band. (A) White cataclastic deformation band in red Navajo Sandstone, sample DB-7. (B) White
cataclastic deformation bands in red Navajo Sandstone, sample DB-8D. (C) White cataclastic deformation band in red Navajo Sandstone, sample DB-2. (D) White cataclastic
deformation band in red Navajo Sandstone, sample DB-6. Bleached eolian foreset beds are offset on the deformation band.
Figure 5. Red sandstone natural color
shown on the left compared with the
color that results from grinding shown on
the right. Color designations are based on
the Munsell Rock Color Chart.
Hematite is present in the deformation bands as 40-mm
grains (Figure 6F).
CLAY MINERALOGY OF DEFORMATION
BANDS AND HOST ROCK
Host rock and deformation bands contain illite and
kaolinite. Most illite in the host rock occurs as coatings along with hematite on the sand grains, but some
illite occurs in pore spaces. Kaolinite in the host rock
occurs as a pore filling. In the deformation bands, illite
coatings on the sand grains are discontinuous and are
commonly almost completely removed. X-ray diffraction analysis of the clay-size separates from host and
deformation bands are shown in Figure 7. Illite and
kaolinite x-ray peaks are much less intense in deformation bands, and the ratio of illite to kaolinite intensities are altered. The decrease in x-ray intensity in
deformation-band clays is caused partly by dilution of
the clay minerals, with less than 2-mm quartz shown
in each deformation-band diffraction pattern (Figure 7).
Comparison of clay mineralogy of deformation
bands with the mineralogy of host rocks shows that
the deformation bands contain less kaolinite and illite,
and the illite/kaolinite ratio is different. For example,
kaolinite 001 x-ray diffraction peaks in samples DB-2,
DB-5, and DB-8 are more intense than illite 001 in the
host rock and less intense than illite 001 in the deformation band (Figure 7). Host rock in sample DB-3
contains a small amount of smectite clay that is not
present in the deformation band.
CHEMICAL COMPOSITION OF DEFORMATION
BANDS AND HOST ROCK
Chemical analyses verify that the deformation bands
are significantly different in composition from their
host rocks (Table 1). Deformation bands 2, 5, 6, 7, and
Parry et al.
183
184
Chemical Bleaching Indicates Episodes of Fluid Flow in Deformation Bands
Table 1. Chemical Composition and Modal Mineralogy*
Sample Number (wt.%)
2A
2B
3A
3B
5A
5B
6A
6B
7A
7B
8A
SiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
TiO2
P2O5
LOI
Total
Color (Munsell)y
92.53
3.01
1.32
0.006
0.14
0.06
0.03
1.35
0.094
0.03
0.40
98.98
10R5/4
94.02
2.36
1.07
0.006
0.14
0.12
0.04
1.39
0.096
0.02
0.37
99.62
5R8/2
95.41
2.16
0.96
0.006
0.11
0.07
0.02
1.06
0.055
0.03
0.31
100.18
10R5/4
95.79
1.87
1.02
0.006
0.11
0.13
nd**
1.01
0.050
0.02
0.27
100.29
N9
95.05
2.04
1.15
0.006
0.10
0.20
0.02
1.04
0.058
0.02
0.18
99.87
10R5/4
95.93
1.71
0.65
0.003
0.07
0.19
nd**
0.99
0.047
0.03
0.26
99.88
N9
94.56
2.10
1.05
0.011
0.16
0.47
0.02
1.05
0.064
0.03
0.63
100.14
10R5/4
95.66
1.70
0.98
0.005
0.10
0.14
0.02
1.02
0.049
0.03
0.21
99.92
N9
95.48
2.10
0.83
0.003
0.11
0.04
0.02
1.00
0.043
0.02
0.39
100.03
10R5/4
95.06
1.70
0.62
0.005
0.08
0.12
0.03
0.94
0.033
0.02
0.27
98.88
N8
96.67
1.37
0.68
0.008
0.05
0.02
0.03
0.68
0.026
0.02
0.16
99.72
10R6/2
Modal Mineralogy (wt.%)
Quartz
Hematite
K-feldspar
Calcite
Illite
Kaolinite
Total
Iron Depletion
87.15
1.33
5.81
0.11
4.09
1.26
99.76
90.42
0.96
4.60
0.12
3.06
0.71
99.87
91.04
1.02
4.78
0.23
2.35
0.42
99.84
+ 0.09
90.60
1.15
4.75
0.36
2.42
0.66
99.94
88.17
1.07
7.82
0.21
1.90
0.36
99.53
0.22
91.74
0.65
4.92
0.34
2.01
0.26
99.92
0.48
89.45
1.05
5.19
0.84
2.64
0.62
99.78
91.39
0.98
5.12
0.25
1.75
0.40
99.89
0.04
*A samples = enclosing red Jurassic Navajo Sandstone; B samples, italicized = white deformation bands.
**nd = not detected.
y
Color: 10R5/4 = pale reddish brown, 10R6/2 = pale red, 10R3/4 = dark reddish brown, 5R8/2 = grayish pink, N9 = white, N8 = very light gray.
90.78
0.83
4.51
0.07
2.90
0.67
99.76
91.94
0.63
4.42
0.22
2.09
0.62
99.92
0.19
93.89
0.68
3.17
0.04
1.55
0.58
99.90
8B
8D-A
8D-B
96.07
93.34
93.76
1.31
3.18
2.90
0.70
0.36
0.25
0.005
0.007
0.004
0.12
0.15
0.14
0.07
0.38
0.37
0.03
0.09
0.10
0.81
1.65
1.61
0.029
0.054
0.057
0.02
0.03
0.03
0.20
1.18
1.05
99.37 100.41 100.27
N9
10R3/4
N8
93.16
0.70
4.21
0.13
1.30
0.26
99.77
0.04
85.51
0.35
7.50
0.67
4.25
0.93
99.21
85.99
0.25
8.58
0.66
3.02
0.70
99.20
0.10
Figure 6. Photomicrographs of sandstone and deformation bands. (A) Polished plug of deformation band in sample DB-3 showing
bleached selvage. (B) Illite coatings (I) on quartz grains (Q), kaolinite (K) pore filling, Navajo Sandstone host to deformation bands,
sample DB-2. (C) Cataclasite in shear deformation-band sample DB-6 typical of all seven sampled deformation bands. (D) K-feldspar
stained yellow (Kf) in deformation band cataclasite, plane light sample DB-3. K-feldspar textures indicate formation from solution in the
deformation band. (E) Pyrite (Py) grain in deformation band cataclasite. Pyrite formed from chemical reduction of iron and sulfur.
Sample DB-8. (F) Hematite (H) in deformation band cataclasite sample DB-2. The hematite is secondary and may be the result of
oxidation of magnetite.
8D are lower in Fe2O3 relative to host rock, but deformation bands 3 and 8 contain more Fe2O3 than host
rock. The average iron depletion in the seven deformation bands studied is 0.15% Fe2O3. Significantly,
some iron remains in all of the bleached deformation
bands. Deformation bands 2, 3, 5, 6, and 8D contain
more SiO2 than corresponding host, but samples 7 and
8 are lower in SiO2. All of the deformation bands
contain less Al2O3 relative to host rock. Deformation
bands 2, 3, 7, and 8 contain more CaO than host rock.
Parry et al.
185
Samples 5 and 8D are similar to host rock CaO, and
sample 6 shows a decrease in CaO.
MODAL MINERALOGY
Calculated mineral abundances in host rock and deformation bands (Table 1) show that more than 99%
of the rock is quartz, K-feldspar, calcite, illite, kaolinite, and hematite. Trace minerals containing TiO2 and
P2O5 were not included in the calculation of mineral abundances. Quartz, the most abundant mineral,
comprises 85.5 – 93.9% of the host sandstone and 86 –
93.2% of the deformation bands. Significant variations between deformation band and host are present
in abundances of hematite, K-feldspar, calcite, illite, and
kaolinite. Host rocks contain 3.2 – 7.5% K-feldspar, and
deformation bands contain 4.2 –8.6% K-feldspar. All
deformation-band samples contain more K-feldspar than
the host rock except sample DB 6. Illite abundance in
the host rocks ranges from 1.6 to 4.3%, and deformation bands contain from 1.3 to 3.0% illite. Host rocks
contain 0.58 – 1.3% kaolinite compared to deformation
bands where kaolinite ranges from 0.26 to 0.70%. All
deformation-band samples contain less illite and kaolinite than corresponding host rocks. Calcite in the host
rock ranges from 0.07 to 0.84%, and deformation bands
contain 0.13 – 0.66% calcite.
Calculated modal mineralogy shows that the abundances of the clays are decreased in the deformation
bands consistent with the decreased x-ray intensities
(Figure 7). Some of the difference could be the result
of introduction and precipitation of quartz in the deformation bands. The deformation bands contain nearly 1 wt.% more quartz than their corresponding host
rocks (Table 1). Deformation bands contain an average of (in wt.%) 90.5% quartz, 0.76% Fe2O3, 5.69%
K-feldspar, 0.29% calcite, 2.09% illite (0.005 mol/100 g),
and 0.43% kaolinite.
GEOCHEMICAL MODELING
Figure 7. X-ray diffraction patterns of clay minerals in deformation bands and red sandstone host. The deformation bands
contain less kaolinite and illite than the corresponding host.
186
Iron occurs in minerals and fluids as Fe2 + and Fe3 + .
The mobility of iron in solutions is dependent on reduction to Fe2 + , pH, and complexing ligands such
as SO42 or Cl . Iron is immobile as Fe3 + at ordinary temperatures and geologically reasonable values
of pH (4 –8). Calculated iron concentrations in oxidizing solutions are typically less than a small fraction
of 1 ppm. Iron in red sediment must be reduced to
Chemical Bleaching Indicates Episodes of Fluid Flow in Deformation Bands
Fe2 + for transport. Hematite may be reduced by chemical reactions with hydrocarbons, organic acids, methane, or hydrogen sulfide.
Diverse compositions of ground water are present
in the Navajo Sandstone today. These diverse fluids include petroleum and natural gas reservoirs, as well as
brines with salinities as much as 20,000 mg/L dissolved
solids and as much as 19 mg/L dissolved iron (Spangler
et al., 1996). Brines have moved from below the Navajo
upward, where artesian pressures are high (Hood and
Patterson, 1984). These fluids contain variable amounts
of sulfate, methane, and other dissolved solids.
The candidate water chosen for geochemical modeling has a composition in the midrange of the diverse
compositions of Navajo ground water. The water contains 3137 mg/kg Cl , 2193 mg/kg Na + , 2028 mg/kg
SO42 , 3720 mg/kg HCO3 , 1148 mg/kg Ca2 + , 242
mg/kg K + , 50 mg/kg Mg2 + , and 2 mg/kg O2. The
chemical interaction of water, methane, and hematite
bearing red sandstone has been modeled using the
Geochemist’s Workbench (Bethke, 1998). The reaction
path (Figure 8A) begins at pH 6 and high oxidation
potential. As methane reacts, the oxidation potential
decreases toward sulfate reduction and pyrite precipitation. The concentration of dissolved iron systematically increases as methane reacts and reaches a maximum value near 14 mg/kg in the hematite stability
field (Figure 8B). The reaction path then crosses into
the pyrite stability field, pyrite precipitates, and dissolved iron decreases to 2 mg/kg. Reduction of Fe3 + in
hematite to Fe2 + in solution requires transfer of one
electron from methane. Reduction of SO42 in solution to S22 in pyrite requires transfer of seven electrons for each atom of S. Precipitation of pyrite also
reduces the concentration of dissolved sulfur and drastically restricts the pyrite stability field, permitting
precipitation of magnetite. Iron may be removed from
the rock in solution in the reaction path above at
higher values of oxidation, where the dissolved concentration is relatively high. Reduction of sulfur and
precipitation of pyrite not only fixes iron in the rock,
but also bleaches the rock in conversion of red hematite to pyrite according to the following reaction:
If deformation-band porosity was initially 15% during
dilatancy, the 1.3 g of pyrite would be distributed
through 6667 cm3 (15.1 kg) of rock.
In order for bleached deformation bands to contain 0.1 wt.% less iron than the host rock, about 100
mg of Fe must be removed per 100 g of rock. For a
rock density of 2.26 g/cm3 and 15 vol.% porosity, a
100-g portion of rock contains 6.6 cm3 (0.40 in.3) of
pore space. The calculated solubility of hematite at the
solubility maximum just outside the sulfide stability
field is about 14 mg/kg of fluid. If iron is removed over
the portion of reaction path where the solubility averages 10 mg/kg (shaded area in Figure 8B), removal of
100 mg of iron (143 mg of Fe2O3) requires 10 kg of
fluid per 100-g portion of rock with 6.6 cm3 of pore
space. Assuming a fluid nominal density of 1 g/cm3, the
fluid flow to remove the iron must amount to 1500
pore volumes.
Concentration of total Al in solution in equilibrium with aluminosilicate minerals is approximately
0.005 mg/kg. The concentration of Al is so low that
alteration of aluminum content of the deformation
band would be difficult with reasonable fluid volumes. The decrease in clay content of the deformation bands is not the result of removal of Al in solution, but likely follows a two-stage reaction path shown
in Figure 8C. The fluid in a deformation band reacts
with K-feldspar, kaolinite, and illite. First, kaolinite
reacts with K-feldspar (point A in Figure 8C) in the
presence of solution to form illite (modeled as muscovite) according to
4 Hematite þ 16 SO4 2 þ 17 Hþ þ 15 CH4 ðgÞ ¼
KAl3 Si3 O10 ðOHÞ2 þ 2Kþ þ 6SiO2 ¼
8 Pyrite þ 31 H2 O þ 15 HCO3 The quantity of pyrite precipitated is dependent on the
total sulfur content of the fluid. For the fluid modeled
here, 1.3 g of pyrite precipitates per kilogram of fluid.
Al2 Si2 O5 ðOHÞ4 þ KAlSi3 O8 ¼
KAl3 Si3 O10 ðOHÞ2 þ 2SiO2 þ H2 O
Following consumption of kaolinite, an additional small
amount of illite is precipitated as the solution reacts
with K-feldspar, until equilibrium with K-feldspar is
established. Then, reaction of illite with K + and SiO2
provided by the solution (point B in Figure 8C) consumes illite and precipitates additional K-feldspar according to
3KAlSi3 O8 þ 2 Hþ
Reaction of 0.005 mol of illite to form K-feldspar
consumes 0.01 mol of K + (391 mg) from solution,
resulting in an increase of 0.942 g of K2O in the rock
Parry et al.
187
Figure 8. (A) Equilibrium mineral stabilities in terms of pH and
log f O2, together with the calculated reaction path for reaction of
CH4 with red sandstone at 50jC. CH4 chemically reduces the
iron in the sandstone, increasing iron solubility. Sulfur is then
reduced, precipitating pyrite. Molality of sulfur = 0.01, activity of
Fe2 + = 10 5. (B) Total concentration of iron species in solution
as a function of log f O2. Shaded area represents the segment
of the reaction path that averages 10 mg/kg dissolved Fe.
(C) Equilibrium activity diagram in terms of log a (K + /H + ) and
log a (Na + /H + ). The heavy black line is the reaction path that
accounts for the decrease in abundance of kaolinite and illite
(modeled as muscovite) in the deformation bands.
analysis. The reaction consumes 0.030 mol of SiO2
(1.80 g). The SiO2 may be derived locally from quartz
grains in the deformation band, resulting in no change
in SiO2 content of the band. Conversion of 0.005 mol
of illite to K-feldspar with the 10 kg of solution required to remove 100 mg of Fe requires 39 mg/kg of
K + in solution. The candidate water used in the geochemical simulation contains 242 mg/kg K + . Paragenetic relationships between illite and kaolinite in the
fine-grained cataclasite of the deformation bands could
not be petrographically recognized. Ribbon and porefilling K-feldspar textures are consistent with its formation during fluid-rock interaction.
188
DISCUSSION
On a broad, regional scale, the Jurassic eolian sandstones and, to a lesser extent, associated fluvial and
sabkha deposits are largely aquifers and reservoir rocks,
separated from petroleum source rocks below by confining aquitards of the fine-grained Triassic Chinle and
Moenkopi formations. High-angle reverse faults associated with the Laramide monoclines are potential
fluid pathways for communication of hydrocarbons
into the red sandstone.
On a smaller, localized scale, deformation bands
provide additional fluid pathways during their initial
Chemical Bleaching Indicates Episodes of Fluid Flow in Deformation Bands
Table 2. Comparison of Color, Texture, Mineralogy, and Chemistry of Deformation Bands with Host Rock
Characteristic
Color
Texture
Quartz
K-feldspar
Hematite
Clay
Calcite
Mineralogy
Clay minerals
K-feldspar
Pyrite
Chemical Composition
Fe2O3
Host Navajo Sandstone
Deformation Band
dark reddish brown, pale
reddish brown, pale red
very light gray, white,
grayish pink
fine grained, well rounded, 0.1– 0.25 mm
cataclastic, fractured,
angular, 0.2 mm to < 4 mm
ribbons, elongate masses,
pore fillings on margin
discreet masses
discontinuous to absent illite
coatings; kaolinite not visible
finely disseminated, veinlike,
crosscuts cataclastic quartz
fine grained, well rounded,
0.1 –0.25 mm
coatings on quartz grains
illite coatings on quartz;
kaolinite pore filling
sporadic cement
1.3 –4.6% illite,
0.58 – 1.3% kaolinite
3.2 –7.5%
absent
0.36 – 1.32%
dilatant stage. Hydrocarbons are inferred to have chemically reduced iron and sulfur in deformation bands.
Access of reducing fluids to the deformation bands
under saturated flow conditions is possible only during
initial, dilatant stages of development. Alternatively,
unsaturated, gravity-driven flow similar to arid and
semiarid vadose zones has been invoked to transport
moisture and solutes in the deformation bands (Sigda
et al., 1999; Sigda and Wilson, 2003). The necessary
chemical reductants could be provided by soil solutions. However, semiarid vadose solutions are chemically immature and would likely react with K-feldspar
to produce clay minerals. Vadose solutions are inconsistent with the observed decrease in clay content
and stability of K-feldspar in the deformation bands.
The shear cataclastic deformation bands are tectonic in origin, and the greatest principal stress in this
portion of the Waterpocket fold was oriented eastnortheast to west-southwest during Laramide compression (Davis, 1999). The deformation bands are permeable in saturated flow conditions only during their
initial, dilatant stage of formation accompanying the
formation of the Waterpocket fold. Bleached deformation bands, therefore, constrain the timing of satu-
less clay than host; 1.3 –3% illite,
0.26 – 0.70% kaolinite
more K-feldspar than host,
4.2 –8.6%
small disseminated grains
average depletion 0.15%
relative to host
rated flow of reducing solutions to the initial stage of
band formation during Laramide regional shortening.
If bleaching, reduction in clay content, and formation
of K-feldspar are formed from unsaturated flow in
the vadose zone, then the bleaching solutions would
have permeated the deformation bands following exhumation of the Waterpocket fold, but we consider
this an unlikely scenario based on mineralogy of the
deformation bands.
The color, texture, chemistry, and mineralogy of
deformation bands are significantly different than the
host rocks from which they form (Table 2). Deformation bands contain less kaolinite, illite, and Fe2O3
than their corresponding host rocks. Removal of iron
requires a chemically reducing fluid. Hydrocarbon is
inferred to be the solution that chemically reduces and
removes iron based on published observations of redbed bleaching from as early as Moulton (1926). Other
chemical reducing agents are possible, including hydrogen sulfide and organic acids. If the reducing agent
is hydrocarbon, organic acid, or methane, chemical reduction also reduces SO42 from ground water, and
iron precipitates as pyrite observed in most of the
deformation bands studied. Reducing agent and SO42 Parry et al.
189
are provided by two separate solutions. If hydrogen
sulfide is the reducing agent, then pyrite precipitates
from sulfide in the reducing fluid. At least two separate fluids must be involved: a solution containing
the reducing agent and, possibly, reduced sulfur and
ground water that initially saturated the Navajo Sandstone. Migrating, reducing solutions likely displaced
the Navajo Sandstone ground water. Interaction of the
two fluids with the rocks produced bleaching and pyrite precipitation, and altered the clay content of the
rock. Precipitation of dissolved iron as hematite nodules implies a third, oxidizing solution discussed in
Chan et al. (2000). The complex spectrum of colors and
mineralogy of the sediments suggests multiple episodes
of fluid-rock interaction.
CONCLUSIONS
Deformation bands and sandstone coloration can be
observed in core as well as outcrop and used to understand fluid movement and timing. The color of
deformation bands in red sandstone is an indication of
fluid flow in the deformation band. Red deformation
bands in red sandstone result when reducing (bleaching) fluids are not present. White deformation bands
in red sandstone suggest movement of chemically reducing fluid in the deformation band. Chemical analyses indicate mobility of Fe during deformation-band
formation. In fact, iron depletions in the white deformation bands indicate 1500 pore volumes of fluid are
required to remove 0.1 wt.% Fe2O3. Geochemical
modeling of reaction of CH4 with the red sandstone
shows that iron is mobile before conditions are sufficiently reducing to precipitate pyrite. Following pyrite precipitation from reduction of ferric iron and
sulfate, the iron is immobilized as pyrite and magnetite, neither of which is colored. More complex patterns of multiple colors and precipitants suggest multiple events and different timing and compositions of
moving fluids.
Small amounts of iron oxides are required to color
sandstone, but iron is a sensitive and useful index to
the timing and paths of fluid flow. This initial study
shows the significance of an early, permeable stage
inferred to be during dilatancy in deformation-band
formation in which fluid (likely hydrocarbon) flows
during tectonic events. Large quantities of reducing
fluid have changed the color, chemical composition,
and mineralogy of cataclastic deformation bands during a tectonic event. This analysis has valuable ap190
plications to petrophysical controls on deformation
and hydrocarbon migration associated with tectonic
activity.
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